eframe: Automatically change theme when system dark/light mode changes (#2750)

* React to ThemeChanged event from winit

* React to theme change using media query change event in WASM

* Share conversion from bool -> Theme

* Suppress too_many_arguments warning

* Document limitations of automatically following the dark vs light mode preference

* Simplify expression

* Conditionally compile code to prevent unused item warnings

* Remove needless borrow

* Remove another needless borrow

* Make associated functions to standalone

* Request repaint after theme has changed

* Only install event listener when `follow_system_theme` is enabled

* Remove dark-light feature gate

* Detect system theme using winit

* Update documentation

* Fix typos

* fix warning about unused argument

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/native/epi_integration.rs

@@ -310,14 +310,17 @@ pub struct EpiIntegration {
     close: bool,
     can_drag_window: bool,
     window_state: WindowState,
+    follow_system_theme: bool,
 }
 
 impl EpiIntegration {
+    #[allow(clippy::too_many_arguments)]
     pub fn new<E>(
         event_loop: &EventLoopWindowTarget<E>,
         max_texture_side: usize,
         window: &winit::window::Window,
         system_theme: Option<Theme>,
+        follow_system_theme: bool,
         storage: Option<Box<dyn epi::Storage>>,
         #[cfg(feature = "glow")] gl: Option<std::sync::Arc<glow::Context>>,
         #[cfg(feature = "wgpu")] wgpu_render_state: Option<egui_wgpu::RenderState>,
@@ -366,6 +369,7 @@ impl EpiIntegration {
             close: false,
             can_drag_window: false,
             window_state,
+            follow_system_theme,
         }
     }
 
@@ -429,6 +433,11 @@ impl EpiIntegration {
             WindowEvent::ScaleFactorChanged { scale_factor, .. } => {
                 self.frame.info.native_pixels_per_point = Some(*scale_factor as _);
             }
+            WindowEvent::ThemeChanged(winit_theme) if self.follow_system_theme => {
+                let theme = theme_from_winit_theme(*winit_theme);
+                self.frame.info.system_theme = Some(theme);
+                self.egui_ctx.set_visuals(theme.egui_visuals());
+            }
             _ => {}
         }
 
@@ -569,3 +578,10 @@ pub fn load_egui_memory(_storage: Option<&dyn epi::Storage>) -> Option<egui::Mem
     #[cfg(not(feature = "persistence"))]
     None
 }
+
+pub(crate) fn theme_from_winit_theme(theme: winit::window::Theme) -> Theme {
+    match theme {
+        winit::window::Theme::Dark => Theme::Dark,
+        winit::window::Theme::Light => Theme::Light,
+    }
+}

crates/eframe/src/native/run.rs

@@ -675,12 +675,13 @@ mod glow_integration {
                 egui_glow::Painter::new(gl.clone(), "", self.native_options.shader_version)
                     .unwrap_or_else(|error| panic!("some OpenGL error occurred {}\n", error));
 
-            let system_theme = self.native_options.system_theme();
+            let system_theme = system_theme(gl_window.window(), &self.native_options);
             let mut integration = epi_integration::EpiIntegration::new(
                 event_loop,
                 painter.max_texture_side(),
                 gl_window.window(),
                 system_theme,
+                self.native_options.follow_system_theme,
                 storage,
                 Some(gl.clone()),
                 #[cfg(feature = "wgpu")]
@@ -1129,12 +1130,13 @@ mod wgpu_integration {
 
             let wgpu_render_state = painter.render_state();
 
-            let system_theme = self.native_options.system_theme();
+            let system_theme = system_theme(&window, &self.native_options);
             let mut integration = epi_integration::EpiIntegration::new(
                 event_loop,
                 painter.max_texture_side().unwrap_or(2048),
                 &window,
                 system_theme,
+                self.native_options.follow_system_theme,
                 storage,
                 #[cfg(feature = "glow")]
                 None,
@@ -1438,3 +1440,22 @@ mod wgpu_integration {
 
 #[cfg(feature = "wgpu")]
 pub use wgpu_integration::run_wgpu;
+
+#[cfg(any(target_os = "windows", target_os = "macos"))]
+fn system_theme(window: &winit::window::Window, options: &NativeOptions) -> Option<crate::Theme> {
+    if options.follow_system_theme {
+        window
+            .theme()
+            .map(super::epi_integration::theme_from_winit_theme)
+    } else {
+        None
+    }
+}
+
+// Winit only reads the system theme on macOS and Windows.
+// On Linux we have to fall back on dark-light (if enabled).
+// See: https://github.com/rust-windowing/winit/issues/1549
+#[cfg(not(any(target_os = "windows", target_os = "macos")))]
+fn system_theme(_window: &winit::window::Window, options: &NativeOptions) -> Option<crate::Theme> {
+    options.system_theme()
+}