egui/crates/eframe/src/native/run.rs

//! Note that this file contains two similar paths - one for [`glow`], one for [`wgpu`].
//! When making changes to one you often also want to apply it to the other.
//!
//! This is also very complex code, and not very pretty.
//! There is a bunch of improvements we could do,
//! like removing a bunch of `unwraps`.

use std::{cell::RefCell, rc::Rc, sync::Arc, time::Instant};

use raw_window_handle::{HasRawDisplayHandle as _, HasRawWindowHandle as _};
use winit::{
    event_loop::{ControlFlow, EventLoop, EventLoopBuilder, EventLoopProxy, EventLoopWindowTarget},
    window::{Window, WindowId},
};

use egui::{epaint::ahash::HashMap, ViewportBuilder, ViewportId};

#[cfg(feature = "accesskit")]
use egui_winit::accesskit_winit;
use egui_winit::winit;

use crate::{epi, Result};

use super::epi_integration::{self, EpiIntegration};

// ----------------------------------------------------------------------------

pub const IS_DESKTOP: bool = cfg!(any(
    target_os = "freebsd",
    target_os = "linux",
    target_os = "macos",
    target_os = "openbsd",
    target_os = "windows",
));

// ----------------------------------------------------------------------------

/// The custom even `eframe` uses with the [`winit`] event loop.
#[derive(Debug)]
pub enum UserEvent {
    /// A repaint is requested.
    RequestRepaint {
        /// What to repaint.
        viewport_id: ViewportId,

        /// When to repaint.
        when: Instant,

        /// What the frame number was when the repaint was _requested_.
        frame_nr: u64,
    },

    /// A request related to [`accesskit`](https://accesskit.dev/).
    #[cfg(feature = "accesskit")]
    AccessKitActionRequest(accesskit_winit::ActionRequestEvent),
}

#[cfg(feature = "accesskit")]
impl From<accesskit_winit::ActionRequestEvent> for UserEvent {
    fn from(inner: accesskit_winit::ActionRequestEvent) -> Self {
        Self::AccessKitActionRequest(inner)
    }
}

// ----------------------------------------------------------------------------

pub use epi::NativeOptions;

#[derive(Debug)]
enum EventResult {
    Wait,

    /// Causes a synchronous repaint inside the event handler. This should only
    /// be used in special situations if the window must be repainted while
    /// handling a specific event. This occurs on Windows when handling resizes.
    ///
    /// `RepaintNow` creates a new frame synchronously, and should therefore
    /// only be used for extremely urgent repaints.
    RepaintNow(WindowId),

    /// Queues a repaint for once the event loop handles its next redraw. Exists
    /// so that multiple input events can be handled in one frame. Does not
    /// cause any delay like `RepaintNow`.
    RepaintNext(WindowId),

    RepaintAt(WindowId, Instant),

    Exit,
}

trait WinitApp {
    /// The current frame number, as reported by egui.
    fn frame_nr(&self, viewport_id: ViewportId) -> u64;

    fn is_focused(&self, window_id: WindowId) -> bool;

    fn integration(&self) -> Option<&EpiIntegration>;

    fn window(&self, window_id: WindowId) -> Option<Rc<Window>>;

    fn window_id_from_viewport_id(&self, id: ViewportId) -> Option<WindowId>;

    fn save_and_destroy(&mut self);

    fn run_ui_and_paint(&mut self, window_id: WindowId) -> EventResult;

    fn on_event(
        &mut self,
        event_loop: &EventLoopWindowTarget<UserEvent>,
        event: &winit::event::Event<'_, UserEvent>,
    ) -> Result<EventResult>;
}

fn create_event_loop_builder(
    native_options: &mut epi::NativeOptions,
) -> EventLoopBuilder<UserEvent> {
    crate::profile_function!();
    let mut event_loop_builder = winit::event_loop::EventLoopBuilder::with_user_event();

    if let Some(hook) = std::mem::take(&mut native_options.event_loop_builder) {
        hook(&mut event_loop_builder);
    }

    event_loop_builder
}

fn create_event_loop(native_options: &mut epi::NativeOptions) -> EventLoop<UserEvent> {
    crate::profile_function!();
    let mut builder = create_event_loop_builder(native_options);

    crate::profile_scope!("EventLoopBuilder::build");
    builder.build()
}

/// Access a thread-local event loop.
///
/// We reuse the event-loop so we can support closing and opening an eframe window
/// multiple times. This is just a limitation of winit.
fn with_event_loop<R>(
    mut native_options: epi::NativeOptions,
    f: impl FnOnce(&mut EventLoop<UserEvent>, NativeOptions) -> R,
) -> R {
    thread_local!(static EVENT_LOOP: RefCell<Option<EventLoop<UserEvent>>> = RefCell::new(None));

    EVENT_LOOP.with(|event_loop| {
        // Since we want to reference NativeOptions when creating the EventLoop we can't
        // do that as part of the lazy thread local storage initialization and so we instead
        // create the event loop lazily here
        let mut event_loop = event_loop.borrow_mut();
        let event_loop = event_loop.get_or_insert_with(|| create_event_loop(&mut native_options));
        f(event_loop, native_options)
    })
}

#[cfg(not(target_os = "ios"))]
fn run_and_return(
    event_loop: &mut EventLoop<UserEvent>,
    mut winit_app: impl WinitApp,
) -> Result<()> {
    use winit::platform::run_return::EventLoopExtRunReturn as _;

    log::debug!("Entering the winit event loop (run_return)…");

    // When to repaint what window
    let mut windows_next_repaint_times = HashMap::default();

    let mut returned_result = Ok(());

    event_loop.run_return(|event, event_loop, control_flow| {
        crate::profile_scope!("winit_event", short_event_description(&event));

        let event_result = match &event {
            winit::event::Event::LoopDestroyed => {
                // On Mac, Cmd-Q we get here and then `run_return` doesn't return (despite its name),
                // so we need to save state now:
                log::debug!("Received Event::LoopDestroyed - saving app state…");
                winit_app.save_and_destroy();
                *control_flow = ControlFlow::Exit;
                return;
            }

            winit::event::Event::RedrawRequested(window_id) => {
                windows_next_repaint_times.remove(window_id);
                winit_app.run_ui_and_paint(*window_id)
            }

            winit::event::Event::UserEvent(UserEvent::RequestRepaint {
                when,
                frame_nr,
                viewport_id,
            }) => {
                let current_frame_nr = winit_app.frame_nr(*viewport_id);
                if current_frame_nr == *frame_nr || current_frame_nr == *frame_nr + 1 {
                    log::trace!("UserEvent::RequestRepaint scheduling repaint at {when:?}");
                    if let Some(window_id) = winit_app.window_id_from_viewport_id(*viewport_id) {
                        EventResult::RepaintAt(window_id, *when)
                    } else {
                        EventResult::Wait
                    }
                } else {
                    log::trace!("Got outdated UserEvent::RequestRepaint");
                    EventResult::Wait // old request - we've already repainted
                }
            }

            winit::event::Event::NewEvents(winit::event::StartCause::ResumeTimeReached {
                ..
            }) => {
                log::trace!("Woke up to check next_repaint_time");
                EventResult::Wait
            }

            event => match winit_app.on_event(event_loop, event) {
                Ok(event_result) => event_result,
                Err(err) => {
                    log::error!("Exiting because of error: {err} during event {event:?}");
                    returned_result = Err(err);
                    EventResult::Exit
                }
            },
        };

        match event_result {
            EventResult::Wait => {
                control_flow.set_wait();
            }
            EventResult::RepaintNow(window_id) => {
                log::trace!("Repaint caused by {}", short_event_description(&event));
                if cfg!(target_os = "windows") {
                    // Fix flickering on Windows, see https://github.com/emilk/egui/pull/2280
                    windows_next_repaint_times.remove(&window_id);

                    winit_app.run_ui_and_paint(window_id);
                } else {
                    // Fix for https://github.com/emilk/egui/issues/2425
                    windows_next_repaint_times.insert(window_id, Instant::now());
                }
            }
            EventResult::RepaintNext(window_id) => {
                log::trace!("Repaint caused by {}", short_event_description(&event));
                windows_next_repaint_times.insert(window_id, Instant::now());
            }
            EventResult::RepaintAt(window_id, repaint_time) => {
                windows_next_repaint_times.insert(
                    window_id,
                    windows_next_repaint_times
                        .get(&window_id)
                        .map_or(repaint_time, |last| (*last).min(repaint_time)),
                );
            }
            EventResult::Exit => {
                log::debug!("Asking to exit event loop…");
                winit_app.save_and_destroy();
                *control_flow = ControlFlow::Exit;
                return;
            }
        }

        let mut next_repaint_time = windows_next_repaint_times.values().min().copied();

        // This is for not duplicating redraw requests
        use winit::event::Event;
        if matches!(
            event,
            Event::RedrawEventsCleared | Event::RedrawRequested(_) | Event::Resumed
        ) {
            windows_next_repaint_times.retain(|window_id, repaint_time| {
                if Instant::now() < *repaint_time {
                    return true;
                };

                next_repaint_time = None;
                control_flow.set_poll();

                if let Some(window) = winit_app.window(*window_id) {
                    log::trace!("request_redraw for {window_id:?}");
                    window.request_redraw();
                    true
                } else {
                    false
                }
            });
        }

        if let Some(next_repaint_time) = next_repaint_time {
            let time_until_next = next_repaint_time.saturating_duration_since(Instant::now());
            if time_until_next < std::time::Duration::from_secs(10_000) {
                log::trace!("WaitUntil {time_until_next:?}");
            }
            control_flow.set_wait_until(next_repaint_time);
        };
    });

    log::debug!("eframe window closed");

    drop(winit_app);

    // On Windows this clears out events so that we can later create another window.
    // See https://github.com/emilk/egui/pull/1889 for details.
    //
    // Note that this approach may cause issues on macOS (emilk/egui#2768); therefore,
    // we only apply this approach on Windows to minimize the affect.
    #[cfg(target_os = "windows")]
    {
        event_loop.run_return(|_, _, control_flow| {
            control_flow.set_exit();
        });
    }

    returned_result
}

fn run_and_exit(event_loop: EventLoop<UserEvent>, mut winit_app: impl WinitApp + 'static) -> ! {
    log::debug!("Entering the winit event loop (run)…");

    // When to repaint what window
    let mut windows_next_repaint_times = HashMap::default();

    event_loop.run(move |event, event_loop, control_flow| {
        crate::profile_scope!("winit_event", short_event_description(&event));

        let event_result = match &event {
            winit::event::Event::LoopDestroyed => {
                log::debug!("Received Event::LoopDestroyed");
                EventResult::Exit
            }

            winit::event::Event::RedrawRequested(window_id) => {
                windows_next_repaint_times.remove(window_id);
                winit_app.run_ui_and_paint(*window_id)
            }

            winit::event::Event::UserEvent(UserEvent::RequestRepaint {
                when,
                frame_nr,
                viewport_id,
            }) => {
                let current_frame_nr = winit_app.frame_nr(*viewport_id);
                if current_frame_nr == *frame_nr || current_frame_nr == *frame_nr + 1 {
                    if let Some(window_id) = winit_app.window_id_from_viewport_id(*viewport_id) {
                        EventResult::RepaintAt(window_id, *when)
                    } else {
                        EventResult::Wait
                    }
                } else {
                    log::trace!("Got outdated UserEvent::RequestRepaint");
                    EventResult::Wait // old request - we've already repainted
                }
            }

            winit::event::Event::NewEvents(winit::event::StartCause::ResumeTimeReached {
                ..
            }) => {
                log::trace!("Woke up to check next_repaint_time");
                EventResult::Wait
            }

            event => match winit_app.on_event(event_loop, event) {
                Ok(event_result) => event_result,
                Err(err) => {
                    panic!("eframe encountered a fatal error: {err} during event {event:?}");
                }
            },
        };

        match event_result {
            EventResult::Wait => {
                control_flow.set_wait();
            }
            EventResult::RepaintNow(window_id) => {
                log::trace!("Repaint caused by {}", short_event_description(&event));
                if cfg!(target_os = "windows") {
                    // Fix flickering on Windows, see https://github.com/emilk/egui/pull/2280
                    windows_next_repaint_times.remove(&window_id);

                    winit_app.run_ui_and_paint(window_id);
                } else {
                    // Fix for https://github.com/emilk/egui/issues/2425
                    windows_next_repaint_times.insert(window_id, Instant::now());
                }
            }
            EventResult::RepaintNext(window_id) => {
                log::trace!("Repaint caused by {}", short_event_description(&event));
                windows_next_repaint_times.insert(window_id, Instant::now());
            }
            EventResult::RepaintAt(window_id, repaint_time) => {
                windows_next_repaint_times.insert(
                    window_id,
                    windows_next_repaint_times
                        .get(&window_id)
                        .map_or(repaint_time, |last| (*last).min(repaint_time)),
                );
            }
            EventResult::Exit => {
                log::debug!("Quitting - saving app state…");
                winit_app.save_and_destroy();
                #[allow(clippy::exit)]
                std::process::exit(0);
            }
        }

        let mut next_repaint_time = windows_next_repaint_times.values().min().copied();

        // This is for not duplicating redraw requests
        use winit::event::Event;
        if matches!(
            event,
            Event::RedrawEventsCleared | Event::RedrawRequested(_) | Event::Resumed
        ) {
            windows_next_repaint_times.retain(|window_id, repaint_time| {
                if Instant::now() < *repaint_time {
                    return true;
                }

                next_repaint_time = None;
                control_flow.set_poll();

                if let Some(window) = winit_app.window(*window_id) {
                    log::trace!("request_redraw for {window_id:?}");
                    window.request_redraw();
                    true
                } else {
                    false
                }
            });
        }

        if let Some(next_repaint_time) = next_repaint_time {
            let time_until_next = next_repaint_time.saturating_duration_since(Instant::now());
            if time_until_next < std::time::Duration::from_secs(10_000) {
                log::trace!("WaitUntil {time_until_next:?}");
            }

            // WaitUntil seems to not work on iOS
            #[cfg(target_os = "ios")]
            winit_app
                .get_window_winit_id(ViewportId::ROOT)
                .map(|window_id| {
                    winit_app
                        .window(window_id)
                        .map(|window| window.request_redraw())
                });

            control_flow.set_wait_until(next_repaint_time);
        };
    })
}

// ----------------------------------------------------------------------------
/// Run an egui app
#[cfg(feature = "glow")]
mod glow_integration {
    use glutin::{
        display::GetGlDisplay,
        prelude::{GlDisplay, NotCurrentGlContextSurfaceAccessor, PossiblyCurrentGlContext},
        surface::GlSurface,
    };

    use egui::{
        epaint::ahash::HashMap, DeferredViewportUiCallback, ImmediateViewport, NumExt as _,
        ViewportClass, ViewportIdMap, ViewportIdPair, ViewportIdSet, ViewportOutput,
    };
    use egui_winit::{create_winit_window_builder, process_viewport_commands, EventResponse};

    use super::*;

    // Note: that the current Glutin API design tightly couples the GL context with
    // the Window which means it's not practically possible to just destroy the
    // window and re-create a new window while continuing to use the same GL context.
    //
    // For now this means it's not possible to support Android as well as we can with
    // wgpu because we're basically forced to destroy and recreate _everything_ when
    // the application suspends and resumes.
    //
    // There is work in progress to improve the Glutin API so it has a separate Surface
    // API that would allow us to just destroy a Window/Surface when suspending, see:
    // https://github.com/rust-windowing/glutin/pull/1435

    /// State that is initialized when the application is first starts running via
    /// a Resumed event. On Android this ensures that any graphics state is only
    /// initialized once the application has an associated `SurfaceView`.
    struct GlowWinitRunning {
        integration: epi_integration::EpiIntegration,
        app: Box<dyn epi::App>,

        // These needs to be shared with the immediate viewport renderer, hence the Rc/Arc/RefCells:
        glutin: Rc<RefCell<GlutinWindowContext>>,
        painter: Rc<RefCell<egui_glow::Painter>>,
    }

    impl GlowWinitRunning {
        fn run_ui_and_paint(
            &mut self,
            window_id: WindowId,
            focused_viewport: Option<ViewportId>,
        ) -> EventResult {
            let Some(viewport_id) = self
                .glutin
                .borrow()
                .viewport_from_window
                .get(&window_id)
                .copied()
            else {
                return EventResult::Wait;
            };

            #[cfg(feature = "puffin")]
            puffin::GlobalProfiler::lock().new_frame();
            crate::profile_scope!("frame");

            {
                let glutin = self.glutin.borrow();
                let viewport = &glutin.viewports[&viewport_id];
                let is_immediate = viewport.viewport_ui_cb.is_none();
                if is_immediate && viewport_id != ViewportId::ROOT {
                    // This will only happen if this is an immediate viewport.
                    // That means that the viewport cannot be rendered by itself and needs his parent to be rendered.
                    if let Some(parent_viewport) = glutin.viewports.get(&viewport.ids.parent) {
                        if let Some(window) = parent_viewport.window.as_ref() {
                            return EventResult::RepaintNext(window.id());
                        }
                    }
                    return EventResult::Wait;
                }
            }

            let (raw_input, viewport_ui_cb) = {
                let mut glutin = self.glutin.borrow_mut();
                let viewport = glutin.viewports.get_mut(&viewport_id).unwrap();
                let window = viewport.window.as_ref().unwrap();

                let egui_winit = viewport.egui_winit.as_mut().unwrap();
                let raw_input = egui_winit.take_egui_input(window, viewport.ids);

                self.integration.pre_update(window);

                (raw_input, viewport.viewport_ui_cb.clone())
            };

            // ------------------------------------------------------------
            // The update function, which could call immediate viewports,
            // so make sure we don't hold any locks here required by the immediate viewports rendeer.

            let full_output =
                self.integration
                    .update(self.app.as_mut(), viewport_ui_cb.as_deref(), raw_input);

            // ------------------------------------------------------------

            let Self {
                integration,
                app,
                glutin,
                painter,
                ..
            } = self;

            let mut glutin = glutin.borrow_mut();
            let mut painter = painter.borrow_mut();

            let egui::FullOutput {
                platform_output,
                textures_delta,
                shapes,
                pixels_per_point,
                viewport_output,
            } = full_output;

            let GlutinWindowContext {
                viewports,
                current_gl_context,
                ..
            } = &mut *glutin;

            let viewport = viewports.get_mut(&viewport_id).unwrap();
            let window = viewport.window.as_ref().unwrap();
            let gl_surface = viewport.gl_surface.as_ref().unwrap();
            let egui_winit = viewport.egui_winit.as_mut().unwrap();

            integration.post_update(app.as_mut(), window);
            integration.handle_platform_output(window, viewport_id, platform_output, egui_winit);

            let clipped_primitives = integration.egui_ctx.tessellate(shapes, pixels_per_point);

            *current_gl_context = Some(
                current_gl_context
                    .take()
                    .unwrap()
                    .make_not_current()
                    .unwrap()
                    .make_current(gl_surface)
                    .unwrap(),
            );

            let screen_size_in_pixels: [u32; 2] = window.inner_size().into();

            painter.clear(
                screen_size_in_pixels,
                app.clear_color(&integration.egui_ctx.style().visuals),
            );

            painter.paint_and_update_textures(
                screen_size_in_pixels,
                pixels_per_point,
                &clipped_primitives,
                &textures_delta,
            );

            {
                let screenshot_requested = &mut integration.frame.output.screenshot_requested;
                if *screenshot_requested {
                    *screenshot_requested = false;
                    let screenshot = painter.read_screen_rgba(screen_size_in_pixels);
                    integration.frame.screenshot.set(Some(screenshot));
                }
                integration.post_rendering(app.as_mut(), window);
            }

            {
                crate::profile_scope!("swap_buffers");
                if let Err(err) = gl_surface.swap_buffers(
                    current_gl_context
                        .as_ref()
                        .expect("failed to get current context to swap buffers"),
                ) {
                    log::error!("swap_buffers failed: {err}");
                }
            }

            integration.post_present(window);

            // give it time to settle:
            #[cfg(feature = "__screenshot")]
            if integration.egui_ctx.frame_nr() == 2 {
                if let Ok(path) = std::env::var("EFRAME_SCREENSHOT_TO") {
                    save_screeshot_and_exit(&path, &painter, screen_size_in_pixels);
                }
            }

            integration.maybe_autosave(app.as_mut(), Some(window));

            if window.is_minimized() == Some(true) {
                // On Mac, a minimized Window uses up all CPU:
                // https://github.com/emilk/egui/issues/325
                crate::profile_scope!("minimized_sleep");
                std::thread::sleep(std::time::Duration::from_millis(10));
            }

            glutin.handle_viewport_output(viewport_output, focused_viewport);

            if integration.should_close() {
                EventResult::Exit
            } else {
                EventResult::Wait
            }
        }

        fn on_window_event(
            &mut self,
            window_id: WindowId,
            event: &winit::event::WindowEvent<'_>,
            focused_viewport: &mut Option<ViewportId>,
        ) -> EventResult {
            let viewport_id = self
                .glutin
                .borrow()
                .viewport_from_window
                .get(&window_id)
                .copied();

            // On Windows, if a window is resized by the user, it should repaint synchronously, inside the
            // event handler.
            //
            // If this is not done, the compositor will assume that the window does not want to redraw,
            // and continue ahead.
            //
            // In eframe's case, that causes the window to rapidly flicker, as it struggles to deliver
            // new frames to the compositor in time.
            //
            // The flickering is technically glutin or glow's fault, but we should be responding properly
            // to resizes anyway, as doing so avoids dropping frames.
            //
            // See: https://github.com/emilk/egui/issues/903
            let mut repaint_asap = false;

            match event {
                winit::event::WindowEvent::Focused(new_focused) => {
                    *focused_viewport = new_focused.then(|| viewport_id).flatten();
                }

                winit::event::WindowEvent::Resized(physical_size) => {
                    // Resize with 0 width and height is used by winit to signal a minimize event on Windows.
                    // See: https://github.com/rust-windowing/winit/issues/208
                    // This solves an issue where the app would panic when minimizing on Windows.
                    if 0 < physical_size.width && 0 < physical_size.height {
                        if let Some(viewport_id) = viewport_id {
                            repaint_asap = true;
                            self.glutin.borrow_mut().resize(viewport_id, *physical_size);
                        }
                    }
                }

                winit::event::WindowEvent::ScaleFactorChanged { new_inner_size, .. } => {
                    if let Some(viewport_id) = viewport_id {
                        repaint_asap = true;
                        self.glutin
                            .borrow_mut()
                            .resize(viewport_id, **new_inner_size);
                    }
                }

                winit::event::WindowEvent::CloseRequested => {
                    let is_root = viewport_id == Some(ViewportId::ROOT);
                    if is_root && self.integration.should_close() {
                        log::debug!("Received WindowEvent::CloseRequested");
                        return EventResult::Exit;
                    }
                }
                _ => {}
            }

            let event_response = 'res: {
                if let Some(viewport_id) = viewport_id {
                    let mut glutin = self.glutin.borrow_mut();
                    if let Some(viewport) = glutin.viewports.get_mut(&viewport_id) {
                        break 'res self.integration.on_event(
                            self.app.as_mut(),
                            event,
                            viewport.egui_winit.as_mut().unwrap(),
                            viewport.ids.this,
                        );
                    }
                }

                EventResponse {
                    consumed: false,
                    repaint: false,
                }
            };

            if self.integration.should_close() {
                EventResult::Exit
            } else if event_response.repaint {
                if repaint_asap {
                    EventResult::RepaintNow(window_id)
                } else {
                    EventResult::RepaintNext(window_id)
                }
            } else {
                EventResult::Wait
            }
        }
    }

    #[cfg(feature = "__screenshot")]
    fn save_screeshot_and_exit(
        path: &str,
        painter: &egui_glow::Painter,
        screen_size_in_pixels: [u32; 2],
    ) {
        assert!(
            path.ends_with(".png"),
            "Expected EFRAME_SCREENSHOT_TO to end with '.png', got {path:?}"
        );
        let screenshot = painter.read_screen_rgba(screen_size_in_pixels);
        image::save_buffer(
            path,
            screenshot.as_raw(),
            screenshot.width() as u32,
            screenshot.height() as u32,
            image::ColorType::Rgba8,
        )
        .unwrap_or_else(|err| {
            panic!("Failed to save screenshot to {path:?}: {err}");
        });
        eprintln!("Screenshot saved to {path:?}.");

        #[allow(clippy::exit)]
        std::process::exit(0);
    }

    struct Viewport {
        ids: ViewportIdPair,
        class: ViewportClass,
        builder: ViewportBuilder,

        /// The user-callback that shows the ui.
        /// None for immediate viewports.
        viewport_ui_cb: Option<Arc<DeferredViewportUiCallback>>,

        gl_surface: Option<glutin::surface::Surface<glutin::surface::WindowSurface>>,
        window: Option<Rc<Window>>,
        egui_winit: Option<egui_winit::State>,
    }

    /// This struct will contain both persistent and temporary glutin state.
    ///
    /// Platform Quirks:
    /// * Microsoft Windows: requires that we create a window before opengl context.
    /// * Android: window and surface should be destroyed when we receive a suspend event. recreate on resume event.
    ///
    /// winit guarantees that we will get a Resumed event on startup on all platforms.
    /// * Before Resumed event: `gl_config`, `gl_context` can be created at any time. on windows, a window must be created to get `gl_context`.
    /// * Resumed: `gl_surface` will be created here. `window` will be re-created here for android.
    /// * Suspended: on android, we drop window + surface.  on other platforms, we don't get Suspended event.
    ///
    /// The setup is divided between the `new` fn and `on_resume` fn. we can just assume that `on_resume` is a continuation of
    /// `new` fn on all platforms. only on android, do we get multiple resumed events because app can be suspended.
    struct GlutinWindowContext {
        swap_interval: glutin::surface::SwapInterval,
        gl_config: glutin::config::Config,

        max_texture_side: Option<usize>,

        current_gl_context: Option<glutin::context::PossiblyCurrentContext>,
        not_current_gl_context: Option<glutin::context::NotCurrentContext>,

        viewports: ViewportIdMap<Viewport>,
        viewport_from_window: HashMap<WindowId, ViewportId>,
        window_from_viewport: ViewportIdMap<WindowId>,
    }

    impl GlutinWindowContext {
        #[allow(unsafe_code)]
        unsafe fn new(
            viewport_builder: ViewportBuilder,
            native_options: &epi::NativeOptions,
            event_loop: &EventLoopWindowTarget<UserEvent>,
        ) -> Result<Self> {
            crate::profile_function!();

            // There is a lot of complexity with opengl creation,
            // so prefer extensive logging to get all the help we can to debug issues.

            use glutin::prelude::*;
            // convert native options to glutin options
            let hardware_acceleration = match native_options.hardware_acceleration {
                crate::HardwareAcceleration::Required => Some(true),
                crate::HardwareAcceleration::Preferred => None,
                crate::HardwareAcceleration::Off => Some(false),
            };
            let swap_interval = if native_options.vsync {
                glutin::surface::SwapInterval::Wait(std::num::NonZeroU32::new(1).unwrap())
            } else {
                glutin::surface::SwapInterval::DontWait
            };
            /*  opengl setup flow goes like this:
                1. we create a configuration for opengl "Display" / "Config" creation
                2. choose between special extensions like glx or egl or wgl and use them to create config/display
                3. opengl context configuration
                4. opengl context creation
            */
            // start building config for gl display
            let config_template_builder = glutin::config::ConfigTemplateBuilder::new()
                .prefer_hardware_accelerated(hardware_acceleration)
                .with_depth_size(native_options.depth_buffer)
                .with_stencil_size(native_options.stencil_buffer)
                .with_transparency(native_options.transparent);
            // we don't know if multi sampling option is set. so, check if its more than 0.
            let config_template_builder = if native_options.multisampling > 0 {
                config_template_builder.with_multisampling(
                    native_options
                        .multisampling
                        .try_into()
                        .expect("failed to fit multisamples option of native_options into u8"),
                )
            } else {
                config_template_builder
            };

            log::debug!("trying to create glutin Display with config: {config_template_builder:?}");

            // Create GL display. This may probably create a window too on most platforms. Definitely on `MS windows`. Never on Android.
            let display_builder = glutin_winit::DisplayBuilder::new()
                // we might want to expose this option to users in the future. maybe using an env var or using native_options.
                .with_preference(glutin_winit::ApiPrefence::FallbackEgl) // https://github.com/emilk/egui/issues/2520#issuecomment-1367841150
                .with_window_builder(Some(create_winit_window_builder(&viewport_builder)));

            let (window, gl_config) = {
                crate::profile_scope!("DisplayBuilder::build");

                display_builder
                    .build(
                        event_loop,
                        config_template_builder.clone(),
                        |mut config_iterator| {
                            let config = config_iterator.next().expect(
                            "failed to find a matching configuration for creating glutin config",
                        );
                            log::debug!(
                                "using the first config from config picker closure. config: {config:?}"
                            );
                            config
                        },
                    )
                    .map_err(|e| {
                        crate::Error::NoGlutinConfigs(config_template_builder.build(), e)
                    })?
            };

            let gl_display = gl_config.display();
            log::debug!(
                "successfully created GL Display with version: {} and supported features: {:?}",
                gl_display.version_string(),
                gl_display.supported_features()
            );
            let raw_window_handle = window.as_ref().map(|w| w.raw_window_handle());
            log::debug!("creating gl context using raw window handle: {raw_window_handle:?}");

            // create gl context. if core context cannot be created, try gl es context as fallback.
            let context_attributes =
                glutin::context::ContextAttributesBuilder::new().build(raw_window_handle);
            let fallback_context_attributes = glutin::context::ContextAttributesBuilder::new()
                .with_context_api(glutin::context::ContextApi::Gles(None))
                .build(raw_window_handle);

            let gl_context_result = {
                crate::profile_scope!("create_context");
                gl_config
                    .display()
                    .create_context(&gl_config, &context_attributes)
            };

            let gl_context = match gl_context_result {
                Ok(it) => it,
                Err(err) => {
                    log::warn!("Failed to create context using default context attributes {context_attributes:?} due to error: {err}");
                    log::debug!("Retrying with fallback context attributes: {fallback_context_attributes:?}");
                    gl_config
                        .display()
                        .create_context(&gl_config, &fallback_context_attributes)?
                }
            };
            let not_current_gl_context = Some(gl_context);

            let mut viewport_from_window = HashMap::default();
            let mut window_from_viewport = ViewportIdMap::default();
            if let Some(window) = &window {
                viewport_from_window.insert(window.id(), ViewportId::ROOT);
                window_from_viewport.insert(ViewportId::ROOT, window.id());
            }

            let mut viewports = ViewportIdMap::default();
            viewports.insert(
                ViewportId::ROOT,
                Viewport {
                    ids: ViewportIdPair::ROOT,
                    class: ViewportClass::Root,
                    builder: viewport_builder,
                    viewport_ui_cb: None,
                    gl_surface: None,
                    window: window.map(Rc::new),
                    egui_winit: None,
                },
            );

            // the fun part with opengl gl is that we never know whether there is an error. the context creation might have failed, but
            // it could keep working until we try to make surface current or swap buffers or something else. future glutin improvements might
            // help us start from scratch again if we fail context creation and go back to preferEgl or try with different config etc..
            // https://github.com/emilk/egui/pull/2541#issuecomment-1370767582

            let mut slf = GlutinWindowContext {
                swap_interval,
                gl_config,
                current_gl_context: None,
                not_current_gl_context,
                viewports,
                viewport_from_window,
                max_texture_side: None,
                window_from_viewport,
            };

            slf.on_resume(event_loop)?;

            Ok(slf)
        }

        /// This will be run after `new`. on android, it might be called multiple times over the course of the app's lifetime.
        /// roughly,
        /// 1. check if window already exists. otherwise, create one now.
        /// 2. create attributes for surface creation.
        /// 3. create surface.
        /// 4. make surface and context current.
        ///
        /// we presently assume that we will
        fn on_resume(&mut self, event_loop: &EventLoopWindowTarget<UserEvent>) -> Result<()> {
            crate::profile_function!();

            let viewports: Vec<ViewportId> = self
                .viewports
                .iter()
                .filter(|(_, viewport)| viewport.gl_surface.is_none())
                .map(|(id, _)| *id)
                .collect();

            for viewport_id in viewports {
                self.init_viewport(viewport_id, event_loop)?;
            }
            Ok(())
        }

        #[allow(unsafe_code)]
        pub(crate) fn init_viewport(
            &mut self,
            viewport_id: ViewportId,
            event_loop: &EventLoopWindowTarget<UserEvent>,
        ) -> Result<()> {
            crate::profile_function!();

            let viewport = self
                .viewports
                .get_mut(&viewport_id)
                .expect("viewport doesn't exist");

            let window = if let Some(window) = &mut viewport.window {
                window
            } else {
                log::trace!("Window doesn't exist yet. Creating one now with finalize_window");
                viewport
                    .window
                    .insert(Rc::new(glutin_winit::finalize_window(
                        event_loop,
                        create_winit_window_builder(&viewport.builder),
                        &self.gl_config,
                    )?))
            };

            {
                // surface attributes
                let (width_px, height_px): (u32, u32) = window.inner_size().into();
                let width_px = std::num::NonZeroU32::new(width_px.at_least(1)).unwrap();
                let height_px = std::num::NonZeroU32::new(height_px.at_least(1)).unwrap();
                let surface_attributes = glutin::surface::SurfaceAttributesBuilder::<
                    glutin::surface::WindowSurface,
                >::new()
                .build(window.raw_window_handle(), width_px, height_px);

                log::trace!("creating surface with attributes: {surface_attributes:?}");
                let gl_surface = unsafe {
                    self.gl_config
                        .display()
                        .create_window_surface(&self.gl_config, &surface_attributes)?
                };

                log::trace!("surface created successfully: {gl_surface:?}. making context current");

                let not_current_gl_context =
                    if let Some(not_current_context) = self.not_current_gl_context.take() {
                        not_current_context
                    } else {
                        self.current_gl_context
                            .take()
                            .unwrap()
                            .make_not_current()
                            .unwrap()
                    };
                let current_gl_context = not_current_gl_context.make_current(&gl_surface)?;

                // try setting swap interval. but its not absolutely necessary, so don't panic on failure.
                log::trace!("made context current. setting swap interval for surface");
                if let Err(err) =
                    gl_surface.set_swap_interval(&current_gl_context, self.swap_interval)
                {
                    log::warn!("Failed to set swap interval due to error: {err}");
                }

                // we will reach this point only once in most platforms except android.
                // create window/surface/make context current once and just use them forever.

                viewport.egui_winit.get_or_insert_with(|| {
                    egui_winit::State::new(
                        event_loop,
                        Some(window.scale_factor() as f32),
                        self.max_texture_side,
                    )
                });

                viewport.gl_surface = Some(gl_surface);
                self.current_gl_context = Some(current_gl_context);
                self.viewport_from_window
                    .insert(window.id(), viewport.ids.this);
                self.window_from_viewport
                    .insert(viewport.ids.this, window.id());
            }

            Ok(())
        }

        /// only applies for android. but we basically drop surface + window and make context not current
        fn on_suspend(&mut self) -> Result<()> {
            log::debug!("received suspend event. dropping window and surface");
            for viewport in self.viewports.values_mut() {
                viewport.gl_surface = None;
                viewport.window = None;
            }
            if let Some(current) = self.current_gl_context.take() {
                log::debug!("context is current, so making it non-current");
                self.not_current_gl_context = Some(current.make_not_current()?);
            } else {
                log::debug!("context is already not current??? could be duplicate suspend event");
            }
            Ok(())
        }

        fn viewport(&self, viewport_id: ViewportId) -> &Viewport {
            self.viewports
                .get(&viewport_id)
                .expect("viewport doesn't exist")
        }

        fn viewport_mut(&mut self, viewport_id: ViewportId) -> &mut Viewport {
            self.viewports
                .get_mut(&viewport_id)
                .expect("viewport doesn't exist")
        }

        fn window(&self, viewport_id: ViewportId) -> Rc<Window> {
            self.viewport(viewport_id)
                .window
                .clone()
                .expect("winit window doesn't exist")
        }

        fn resize(
            &mut self,
            viewport_id: ViewportId,
            physical_size: winit::dpi::PhysicalSize<u32>,
        ) {
            let width_px = std::num::NonZeroU32::new(physical_size.width.at_least(1)).unwrap();
            let height_px = std::num::NonZeroU32::new(physical_size.height.at_least(1)).unwrap();

            if let Some(viewport) = self.viewports.get(&viewport_id) {
                if let Some(gl_surface) = &viewport.gl_surface {
                    self.current_gl_context = Some(
                        self.current_gl_context
                            .take()
                            .unwrap()
                            .make_not_current()
                            .unwrap()
                            .make_current(gl_surface)
                            .unwrap(),
                    );
                    gl_surface.resize(
                        self.current_gl_context
                            .as_ref()
                            .expect("failed to get current context to resize surface"),
                        width_px,
                        height_px,
                    );
                }
            }
        }

        fn get_proc_address(&self, addr: &std::ffi::CStr) -> *const std::ffi::c_void {
            self.gl_config.display().get_proc_address(addr)
        }

        fn handle_viewport_output(
            &mut self,
            viewport_output: ViewportIdMap<ViewportOutput>,
            focused_viewport: Option<ViewportId>,
        ) {
            crate::profile_function!();

            let active_viewports_ids: ViewportIdSet = viewport_output.keys().copied().collect();

            for (
                viewport_id,
                ViewportOutput {
                    parent,
                    class,
                    builder,
                    viewport_ui_cb,
                    commands,
                    repaint_delay: _, // ignored - we listened to the repaint callback instead
                },
            ) in viewport_output
            {
                let ids = ViewportIdPair::from_self_and_parent(viewport_id, parent);

                initialize_or_update_viewport(
                    &mut self.viewports,
                    ids,
                    class,
                    builder,
                    viewport_ui_cb,
                    focused_viewport,
                );

                if let Some(viewport) = self.viewports.get(&viewport_id) {
                    if let Some(window) = &viewport.window {
                        let is_viewport_focused = focused_viewport == Some(viewport_id);
                        egui_winit::process_viewport_commands(
                            commands,
                            window,
                            is_viewport_focused,
                        );
                    }
                }
            }

            // GC old viewports
            self.viewports
                .retain(|id, _| active_viewports_ids.contains(id));
            self.viewport_from_window
                .retain(|_, id| active_viewports_ids.contains(id));
            self.window_from_viewport
                .retain(|id, _| active_viewports_ids.contains(id));
        }
    }

    fn initialize_or_update_viewport(
        viewports: &mut ViewportIdMap<Viewport>,
        ids: ViewportIdPair,
        class: ViewportClass,
        mut builder: ViewportBuilder,
        viewport_ui_cb: Option<Arc<dyn Fn(&egui::Context) + Send + Sync>>,
        focused_viewport: Option<ViewportId>,
    ) -> &mut Viewport {
        crate::profile_function!();

        if builder.icon.is_none() {
            // Inherit icon from parent
            builder.icon = viewports
                .get_mut(&ids.parent)
                .and_then(|vp| vp.builder.icon.clone());
        }

        match viewports.entry(ids.this) {
            std::collections::hash_map::Entry::Vacant(entry) => {
                // New viewport:
                log::debug!("Creating new viewport {:?} ({:?})", ids.this, builder.title);
                entry.insert(Viewport {
                    ids,
                    class,
                    builder,
                    viewport_ui_cb,
                    window: None,
                    egui_winit: None,
                    gl_surface: None,
                })
            }

            std::collections::hash_map::Entry::Occupied(mut entry) => {
                // Patch an existing viewport:
                let viewport = entry.get_mut();

                viewport.ids.parent = ids.parent;
                viewport.class = class;
                viewport.viewport_ui_cb = viewport_ui_cb;

                let (delta_commands, recreate) = viewport.builder.patch(&builder);

                if recreate {
                    log::debug!(
                        "Recreating window for viewport {:?} ({:?})",
                        ids.this,
                        builder.title
                    );
                    viewport.window = None;
                    viewport.egui_winit = None;
                } else if let Some(window) = &viewport.window {
                    let is_viewport_focused = focused_viewport == Some(ids.this);
                    process_viewport_commands(delta_commands, window, is_viewport_focused);
                }

                entry.into_mut()
            }
        }
    }

    struct GlowWinitApp {
        repaint_proxy: Arc<egui::mutex::Mutex<EventLoopProxy<UserEvent>>>,
        app_name: String,
        native_options: epi::NativeOptions,
        running: Option<GlowWinitRunning>,

        // Note that since this `AppCreator` is FnOnce we are currently unable to support
        // re-initializing the `GlowWinitRunning` state on Android if the application
        // suspends and resumes.
        app_creator: Option<epi::AppCreator>,

        focused_viewport: Option<ViewportId>,
    }

    impl GlowWinitApp {
        fn new(
            event_loop: &EventLoop<UserEvent>,
            app_name: &str,
            native_options: epi::NativeOptions,
            app_creator: epi::AppCreator,
        ) -> Self {
            crate::profile_function!();
            Self {
                repaint_proxy: Arc::new(egui::mutex::Mutex::new(event_loop.create_proxy())),
                app_name: app_name.to_owned(),
                native_options,
                running: None,
                app_creator: Some(app_creator),
                focused_viewport: Some(ViewportId::ROOT),
            }
        }

        #[allow(unsafe_code)]
        fn create_glutin_windowed_context(
            event_loop: &EventLoopWindowTarget<UserEvent>,
            storage: Option<&dyn epi::Storage>,
            title: &str,
            native_options: &mut NativeOptions,
        ) -> Result<(GlutinWindowContext, egui_glow::Painter)> {
            crate::profile_function!();

            let window_settings = epi_integration::load_window_settings(storage);

            let winit_window_builder =
                epi_integration::window_builder(event_loop, title, native_options, window_settings);

            let mut glutin_window_context = unsafe {
                GlutinWindowContext::new(winit_window_builder, native_options, event_loop)?
            };

            // Creates the window - must come before we create our glow context
            glutin_window_context.on_resume(event_loop)?;

            if let Some(viewport) = glutin_window_context.viewports.get(&ViewportId::ROOT) {
                if let Some(window) = &viewport.window {
                    epi_integration::apply_native_options_to_window(
                        window,
                        native_options,
                        window_settings,
                    );
                }
            }

            let gl = unsafe {
                crate::profile_scope!("glow::Context::from_loader_function");
                Arc::new(glow::Context::from_loader_function(|s| {
                    let s = std::ffi::CString::new(s)
                        .expect("failed to construct C string from string for gl proc address");

                    glutin_window_context.get_proc_address(&s)
                }))
            };

            let painter = egui_glow::Painter::new(gl, "", native_options.shader_version)?;

            Ok((glutin_window_context, painter))
        }

        fn init_run_state(
            &mut self,
            event_loop: &EventLoopWindowTarget<UserEvent>,
        ) -> Result<&mut GlowWinitRunning> {
            crate::profile_function!();

            let storage = epi_integration::create_storage(
                self.native_options
                    .app_id
                    .as_ref()
                    .unwrap_or(&self.app_name),
            );

            let (mut glutin, painter) = Self::create_glutin_windowed_context(
                event_loop,
                storage.as_deref(),
                &self.app_name,
                &mut self.native_options,
            )?;
            let gl = painter.gl().clone();

            let max_texture_side = painter.max_texture_side();
            glutin.max_texture_side = Some(max_texture_side);
            for viewport in glutin.viewports.values_mut() {
                if let Some(egui_winit) = viewport.egui_winit.as_mut() {
                    egui_winit.set_max_texture_side(max_texture_side);
                }
            }

            let system_theme = system_theme(&glutin.window(ViewportId::ROOT), &self.native_options);

            let mut integration = epi_integration::EpiIntegration::new(
                &glutin.window(ViewportId::ROOT),
                system_theme,
                &self.app_name,
                &self.native_options,
                storage,
                IS_DESKTOP,
                Some(gl.clone()),
                #[cfg(feature = "wgpu")]
                None,
            );

            {
                let event_loop_proxy = self.repaint_proxy.clone();
                integration
                    .egui_ctx
                    .set_request_repaint_callback(move |info| {
                        log::trace!("request_repaint_callback: {info:?}");
                        let when = Instant::now() + info.delay;
                        let frame_nr = info.current_frame_nr;
                        event_loop_proxy
                            .lock()
                            .send_event(UserEvent::RequestRepaint {
                                viewport_id: info.viewport_id,
                                when,
                                frame_nr,
                            })
                            .ok();
                    });
            }

            #[cfg(feature = "accesskit")]
            {
                let event_loop_proxy = self.repaint_proxy.lock().clone();
                let viewport = glutin.viewports.get_mut(&ViewportId::ROOT).unwrap();
                if let Viewport {
                    window: Some(window),
                    egui_winit: Some(egui_winit),
                    ..
                } = viewport
                {
                    integration.init_accesskit(egui_winit, window, event_loop_proxy);
                }
            }

            let theme = system_theme.unwrap_or(self.native_options.default_theme);
            integration.egui_ctx.set_visuals(theme.egui_visuals());

            if self.native_options.mouse_passthrough {
                if let Err(err) = glutin.window(ViewportId::ROOT).set_cursor_hittest(false) {
                    log::warn!("set_cursor_hittest(false) failed: {err}");
                }
            }

            let app_creator = std::mem::take(&mut self.app_creator)
                .expect("Single-use AppCreator has unexpectedly already been taken");

            let app = {
                let window = glutin.window(ViewportId::ROOT);
                let mut app = app_creator(&epi::CreationContext {
                    egui_ctx: integration.egui_ctx.clone(),
                    integration_info: integration.frame.info().clone(),
                    storage: integration.frame.storage(),
                    gl: Some(gl),
                    #[cfg(feature = "wgpu")]
                    wgpu_render_state: None,
                    raw_display_handle: window.raw_display_handle(),
                    raw_window_handle: window.raw_window_handle(),
                });

                if app.warm_up_enabled() {
                    let viewport = glutin.viewport_mut(ViewportId::ROOT);
                    integration.warm_up(
                        app.as_mut(),
                        &window,
                        viewport.egui_winit.as_mut().unwrap(),
                    );
                }

                app
            };

            let glutin = Rc::new(RefCell::new(glutin));
            let painter = Rc::new(RefCell::new(painter));

            {
                // Create weak pointers so that we don't keep
                // state alive for too long.
                let glutin = Rc::downgrade(&glutin);
                let painter = Rc::downgrade(&painter);
                let beginning = integration.beginning;

                let event_loop: *const EventLoopWindowTarget<UserEvent> = event_loop;

                egui::Context::set_immediate_viewport_renderer(
                    move |egui_ctx, immediate_viewport| {
                        if let (Some(glutin), Some(painter)) = (glutin.upgrade(), painter.upgrade())
                        {
                            // SAFETY: the event loop lives longer than
                            // the Rc:s we just upgraded above.
                            #[allow(unsafe_code)]
                            let event_loop = unsafe { event_loop.as_ref().unwrap() };

                            render_immediate_viewport(
                                event_loop,
                                egui_ctx,
                                &glutin,
                                &painter,
                                beginning,
                                immediate_viewport,
                            );
                        } else {
                            log::warn!("render_sync_callback called after window closed");
                        }
                    },
                );
            }

            Ok(self.running.insert(GlowWinitRunning {
                glutin,
                painter,
                integration,
                app,
            }))
        }
    }

    /// This is called (via a callback) by user code to render immediate viewports,
    /// i.e. viewport that are directly nested inside a parent viewport.
    fn render_immediate_viewport(
        event_loop: &EventLoopWindowTarget<UserEvent>,
        egui_ctx: &egui::Context,
        glutin: &RefCell<GlutinWindowContext>,
        painter: &RefCell<egui_glow::Painter>,
        beginning: Instant,
        immediate_viewport: ImmediateViewport<'_>,
    ) {
        crate::profile_function!();

        let ImmediateViewport {
            ids,
            builder,
            viewport_ui_cb,
        } = immediate_viewport;

        {
            let mut glutin = glutin.borrow_mut();

            let viewport = initialize_or_update_viewport(
                &mut glutin.viewports,
                ids,
                ViewportClass::Immediate,
                builder,
                None,
                None,
            );

            if viewport.gl_surface.is_none() {
                glutin.init_viewport(ids.this, event_loop).expect(
                    "Failed to initialize window in egui::Context::show_viewport_immediate",
                );
            }
        }

        let input = {
            let mut glutin = glutin.borrow_mut();

            let Some(viewport) = glutin.viewports.get_mut(&ids.this) else {
                return;
            };

            let Some(winit_state) = &mut viewport.egui_winit else {
                return;
            };
            let Some(window) = &viewport.window else {
                return;
            };

            let mut input = winit_state.take_egui_input(window, ids);
            input.time = Some(beginning.elapsed().as_secs_f64());
            input
        };

        // ---------------------------------------------------
        // Call the user ui-code, which could re-entrantly call this function again!
        // No locks may be hold while calling this function.

        let egui::FullOutput {
            platform_output,
            textures_delta,
            shapes,
            pixels_per_point,
            viewport_output,
        } = egui_ctx.run(input, |ctx| {
            viewport_ui_cb(ctx);
        });

        // ---------------------------------------------------

        let mut glutin = glutin.borrow_mut();

        let GlutinWindowContext {
            current_gl_context,
            viewports,
            ..
        } = &mut *glutin;

        let Some(viewport) = viewports.get_mut(&ids.this) else {
            return;
        };

        let Some(winit_state) = &mut viewport.egui_winit else {
            return;
        };
        let (Some(window), Some(gl_surface)) = (&viewport.window, &viewport.gl_surface) else {
            return;
        };

        let screen_size_in_pixels: [u32; 2] = window.inner_size().into();

        let clipped_primitives = egui_ctx.tessellate(shapes, pixels_per_point);

        let mut painter = painter.borrow_mut();

        *current_gl_context = Some(
            current_gl_context
                .take()
                .unwrap()
                .make_not_current()
                .unwrap()
                .make_current(gl_surface)
                .unwrap(),
        );

        let current_gl_context = current_gl_context.as_ref().unwrap();

        if !gl_surface.is_current(current_gl_context) {
            log::error!("egui::show_viewport_immediate: viewport {:?}  ({:?}) is not created in main thread, try to use wgpu!", viewport.ids.this, viewport.builder.title);
        }

        let gl = &painter.gl().clone();
        egui_glow::painter::clear(gl, screen_size_in_pixels, [0.0, 0.0, 0.0, 0.0]);

        painter.paint_and_update_textures(
            screen_size_in_pixels,
            pixels_per_point,
            &clipped_primitives,
            &textures_delta,
        );

        {
            crate::profile_scope!("swap_buffers");
            if let Err(err) = gl_surface.swap_buffers(current_gl_context) {
                log::error!("swap_buffers failed: {err}");
            }
        }

        winit_state.handle_platform_output(window, ids.this, egui_ctx, platform_output);

        let focused_viewport = None; // TODO
        glutin.handle_viewport_output(viewport_output, focused_viewport);
    }

    impl WinitApp for GlowWinitApp {
        fn frame_nr(&self, viewport_id: ViewportId) -> u64 {
            self.running
                .as_ref()
                .map_or(0, |r| r.integration.egui_ctx.frame_nr_for(viewport_id))
        }

        fn is_focused(&self, window_id: WindowId) -> bool {
            if let Some(focused_viewport) = self.focused_viewport {
                if let Some(running) = &self.running {
                    if let Some(window_id) =
                        running.glutin.borrow().viewport_from_window.get(&window_id)
                    {
                        return focused_viewport == *window_id;
                    }
                }
            }
            false
        }

        fn integration(&self) -> Option<&EpiIntegration> {
            self.running.as_ref().map(|r| &r.integration)
        }

        fn window(&self, window_id: WindowId) -> Option<Rc<Window>> {
            let running = self.running.as_ref()?;
            let glutin = running.glutin.borrow();
            let viewport_id = *glutin.viewport_from_window.get(&window_id)?;
            if let Some(viewport) = glutin.viewports.get(&viewport_id) {
                viewport.window.clone()
            } else {
                None
            }
        }

        fn window_id_from_viewport_id(&self, id: ViewportId) -> Option<WindowId> {
            self.running
                .as_ref()
                .and_then(|r| r.glutin.borrow().window_from_viewport.get(&id).copied())
        }

        fn save_and_destroy(&mut self) {
            if let Some(mut running) = self.running.take() {
                crate::profile_function!();

                running.integration.save(
                    running.app.as_mut(),
                    Some(&running.glutin.borrow().window(ViewportId::ROOT)),
                );
                running.app.on_exit(Some(running.painter.borrow().gl()));
                running.painter.borrow_mut().destroy();
            }
        }

        fn run_ui_and_paint(&mut self, window_id: WindowId) -> EventResult {
            if let Some(running) = &mut self.running {
                running.run_ui_and_paint(window_id, self.focused_viewport)
            } else {
                EventResult::Wait
            }
        }

        fn on_event(
            &mut self,
            event_loop: &EventLoopWindowTarget<UserEvent>,
            event: &winit::event::Event<'_, UserEvent>,
        ) -> Result<EventResult> {
            crate::profile_function!();

            Ok(match event {
                winit::event::Event::Resumed => {
                    let running = if let Some(running) = &mut self.running {
                        // not the first resume event. create whatever you need.
                        running.glutin.borrow_mut().on_resume(event_loop)?;
                        running
                    } else {
                        // first resume event.
                        // we can actually move this outside of event loop.
                        // and just run the on_resume fn of gl_window
                        self.init_run_state(event_loop)?
                    };
                    let window_id = running
                        .glutin
                        .borrow()
                        .window_from_viewport
                        .get(&ViewportId::ROOT)
                        .copied();
                    EventResult::RepaintNow(window_id.unwrap())
                }

                winit::event::Event::Suspended => {
                    if let Some(running) = &mut self.running {
                        running.glutin.borrow_mut().on_suspend()?;
                    }
                    EventResult::Wait
                }

                winit::event::Event::MainEventsCleared => {
                    if let Some(running) = &self.running {
                        if let Err(err) = running.glutin.borrow_mut().on_resume(event_loop) {
                            log::warn!("on_resume failed {err}");
                        }
                    }
                    EventResult::Wait
                }

                winit::event::Event::WindowEvent { event, window_id } => {
                    if let Some(running) = &mut self.running {
                        running.on_window_event(*window_id, event, &mut self.focused_viewport)
                    } else {
                        EventResult::Wait
                    }
                }

                #[cfg(feature = "accesskit")]
                winit::event::Event::UserEvent(UserEvent::AccessKitActionRequest(
                    accesskit_winit::ActionRequestEvent { request, window_id },
                )) => {
                    if let Some(running) = &self.running {
                        let mut glutin = running.glutin.borrow_mut();
                        if let Some(viewport_id) =
                            glutin.viewport_from_window.get(window_id).copied()
                        {
                            if let Some(viewport) = glutin.viewports.get_mut(&viewport_id) {
                                if let Some(egui_winit) = &mut viewport.egui_winit {
                                    crate::profile_scope!("on_accesskit_action_request");
                                    egui_winit.on_accesskit_action_request(request.clone());
                                }
                            }
                        }
                        // As a form of user input, accessibility actions should
                        // lead to a repaint.
                        EventResult::RepaintNext(*window_id)
                    } else {
                        EventResult::Wait
                    }
                }
                _ => EventResult::Wait,
            })
        }
    }

    pub fn run_glow(
        app_name: &str,
        mut native_options: epi::NativeOptions,
        app_creator: epi::AppCreator,
    ) -> Result<()> {
        #[cfg(not(target_os = "ios"))]
        if native_options.run_and_return {
            return with_event_loop(native_options, |event_loop, native_options| {
                let glow_eframe =
                    GlowWinitApp::new(event_loop, app_name, native_options, app_creator);
                run_and_return(event_loop, glow_eframe)
            });
        }

        let event_loop = create_event_loop(&mut native_options);
        let glow_eframe = GlowWinitApp::new(&event_loop, app_name, native_options, app_creator);
        run_and_exit(event_loop, glow_eframe);
    }
}

#[cfg(feature = "glow")]
pub use glow_integration::run_glow;
// ----------------------------------------------------------------------------

#[cfg(feature = "wgpu")]
mod wgpu_integration {
    use parking_lot::Mutex;

    use egui::{
        DeferredViewportUiCallback, FullOutput, ImmediateViewport, ViewportClass, ViewportIdMap,
        ViewportIdPair, ViewportIdSet, ViewportOutput,
    };
    use egui_winit::{create_winit_window_builder, process_viewport_commands};

    use super::*;

    pub struct Viewport {
        ids: ViewportIdPair,

        class: ViewportClass,

        builder: ViewportBuilder,

        /// `None` for sync viewports.
        viewport_ui_cb: Option<Arc<DeferredViewportUiCallback>>,

        /// Window surface state that's initialized when the app starts running via a Resumed event
        /// and on Android will also be destroyed if the application is paused.
        window: Option<Rc<Window>>,

        /// `window` and `egui_winit` are initialized together.
        egui_winit: Option<egui_winit::State>,
    }

    impl Viewport {
        fn init_window(
            &mut self,
            windows_id: &mut HashMap<WindowId, ViewportId>,
            painter: &mut egui_wgpu::winit::Painter,
            event_loop: &EventLoopWindowTarget<UserEvent>,
        ) {
            crate::profile_function!();

            let viewport_id = self.ids.this;

            match create_winit_window_builder(&self.builder).build(event_loop) {
                Ok(new_window) => {
                    windows_id.insert(new_window.id(), viewport_id);

                    if let Err(err) =
                        pollster::block_on(painter.set_window(viewport_id, Some(&new_window)))
                    {
                        log::error!("on set_window: viewport_id {viewport_id:?} {err}");
                    }

                    self.egui_winit = Some(egui_winit::State::new(
                        event_loop,
                        Some(new_window.scale_factor() as f32),
                        painter.max_texture_side(),
                    ));

                    self.window = Some(Rc::new(new_window));
                }
                Err(err) => {
                    log::error!("Failed to create window: {err}");
                }
            }
        }
    }

    pub type Viewports = ViewportIdMap<Viewport>;

    /// Everything needed by the immediate viewport renderer.
    ///
    /// Wrapped in an `Rc<RefCell<…>>` so it can be re-entrantly shared via a weak-pointer.
    pub struct SharedState {
        viewports: Viewports,
        painter: egui_wgpu::winit::Painter,
        viewport_from_window: HashMap<WindowId, ViewportId>,
    }

    /// State that is initialized when the application is first starts running via
    /// a Resumed event. On Android this ensures that any graphics state is only
    /// initialized once the application has an associated `SurfaceView`.
    struct WgpuWinitRunning {
        integration: epi_integration::EpiIntegration,

        /// The users application.
        app: Box<dyn epi::App>,

        /// Wrapped in an `Rc<RefCell<…>>` so it can be re-entrantly shared via a weak-pointer.
        shared: Rc<RefCell<SharedState>>,
    }

    struct WgpuWinitApp {
        repaint_proxy: Arc<Mutex<EventLoopProxy<UserEvent>>>,
        app_name: String,
        native_options: epi::NativeOptions,

        /// Set at initialization, then taken and set to `None` in `init_run_state`.
        app_creator: Option<epi::AppCreator>,

        /// Set when we are actually up and running.
        running: Option<WgpuWinitRunning>,

        focused_viewport: Option<ViewportId>,
    }

    impl WgpuWinitApp {
        fn new(
            event_loop: &EventLoop<UserEvent>,
            app_name: &str,
            native_options: epi::NativeOptions,
            app_creator: epi::AppCreator,
        ) -> Self {
            crate::profile_function!();

            #[cfg(feature = "__screenshot")]
            assert!(
                std::env::var("EFRAME_SCREENSHOT_TO").is_err(),
                "EFRAME_SCREENSHOT_TO not yet implemented for wgpu backend"
            );

            Self {
                repaint_proxy: Arc::new(Mutex::new(event_loop.create_proxy())),
                app_name: app_name.to_owned(),
                native_options,
                running: None,
                app_creator: Some(app_creator),
                focused_viewport: Some(ViewportId::ROOT),
            }
        }

        fn build_windows(&mut self, event_loop: &EventLoopWindowTarget<UserEvent>) {
            let Some(running) = &mut self.running else {
                return;
            };
            let mut shared = running.shared.borrow_mut();
            let SharedState {
                viewports,
                painter,
                viewport_from_window,
            } = &mut *shared;

            for viewport in viewports.values_mut() {
                if viewport.window.is_none() {
                    viewport.init_window(viewport_from_window, painter, event_loop);
                }
            }
        }

        #[cfg(target_os = "android")]
        fn drop_window(&mut self) -> Result<(), egui_wgpu::WgpuError> {
            if let Some(running) = &mut self.running {
                let mut shared = running.shared.borrow_mut();
                shared.viewports.remove(&ViewportId::ROOT);
                pollster::block_on(shared.painter.set_window(ViewportId::ROOT, None))?;
            }
            Ok(())
        }

        fn init_run_state(
            &mut self,
            event_loop: &EventLoopWindowTarget<UserEvent>,
            storage: Option<Box<dyn epi::Storage>>,
            window: Window,
            builder: ViewportBuilder,
        ) -> Result<&mut WgpuWinitRunning, egui_wgpu::WgpuError> {
            crate::profile_function!();

            #[allow(unsafe_code, unused_mut, unused_unsafe)]
            let mut painter = egui_wgpu::winit::Painter::new(
                self.native_options.wgpu_options.clone(),
                self.native_options.multisampling.max(1) as _,
                egui_wgpu::depth_format_from_bits(
                    self.native_options.depth_buffer,
                    self.native_options.stencil_buffer,
                ),
                self.native_options.transparent,
            );
            pollster::block_on(painter.set_window(ViewportId::ROOT, Some(&window)))?;

            let wgpu_render_state = painter.render_state();

            let system_theme = system_theme(&window, &self.native_options);
            let mut integration = epi_integration::EpiIntegration::new(
                &window,
                system_theme,
                &self.app_name,
                &self.native_options,
                storage,
                IS_DESKTOP,
                #[cfg(feature = "glow")]
                None,
                wgpu_render_state.clone(),
            );

            {
                let event_loop_proxy = self.repaint_proxy.clone();

                integration
                    .egui_ctx
                    .set_request_repaint_callback(move |info| {
                        log::trace!("request_repaint_callback: {info:?}");
                        let when = Instant::now() + info.delay;
                        let frame_nr = info.current_frame_nr;

                        event_loop_proxy
                            .lock()
                            .send_event(UserEvent::RequestRepaint {
                                when,
                                frame_nr,
                                viewport_id: info.viewport_id,
                            })
                            .ok();
                    });
            }

            let mut egui_winit = egui_winit::State::new(
                event_loop,
                Some(window.scale_factor() as f32),
                painter.max_texture_side(),
            );

            #[cfg(feature = "accesskit")]
            {
                let event_loop_proxy = self.repaint_proxy.lock().clone();
                integration.init_accesskit(&mut egui_winit, &window, event_loop_proxy);
            }
            let theme = system_theme.unwrap_or(self.native_options.default_theme);
            integration.egui_ctx.set_visuals(theme.egui_visuals());

            let app_creator = std::mem::take(&mut self.app_creator)
                .expect("Single-use AppCreator has unexpectedly already been taken");
            let cc = epi::CreationContext {
                egui_ctx: integration.egui_ctx.clone(),
                integration_info: integration.frame.info().clone(),
                storage: integration.frame.storage(),
                #[cfg(feature = "glow")]
                gl: None,
                wgpu_render_state,
                raw_display_handle: window.raw_display_handle(),
                raw_window_handle: window.raw_window_handle(),
            };
            let mut app = {
                crate::profile_scope!("user_app_creator");
                app_creator(&cc)
            };

            if app.warm_up_enabled() {
                integration.warm_up(app.as_mut(), &window, &mut egui_winit);
            }

            let mut viewport_from_window = HashMap::default();
            viewport_from_window.insert(window.id(), ViewportId::ROOT);

            let mut viewports = Viewports::default();
            viewports.insert(
                ViewportId::ROOT,
                Viewport {
                    ids: ViewportIdPair::ROOT,
                    class: ViewportClass::Root,
                    builder,
                    viewport_ui_cb: None,
                    window: Some(Rc::new(window)),
                    egui_winit: Some(egui_winit),
                },
            );

            let shared = Rc::new(RefCell::new(SharedState {
                viewport_from_window,
                viewports,
                painter,
            }));

            {
                // Create a weak pointer so that we don't keep state alive for too long.
                let shared = Rc::downgrade(&shared);
                let beginning = integration.beginning;

                let event_loop: *const EventLoopWindowTarget<UserEvent> = event_loop;

                egui::Context::set_immediate_viewport_renderer(
                    move |egui_ctx, immediate_viewport| {
                        if let Some(shared) = shared.upgrade() {
                            // SAFETY: the event loop lives longer than
                            // the Rc:s we just upgraded above.
                            #[allow(unsafe_code)]
                            let event_loop = unsafe { event_loop.as_ref().unwrap() };

                            render_immediate_viewport(
                                event_loop,
                                egui_ctx,
                                beginning,
                                &shared,
                                immediate_viewport,
                            );
                        } else {
                            log::warn!("render_sync_callback called after window closed");
                        }
                    },
                );
            }

            Ok(self.running.insert(WgpuWinitRunning {
                integration,
                app,
                shared,
            }))
        }
    }

    fn create_window(
        event_loop: &EventLoopWindowTarget<UserEvent>,
        storage: Option<&dyn epi::Storage>,
        title: &str,
        native_options: &mut NativeOptions,
    ) -> Result<(Window, ViewportBuilder), winit::error::OsError> {
        crate::profile_function!();

        let window_settings = epi_integration::load_window_settings(storage);
        let window_builder =
            epi_integration::window_builder(event_loop, title, native_options, window_settings);
        let window = {
            crate::profile_scope!("WindowBuilder::build");
            create_winit_window_builder(&window_builder).build(event_loop)?
        };
        epi_integration::apply_native_options_to_window(&window, native_options, window_settings);
        Ok((window, window_builder))
    }

    fn render_immediate_viewport(
        event_loop: &EventLoopWindowTarget<UserEvent>,
        egui_ctx: &egui::Context,
        beginning: Instant,
        shared: &RefCell<SharedState>,
        immediate_viewport: ImmediateViewport<'_>,
    ) {
        crate::profile_function!();

        let ImmediateViewport {
            ids,
            builder,
            viewport_ui_cb,
        } = immediate_viewport;

        let input = {
            let SharedState {
                viewports,
                painter,
                viewport_from_window,
            } = &mut *shared.borrow_mut();
            let viewport = initialize_or_update_viewport(
                viewports,
                ids,
                ViewportClass::Immediate,
                builder,
                None,
                None,
            );

            if viewport.window.is_none() {
                viewport.init_window(viewport_from_window, painter, event_loop);
            }

            let (Some(window), Some(winit_state)) = (&viewport.window, &mut viewport.egui_winit)
            else {
                return;
            };

            let mut input = winit_state.take_egui_input(window, ids);
            input.time = Some(beginning.elapsed().as_secs_f64());
            input
        };

        // ------------------------------------------

        // Run the user code, which could re-entrantly call this function again (!).
        // Make sure no locks are held during this call.
        let egui::FullOutput {
            platform_output,
            textures_delta,
            shapes,
            pixels_per_point,
            viewport_output,
        } = egui_ctx.run(input, |ctx| {
            viewport_ui_cb(ctx);
        });

        // ------------------------------------------

        let mut shared = shared.borrow_mut();
        let SharedState {
            viewports, painter, ..
        } = &mut *shared;

        let Some(viewport) = viewports.get_mut(&ids.this) else {
            return;
        };
        let Some(winit_state) = &mut viewport.egui_winit else {
            return;
        };
        let Some(window) = &viewport.window else {
            return;
        };

        if let Err(err) = pollster::block_on(painter.set_window(ids.this, Some(window))) {
            log::error!(
                "when rendering viewport_id={:?}, set_window Error {err}",
                ids.this
            );
        }

        let clipped_primitives = egui_ctx.tessellate(shapes, pixels_per_point);
        painter.paint_and_update_textures(
            ids.this,
            pixels_per_point,
            [0.0, 0.0, 0.0, 0.0],
            &clipped_primitives,
            &textures_delta,
            false,
        );

        winit_state.handle_platform_output(window, ids.this, egui_ctx, platform_output);

        let focused_viewport = None; // TODO
        handle_viewport_output(viewport_output, viewports, focused_viewport);
    }

    impl WinitApp for WgpuWinitApp {
        fn frame_nr(&self, viewport_id: ViewportId) -> u64 {
            self.running
                .as_ref()
                .map_or(0, |r| r.integration.egui_ctx.frame_nr_for(viewport_id))
        }

        fn is_focused(&self, window_id: WindowId) -> bool {
            let viewport_id = self.running.as_ref().and_then(|r| {
                r.shared
                    .borrow()
                    .viewport_from_window
                    .get(&window_id)
                    .copied()
            });

            self.focused_viewport.is_some() && self.focused_viewport == viewport_id
        }

        fn integration(&self) -> Option<&EpiIntegration> {
            self.running.as_ref().map(|r| &r.integration)
        }

        fn window(&self, window_id: WindowId) -> Option<Rc<Window>> {
            self.running
                .as_ref()
                .and_then(|r| {
                    let shared = r.shared.borrow();
                    shared
                        .viewport_from_window
                        .get(&window_id)
                        .and_then(|id| shared.viewports.get(id).map(|v| v.window.clone()))
                })
                .flatten()
        }

        fn window_id_from_viewport_id(&self, id: ViewportId) -> Option<WindowId> {
            Some(
                self.running
                    .as_ref()?
                    .shared
                    .borrow()
                    .viewports
                    .get(&id)?
                    .window
                    .as_ref()?
                    .id(),
            )
        }

        fn save_and_destroy(&mut self) {
            if let Some(mut running) = self.running.take() {
                running.save_and_destroy();
            }
        }

        fn run_ui_and_paint(&mut self, window_id: WindowId) -> EventResult {
            if let Some(running) = &mut self.running {
                running.run_ui_and_paint(window_id, self.focused_viewport)
            } else {
                EventResult::Wait
            }
        }

        fn on_event(
            &mut self,
            event_loop: &EventLoopWindowTarget<UserEvent>,
            event: &winit::event::Event<'_, UserEvent>,
        ) -> Result<EventResult> {
            crate::profile_function!();

            self.build_windows(event_loop);

            Ok(match event {
                winit::event::Event::Resumed => {
                    let running = if let Some(running) = &self.running {
                        if !running
                            .shared
                            .borrow()
                            .viewports
                            .contains_key(&ViewportId::ROOT)
                        {
                            create_window(
                                event_loop,
                                running.integration.frame.storage(),
                                &self.app_name,
                                &mut self.native_options,
                            )?;
                            running.set_window(ViewportId::ROOT)?;
                        }
                        running
                    } else {
                        let storage = epi_integration::create_storage(
                            self.native_options
                                .app_id
                                .as_ref()
                                .unwrap_or(&self.app_name),
                        );
                        let (window, builder) = create_window(
                            event_loop,
                            storage.as_deref(),
                            &self.app_name,
                            &mut self.native_options,
                        )?;
                        self.init_run_state(event_loop, storage, window, builder)?
                    };

                    EventResult::RepaintNow(
                        running.shared.borrow().viewports[&ViewportId::ROOT]
                            .window
                            .as_ref()
                            .unwrap()
                            .id(),
                    )
                }

                winit::event::Event::Suspended => {
                    #[cfg(target_os = "android")]
                    self.drop_window()?;
                    EventResult::Wait
                }

                winit::event::Event::WindowEvent { event, window_id } => {
                    if let Some(running) = &mut self.running {
                        running.on_window_event(*window_id, event, &mut self.focused_viewport)
                    } else {
                        EventResult::Wait
                    }
                }

                #[cfg(feature = "accesskit")]
                winit::event::Event::UserEvent(UserEvent::AccessKitActionRequest(
                    accesskit_winit::ActionRequestEvent { request, window_id },
                )) => {
                    if let Some(running) = &mut self.running {
                        let mut shared_lock = running.shared.borrow_mut();
                        let SharedState {
                            viewport_from_window,
                            viewports,
                            ..
                        } = &mut *shared_lock;
                        if let Some(viewport) = viewport_from_window
                            .get(window_id)
                            .and_then(|id| viewports.get_mut(id))
                        {
                            if let Some(egui_winit) = &mut viewport.egui_winit {
                                egui_winit.on_accesskit_action_request(request.clone());
                            }
                        }
                        // As a form of user input, accessibility actions should
                        // lead to a repaint.
                        EventResult::RepaintNext(*window_id)
                    } else {
                        EventResult::Wait
                    }
                }
                _ => EventResult::Wait,
            })
        }
    }

    impl WgpuWinitRunning {
        fn set_window(&self, id: ViewportId) -> Result<(), egui_wgpu::WgpuError> {
            crate::profile_function!();
            let mut shared = self.shared.borrow_mut();
            let SharedState {
                viewports, painter, ..
            } = &mut *shared;
            if let Some(Viewport { window, .. }) = viewports.get(&id) {
                return pollster::block_on(painter.set_window(id, window.as_deref()));
            }
            Ok(())
        }

        fn save_and_destroy(&mut self) {
            crate::profile_function!();

            let mut shared = self.shared.borrow_mut();
            if let Some(Viewport { window, .. }) = shared.viewports.get(&ViewportId::ROOT) {
                self.integration.save(self.app.as_mut(), window.as_deref());
            }

            #[cfg(feature = "glow")]
            self.app.on_exit(None);

            #[cfg(not(feature = "glow"))]
            self.app.on_exit();

            shared.painter.destroy();
        }

        /// This is called both for the root viewport, and all deferred viewports
        fn run_ui_and_paint(
            &mut self,
            window_id: WindowId,
            focused_viewport: Option<ViewportId>,
        ) -> EventResult {
            let Some(viewport_id) = self
                .shared
                .borrow()
                .viewport_from_window
                .get(&window_id)
                .copied()
            else {
                return EventResult::Wait;
            };

            #[cfg(feature = "puffin")]
            puffin::GlobalProfiler::lock().new_frame();

            crate::profile_scope!("frame");

            let WgpuWinitRunning {
                app,
                integration,
                shared,
            } = self;

            let (viewport_ui_cb, raw_input) = {
                let mut shared_lock = shared.borrow_mut();

                let SharedState {
                    viewports, painter, ..
                } = &mut *shared_lock;

                let Some(viewport) = viewports.get(&viewport_id) else {
                    return EventResult::Wait;
                };

                if viewport_id != ViewportId::ROOT && viewport.viewport_ui_cb.is_none() {
                    // This will only happen if this is an immediate viewport.
                    // That means that the viewport cannot be rendered by itself and needs his parent to be rendered.
                    if let Some(viewport) = viewports.get(&viewport.ids.parent) {
                        if let Some(window) = viewport.window.as_ref() {
                            return EventResult::RepaintNext(window.id());
                        }
                    }
                    return EventResult::Wait;
                }

                let Some(viewport) = viewports.get_mut(&viewport_id) else {
                    return EventResult::Wait;
                };

                let Viewport {
                    ids,
                    viewport_ui_cb,
                    window,
                    egui_winit,
                    ..
                } = viewport;

                let Some(window) = window else {
                    return EventResult::Wait;
                };

                if let Err(err) = pollster::block_on(painter.set_window(viewport_id, Some(window)))
                {
                    log::warn!("Failed to set window: {err}");
                }

                let raw_input = egui_winit.as_mut().unwrap().take_egui_input(
                    window,
                    ViewportIdPair::from_self_and_parent(viewport_id, ids.parent),
                );

                integration.pre_update(window);

                (viewport_ui_cb.clone(), raw_input)
            };

            // ------------------------------------------------------------

            // Runs the update, which could call immediate viewports,
            // so make sure we hold no locks here!
            let full_output =
                integration.update(app.as_mut(), viewport_ui_cb.as_deref(), raw_input);

            // ------------------------------------------------------------

            let mut shared = shared.borrow_mut();

            let SharedState {
                viewports,
                painter,
                viewport_from_window,
            } = &mut *shared;

            let Some(viewport) = viewports.get_mut(&viewport_id) else {
                return EventResult::Wait;
            };

            let Viewport {
                window: Some(window),
                egui_winit: Some(egui_winit),
                ..
            } = viewport
            else {
                return EventResult::Wait;
            };

            integration.post_update(app.as_mut(), window);

            let FullOutput {
                platform_output,
                textures_delta,
                shapes,
                pixels_per_point,
                viewport_output,
            } = full_output;

            integration.handle_platform_output(window, viewport_id, platform_output, egui_winit);

            {
                let clipped_primitives = integration.egui_ctx.tessellate(shapes, pixels_per_point);

                let screenshot_requested = &mut integration.frame.output.screenshot_requested;
                let screenshot = painter.paint_and_update_textures(
                    viewport_id,
                    pixels_per_point,
                    app.clear_color(&integration.egui_ctx.style().visuals),
                    &clipped_primitives,
                    &textures_delta,
                    *screenshot_requested,
                );
                *screenshot_requested = false;
                integration.frame.screenshot.set(screenshot);
            }

            integration.post_rendering(app.as_mut(), window);
            integration.post_present(window);

            let active_viewports_ids: ViewportIdSet = viewport_output.keys().copied().collect();

            handle_viewport_output(viewport_output, viewports, focused_viewport);

            // Prune dead viewports:
            viewports.retain(|id, _| active_viewports_ids.contains(id));
            viewport_from_window.retain(|_, id| active_viewports_ids.contains(id));
            painter.gc_viewports(&active_viewports_ids);

            let window = viewport_from_window
                .get(&window_id)
                .and_then(|id| viewports.get(id))
                .and_then(|vp| vp.window.as_ref());

            integration.maybe_autosave(app.as_mut(), window.map(|w| w.as_ref()));

            if let Some(window) = window {
                if window.is_minimized() == Some(true) {
                    // On Mac, a minimized Window uses up all CPU:
                    // https://github.com/emilk/egui/issues/325
                    crate::profile_scope!("minimized_sleep");
                    std::thread::sleep(std::time::Duration::from_millis(10));
                }
            }

            if integration.should_close() {
                EventResult::Exit
            } else {
                EventResult::Wait
            }
        }

        fn on_window_event(
            &mut self,
            window_id: WindowId,
            event: &winit::event::WindowEvent<'_>,
            focused_viewport: &mut Option<ViewportId>,
        ) -> EventResult {
            let Self {
                integration,
                app,
                shared,
            } = self;
            let mut shared = shared.borrow_mut();

            let viewport_id = shared.viewport_from_window.get(&window_id).copied();

            // On Windows, if a window is resized by the user, it should repaint synchronously, inside the
            // event handler.
            //
            // If this is not done, the compositor will assume that the window does not want to redraw,
            // and continue ahead.
            //
            // In eframe's case, that causes the window to rapidly flicker, as it struggles to deliver
            // new frames to the compositor in time.
            //
            // The flickering is technically glutin or glow's fault, but we should be responding properly
            // to resizes anyway, as doing so avoids dropping frames.
            //
            // See: https://github.com/emilk/egui/issues/903
            let mut repaint_asap = false;

            match event {
                winit::event::WindowEvent::Focused(new_focused) => {
                    *focused_viewport = new_focused.then(|| viewport_id).flatten();
                }
                winit::event::WindowEvent::Resized(physical_size) => {
                    // Resize with 0 width and height is used by winit to signal a minimize event on Windows.
                    // See: https://github.com/rust-windowing/winit/issues/208
                    // This solves an issue where the app would panic when minimizing on Windows.
                    if let Some(viewport_id) = viewport_id {
                        use std::num::NonZeroU32;
                        if let (Some(width), Some(height)) = (
                            NonZeroU32::new(physical_size.width),
                            NonZeroU32::new(physical_size.height),
                        ) {
                            repaint_asap = true;
                            shared.painter.on_window_resized(viewport_id, width, height);
                        }
                    }
                }
                winit::event::WindowEvent::ScaleFactorChanged { new_inner_size, .. } => {
                    use std::num::NonZeroU32;
                    if let (Some(width), Some(height), Some(viewport_id)) = (
                        NonZeroU32::new(new_inner_size.width),
                        NonZeroU32::new(new_inner_size.height),
                        shared.viewport_from_window.get(&window_id).copied(),
                    ) {
                        repaint_asap = true;
                        shared.painter.on_window_resized(viewport_id, width, height);
                    }
                }
                winit::event::WindowEvent::CloseRequested if integration.should_close() => {
                    log::debug!("Received WindowEvent::CloseRequested");
                    return EventResult::Exit;
                }
                _ => {}
            };

            let event_response = viewport_id.and_then(|viewport_id| {
                shared.viewports.get_mut(&viewport_id).and_then(|viewport| {
                    viewport.egui_winit.as_mut().map(|egui_winit| {
                        integration.on_event(app.as_mut(), event, egui_winit, viewport_id)
                    })
                })
            });

            if integration.should_close() {
                EventResult::Exit
            } else if let Some(event_response) = event_response {
                if event_response.repaint {
                    if repaint_asap {
                        EventResult::RepaintNow(window_id)
                    } else {
                        EventResult::RepaintNext(window_id)
                    }
                } else {
                    EventResult::Wait
                }
            } else {
                EventResult::Wait
            }
        }
    }

    /// Add new viewports, and update existing ones:
    fn handle_viewport_output(
        viewport_output: ViewportIdMap<ViewportOutput>,
        viewports: &mut ViewportIdMap<Viewport>,
        focused_viewport: Option<ViewportId>,
    ) {
        for (
            viewport_id,
            ViewportOutput {
                parent,
                class,
                builder,
                viewport_ui_cb,
                commands,
                repaint_delay: _, // ignored - we listened to the repaint callback instead
            },
        ) in viewport_output
        {
            let ids = ViewportIdPair::from_self_and_parent(viewport_id, parent);

            initialize_or_update_viewport(
                viewports,
                ids,
                class,
                builder,
                viewport_ui_cb,
                focused_viewport,
            );

            if let Some(window) = viewports
                .get(&viewport_id)
                .and_then(|vp| vp.window.as_ref())
            {
                let is_viewport_focused = focused_viewport == Some(viewport_id);
                egui_winit::process_viewport_commands(commands, window, is_viewport_focused);
            }
        }
    }

    fn initialize_or_update_viewport(
        viewports: &mut Viewports,
        ids: ViewportIdPair,
        class: ViewportClass,
        mut builder: ViewportBuilder,
        viewport_ui_cb: Option<Arc<dyn Fn(&egui::Context) + Send + Sync>>,
        focused_viewport: Option<ViewportId>,
    ) -> &mut Viewport {
        if builder.icon.is_none() {
            // Inherit icon from parent
            builder.icon = viewports
                .get_mut(&ids.parent)
                .and_then(|vp| vp.builder.icon.clone());
        }

        match viewports.entry(ids.this) {
            std::collections::hash_map::Entry::Vacant(entry) => {
                // New viewport:
                log::debug!("Creating new viewport {:?} ({:?})", ids.this, builder.title);
                entry.insert(Viewport {
                    ids,
                    class,
                    builder,
                    viewport_ui_cb,
                    window: None,
                    egui_winit: None,
                })
            }

            std::collections::hash_map::Entry::Occupied(mut entry) => {
                // Patch an existing viewport:
                let viewport = entry.get_mut();

                viewport.class = class;
                viewport.ids.parent = ids.parent;
                viewport.viewport_ui_cb = viewport_ui_cb;

                let (delta_commands, recreate) = viewport.builder.patch(&builder);

                if recreate {
                    log::debug!(
                        "Recreating window for viewport {:?} ({:?})",
                        ids.this,
                        builder.title
                    );
                    viewport.window = None;
                    viewport.egui_winit = None;
                } else if let Some(window) = &viewport.window {
                    let is_viewport_focused = focused_viewport == Some(ids.this);
                    process_viewport_commands(delta_commands, window, is_viewport_focused);
                }

                entry.into_mut()
            }
        }
    }

    pub fn run_wgpu(
        app_name: &str,
        mut native_options: epi::NativeOptions,
        app_creator: epi::AppCreator,
    ) -> Result<()> {
        #[cfg(not(target_os = "ios"))]
        if native_options.run_and_return {
            return with_event_loop(native_options, |event_loop, native_options| {
                let wgpu_eframe =
                    WgpuWinitApp::new(event_loop, app_name, native_options, app_creator);
                run_and_return(event_loop, wgpu_eframe)
            });
        }

        let event_loop = create_event_loop(&mut native_options);
        let wgpu_eframe = WgpuWinitApp::new(&event_loop, app_name, native_options, app_creator);
        run_and_exit(event_loop, wgpu_eframe);
    }
}

#[cfg(feature = "wgpu")]
pub use wgpu_integration::run_wgpu;

// ----------------------------------------------------------------------------

fn system_theme(window: &Window, options: &NativeOptions) -> Option<crate::Theme> {
    if options.follow_system_theme {
        window
            .theme()
            .map(super::epi_integration::theme_from_winit_theme)
    } else {
        None
    }
}

// For the puffin profiler!
#[allow(dead_code)] // Only used for profiling
fn short_event_description(event: &winit::event::Event<'_, UserEvent>) -> &'static str {
    use winit::event::{DeviceEvent, Event, StartCause, WindowEvent};

    match event {
        Event::Suspended => "Event::Suspended",
        Event::Resumed => "Event::Resumed",
        Event::MainEventsCleared => "Event::MainEventsCleared",
        Event::RedrawRequested(_) => "Event::RedrawRequested",
        Event::RedrawEventsCleared => "Event::RedrawEventsCleared",
        Event::LoopDestroyed => "Event::LoopDestroyed",
        Event::UserEvent(user_event) => match user_event {
            UserEvent::RequestRepaint { .. } => "UserEvent::RequestRepaint",
            #[cfg(feature = "accesskit")]
            UserEvent::AccessKitActionRequest(_) => "UserEvent::AccessKitActionRequest",
        },
        Event::DeviceEvent { event, .. } => match event {
            DeviceEvent::Added { .. } => "DeviceEvent::Added",
            DeviceEvent::Removed { .. } => "DeviceEvent::Removed",
            DeviceEvent::MouseMotion { .. } => "DeviceEvent::MouseMotion",
            DeviceEvent::MouseWheel { .. } => "DeviceEvent::MouseWheel",
            DeviceEvent::Motion { .. } => "DeviceEvent::Motion",
            DeviceEvent::Button { .. } => "DeviceEvent::Button",
            DeviceEvent::Key { .. } => "DeviceEvent::Key",
            DeviceEvent::Text { .. } => "DeviceEvent::Text",
        },
        Event::NewEvents(start_cause) => match start_cause {
            StartCause::ResumeTimeReached { .. } => "NewEvents::ResumeTimeReached",
            StartCause::WaitCancelled { .. } => "NewEvents::WaitCancelled",
            StartCause::Poll => "NewEvents::Poll",
            StartCause::Init => "NewEvents::Init",
        },
        Event::WindowEvent { event, .. } => match event {
            WindowEvent::Resized { .. } => "WindowEvent::Resized",
            WindowEvent::Moved { .. } => "WindowEvent::Moved",
            WindowEvent::CloseRequested { .. } => "WindowEvent::CloseRequested",
            WindowEvent::Destroyed { .. } => "WindowEvent::Destroyed",
            WindowEvent::DroppedFile { .. } => "WindowEvent::DroppedFile",
            WindowEvent::HoveredFile { .. } => "WindowEvent::HoveredFile",
            WindowEvent::HoveredFileCancelled { .. } => "WindowEvent::HoveredFileCancelled",
            WindowEvent::ReceivedCharacter { .. } => "WindowEvent::ReceivedCharacter",
            WindowEvent::Focused { .. } => "WindowEvent::Focused",
            WindowEvent::KeyboardInput { .. } => "WindowEvent::KeyboardInput",
            WindowEvent::ModifiersChanged { .. } => "WindowEvent::ModifiersChanged",
            WindowEvent::Ime { .. } => "WindowEvent::Ime",
            WindowEvent::CursorMoved { .. } => "WindowEvent::CursorMoved",
            WindowEvent::CursorEntered { .. } => "WindowEvent::CursorEntered",
            WindowEvent::CursorLeft { .. } => "WindowEvent::CursorLeft",
            WindowEvent::MouseWheel { .. } => "WindowEvent::MouseWheel",
            WindowEvent::MouseInput { .. } => "WindowEvent::MouseInput",
            WindowEvent::TouchpadMagnify { .. } => "WindowEvent::TouchpadMagnify",
            WindowEvent::SmartMagnify { .. } => "WindowEvent::SmartMagnify",
            WindowEvent::TouchpadRotate { .. } => "WindowEvent::TouchpadRotate",
            WindowEvent::TouchpadPressure { .. } => "WindowEvent::TouchpadPressure",
            WindowEvent::AxisMotion { .. } => "WindowEvent::AxisMotion",
            WindowEvent::Touch { .. } => "WindowEvent::Touch",
            WindowEvent::ScaleFactorChanged { .. } => "WindowEvent::ScaleFactorChanged",
            WindowEvent::ThemeChanged { .. } => "WindowEvent::ThemeChanged",
            WindowEvent::Occluded { .. } => "WindowEvent::Occluded",
        },
    }
}