eframe web: detect and report panics during startup (#2992)

* Detect panics during initialization and show them to the user

* PanicHandler now also logs the panics

* Add example of how to call into your app from JS

* Refactor: break out AppRunner and AppRunnerRef to own files

* Hide AppRunner

* Simplify user code

* AppRunnerRef -> WebRunner

* Better docs

* Don't paint until first animation frame

* Update multiple_apps.html

* Update web demo

* Cleanup and fixes

* left-align panic message in html

crates/eframe/src/web/app_runner.rs

@@ -0,0 +1,273 @@
+use egui::TexturesDelta;
+use wasm_bindgen::JsValue;
+
+use crate::{epi, App};
+
+use super::{now_sec, web_painter::WebPainter, NeedRepaint};
+
+pub struct AppRunner {
+    pub(crate) frame: epi::Frame,
+    egui_ctx: egui::Context,
+    painter: super::ActiveWebPainter,
+    pub(crate) input: super::WebInput,
+    app: Box<dyn epi::App>,
+    pub(crate) needs_repaint: std::sync::Arc<NeedRepaint>,
+    last_save_time: f64,
+    screen_reader: super::screen_reader::ScreenReader,
+    pub(crate) text_cursor_pos: Option<egui::Pos2>,
+    pub(crate) mutable_text_under_cursor: bool,
+    textures_delta: TexturesDelta,
+}
+
+impl Drop for AppRunner {
+    fn drop(&mut self) {
+        log::debug!("AppRunner has fully dropped");
+    }
+}
+
+impl AppRunner {
+    /// # Errors
+    /// Failure to initialize WebGL renderer.
+    pub async fn new(
+        canvas_id: &str,
+        web_options: crate::WebOptions,
+        app_creator: epi::AppCreator,
+    ) -> Result<Self, String> {
+        let painter = super::ActiveWebPainter::new(canvas_id, &web_options).await?;
+
+        let system_theme = if web_options.follow_system_theme {
+            super::system_theme()
+        } else {
+            None
+        };
+
+        let info = epi::IntegrationInfo {
+            web_info: epi::WebInfo {
+                user_agent: super::user_agent().unwrap_or_default(),
+                location: super::web_location(),
+            },
+            system_theme,
+            cpu_usage: None,
+            native_pixels_per_point: Some(super::native_pixels_per_point()),
+        };
+        let storage = LocalStorage::default();
+
+        let egui_ctx = egui::Context::default();
+        egui_ctx.set_os(egui::os::OperatingSystem::from_user_agent(
+            &super::user_agent().unwrap_or_default(),
+        ));
+        super::load_memory(&egui_ctx);
+
+        let theme = system_theme.unwrap_or(web_options.default_theme);
+        egui_ctx.set_visuals(theme.egui_visuals());
+
+        let app = app_creator(&epi::CreationContext {
+            egui_ctx: egui_ctx.clone(),
+            integration_info: info.clone(),
+            storage: Some(&storage),
+
+            #[cfg(feature = "glow")]
+            gl: Some(painter.gl().clone()),
+
+            #[cfg(all(feature = "wgpu", not(feature = "glow")))]
+            wgpu_render_state: painter.render_state(),
+            #[cfg(all(feature = "wgpu", feature = "glow"))]
+            wgpu_render_state: None,
+        });
+
+        let frame = epi::Frame {
+            info,
+            output: Default::default(),
+            storage: Some(Box::new(storage)),
+
+            #[cfg(feature = "glow")]
+            gl: Some(painter.gl().clone()),
+
+            #[cfg(all(feature = "wgpu", not(feature = "glow")))]
+            wgpu_render_state: painter.render_state(),
+            #[cfg(all(feature = "wgpu", feature = "glow"))]
+            wgpu_render_state: None,
+        };
+
+        let needs_repaint: std::sync::Arc<NeedRepaint> = Default::default();
+        {
+            let needs_repaint = needs_repaint.clone();
+            egui_ctx.set_request_repaint_callback(move |info| {
+                needs_repaint.repaint_after(info.after.as_secs_f64());
+            });
+        }
+
+        let mut runner = Self {
+            frame,
+            egui_ctx,
+            painter,
+            input: Default::default(),
+            app,
+            needs_repaint,
+            last_save_time: now_sec(),
+            screen_reader: Default::default(),
+            text_cursor_pos: None,
+            mutable_text_under_cursor: false,
+            textures_delta: Default::default(),
+        };
+
+        runner.input.raw.max_texture_side = Some(runner.painter.max_texture_side());
+
+        Ok(runner)
+    }
+
+    pub fn egui_ctx(&self) -> &egui::Context {
+        &self.egui_ctx
+    }
+
+    /// Get mutable access to the concrete [`App`] we enclose.
+    ///
+    /// This will panic if your app does not implement [`App::as_any_mut`].
+    pub fn app_mut<ConcreteApp: 'static + App>(&mut self) -> &mut ConcreteApp {
+        self.app
+            .as_any_mut()
+            .expect("Your app must implement `as_any_mut`, but it doesn't")
+            .downcast_mut::<ConcreteApp>()
+            .expect("app_mut got the wrong type of App")
+    }
+
+    pub fn auto_save_if_needed(&mut self) {
+        let time_since_last_save = now_sec() - self.last_save_time;
+        if time_since_last_save > self.app.auto_save_interval().as_secs_f64() {
+            self.save();
+        }
+    }
+
+    pub fn save(&mut self) {
+        if self.app.persist_egui_memory() {
+            super::save_memory(&self.egui_ctx);
+        }
+        if let Some(storage) = self.frame.storage_mut() {
+            self.app.save(storage);
+        }
+        self.last_save_time = now_sec();
+    }
+
+    pub fn canvas_id(&self) -> &str {
+        self.painter.canvas_id()
+    }
+
+    pub fn warm_up(&mut self) {
+        if self.app.warm_up_enabled() {
+            let saved_memory: egui::Memory = self.egui_ctx.memory(|m| m.clone());
+            self.egui_ctx
+                .memory_mut(|m| m.set_everything_is_visible(true));
+            self.logic();
+            self.egui_ctx.memory_mut(|m| *m = saved_memory); // We don't want to remember that windows were huge.
+            self.egui_ctx.clear_animations();
+        }
+    }
+
+    pub fn destroy(mut self) {
+        log::debug!("Destroying AppRunner");
+        self.painter.destroy();
+    }
+
+    /// Returns how long to wait until the next repaint.
+    ///
+    /// Call [`Self::paint`] later to paint
+    pub fn logic(&mut self) -> (std::time::Duration, Vec<egui::ClippedPrimitive>) {
+        let frame_start = now_sec();
+
+        super::resize_canvas_to_screen_size(self.canvas_id(), self.app.max_size_points());
+        let canvas_size = super::canvas_size_in_points(self.canvas_id());
+        let raw_input = self.input.new_frame(canvas_size);
+
+        let full_output = self.egui_ctx.run(raw_input, |egui_ctx| {
+            self.app.update(egui_ctx, &mut self.frame);
+        });
+        let egui::FullOutput {
+            platform_output,
+            repaint_after,
+            textures_delta,
+            shapes,
+        } = full_output;
+
+        self.handle_platform_output(platform_output);
+        self.textures_delta.append(textures_delta);
+        let clipped_primitives = self.egui_ctx.tessellate(shapes);
+
+        {
+            let app_output = self.frame.take_app_output();
+            let epi::backend::AppOutput {} = app_output;
+        }
+
+        self.frame.info.cpu_usage = Some((now_sec() - frame_start) as f32);
+
+        (repaint_after, clipped_primitives)
+    }
+
+    /// Paint the results of the last call to [`Self::logic`].
+    pub fn paint(&mut self, clipped_primitives: &[egui::ClippedPrimitive]) -> Result<(), JsValue> {
+        let textures_delta = std::mem::take(&mut self.textures_delta);
+
+        self.painter.paint_and_update_textures(
+            self.app.clear_color(&self.egui_ctx.style().visuals),
+            clipped_primitives,
+            self.egui_ctx.pixels_per_point(),
+            &textures_delta,
+        )?;
+
+        Ok(())
+    }
+
+    fn handle_platform_output(&mut self, platform_output: egui::PlatformOutput) {
+        if self.egui_ctx.options(|o| o.screen_reader) {
+            self.screen_reader
+                .speak(&platform_output.events_description());
+        }
+
+        let egui::PlatformOutput {
+            cursor_icon,
+            open_url,
+            copied_text,
+            events: _, // already handled
+            mutable_text_under_cursor,
+            text_cursor_pos,
+            #[cfg(feature = "accesskit")]
+                accesskit_update: _, // not currently implemented
+        } = platform_output;
+
+        super::set_cursor_icon(cursor_icon);
+        if let Some(open) = open_url {
+            super::open_url(&open.url, open.new_tab);
+        }
+
+        #[cfg(web_sys_unstable_apis)]
+        if !copied_text.is_empty() {
+            super::set_clipboard_text(&copied_text);
+        }
+
+        #[cfg(not(web_sys_unstable_apis))]
+        let _ = copied_text;
+
+        self.mutable_text_under_cursor = mutable_text_under_cursor;
+
+        if self.text_cursor_pos != text_cursor_pos {
+            super::text_agent::move_text_cursor(text_cursor_pos, self.canvas_id());
+            self.text_cursor_pos = text_cursor_pos;
+        }
+    }
+}
+
+// ----------------------------------------------------------------------------
+
+#[derive(Default)]
+struct LocalStorage {}
+
+impl epi::Storage for LocalStorage {
+    fn get_string(&self, key: &str) -> Option<String> {
+        super::local_storage_get(key)
+    }
+
+    fn set_string(&mut self, key: &str, value: String) {
+        super::local_storage_set(key, &value);
+    }
+
+    fn flush(&mut self) {}
+}