Use dark-light on Mac and Windows

dark-light has a nasty problem on Linux: frewsxcv/rust-dark-light#17

So we made dark-light opt-in in #1437

This PR makes dark-light a default dependency again,
but only use it on Max and Windows.

This is controlled with the new NativeOptions::follow_system_theme.
If this isn't enabled, then NativeOptions::default_theme is used.

eframe/Cargo.toml

@@ -20,9 +20,11 @@ all-features = true
 
 
 [features]
-default = ["default_fonts", "glow"]
+default = ["dark-light", "default_fonts", "glow"]
 
 ## Detect dark mode system preference using [`dark-light`](https://docs.rs/dark-light).
+##
+## See also [`NativeOptions::follow_system_theme`] and [`NativeOptions::default_theme`].
 dark-light = ["dep:dark-light"]
 
 ## If set, egui will use `include_bytes!` to bundle some fonts.

eframe/src/epi.rs

@@ -243,6 +243,23 @@ pub struct NativeOptions {
 
     /// What rendering backend to use.
     pub renderer: Renderer,
+
+    /// If the `dark-light` feature is enabled:
+    ///
+    /// Try to detect and follow the system preferred setting for dark vs light mode.
+    ///
+    /// By default, this is `true` on Mac and Windows, but `false` on Linux
+    /// due to <https://github.com/frewsxcv/rust-dark-light/issues/17>.
+    ///
+    /// See also [`Self::default_theme`].
+    pub follow_system_theme: bool,
+
+    /// Use the dark mode theme if:
+    /// * the `dark-light` feature is disabled
+    /// * OR [`Self::follow_system_theme`] is `false`.
+    ///
+    /// Default: `Theme::Dark` (default to dark theme).
+    pub default_theme: Theme,
 }
 
 impl Default for NativeOptions {
@@ -265,6 +282,49 @@ impl Default for NativeOptions {
             stencil_buffer: 0,
             hardware_acceleration: HardwareAcceleration::Preferred,
             renderer: Renderer::default(),
+            follow_system_theme: cfg!(target_os = "macos") || cfg!(target_os = "windows"),
+            default_theme: Theme::Dark,
+        }
+    }
+}
+
+impl NativeOptions {
+    /// The theme used by the system.
+    #[cfg(feature = "dark-light")]
+    pub fn system_theme(&self) -> Option<Theme> {
+        if self.follow_system_theme {
+            crate::profile_scope!("dark_light::detect");
+            match dark_light::detect() {
+                dark_light::Mode::Dark => Some(Theme::Dark),
+                dark_light::Mode::Light => Some(Theme::Light),
+            }
+        } else {
+            None
+        }
+    }
+
+    /// The theme used by the system.
+    #[cfg(not(feature = "dark-light"))]
+    pub fn system_theme(&self) -> Option<Theme> {
+        None
+    }
+}
+
+/// Dark or Light theme.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+pub enum Theme {
+    /// Dark mode: light text on a dark background.
+    Dark,
+    /// Light mode: dark text on a light background.
+    Light,
+}
+
+impl Theme {
+    pub(crate) fn egui_visuals(self) -> egui::Visuals {
+        match self {
+            Self::Dark => egui::Visuals::dark(),
+            Self::Light => egui::Visuals::light(),
         }
     }
 }
@@ -531,9 +591,10 @@ pub struct IntegrationInfo {
     /// If the app is running in a Web context, this returns information about the environment.
     pub web_info: Option<WebInfo>,
 
-    /// Does the system prefer dark mode (over light mode)?
+    /// Does the OS use dark or light mode?
+    ///
     /// `None` means "don't know".
-    pub prefer_dark_mode: Option<bool>,
+    pub system_theme: Option<Theme>,
 
     /// Seconds of cpu usage (in seconds) of UI code on the previous frame.
     /// `None` if this is the first frame.

eframe/src/native/epi_integration.rs

@@ -1,4 +1,4 @@
-use crate::{epi, WindowInfo};
+use crate::{epi, Theme, WindowInfo};
 use egui_winit::{native_pixels_per_point, WindowSettings};
 use winit::event_loop::EventLoopWindowTarget;
 
@@ -47,12 +47,7 @@ pub fn window_builder(
         max_window_size,
         resizable,
         transparent,
-        vsync: _,                 // used in `fn create_display`
-        multisampling: _,         // used in `fn create_display`
-        depth_buffer: _,          // used in `fn create_display`
-        stencil_buffer: _,        // used in `fn create_display`
-        hardware_acceleration: _, // used in `fn create_display`
-        renderer: _,              // used in `fn run_native`
+        ..
     } = native_options;
 
     let window_icon = icon_data.clone().and_then(load_icon);
@@ -187,6 +182,7 @@ impl EpiIntegration {
         event_loop: &EventLoopWindowTarget<E>,
         max_texture_side: usize,
         window: &winit::window::Window,
+        system_theme: Option<Theme>,
         storage: Option<Box<dyn epi::Storage>>,
         #[cfg(feature = "glow")] gl: Option<std::sync::Arc<glow::Context>>,
         #[cfg(feature = "wgpu")] render_state: Option<egui_wgpu::RenderState>,
@@ -195,12 +191,10 @@ impl EpiIntegration {
 
         *egui_ctx.memory() = load_egui_memory(storage.as_deref()).unwrap_or_default();
 
-        let prefer_dark_mode = prefer_dark_mode();
-
         let frame = epi::Frame {
             info: epi::IntegrationInfo {
                 web_info: None,
-                prefer_dark_mode,
+                system_theme,
                 cpu_usage: None,
                 native_pixels_per_point: Some(native_pixels_per_point(window)),
                 window_info: read_window_info(window, egui_ctx.pixels_per_point()),
@@ -213,12 +207,6 @@ impl EpiIntegration {
             render_state,
         };
 
-        if prefer_dark_mode == Some(true) {
-            egui_ctx.set_visuals(egui::Visuals::dark());
-        } else {
-            egui_ctx.set_visuals(egui::Visuals::light());
-        }
-
         let mut egui_winit = egui_winit::State::new(event_loop);
         egui_winit.set_max_texture_side(max_texture_side);
         let pixels_per_point = window.scale_factor() as f32;
@@ -376,16 +364,3 @@ pub fn load_egui_memory(_storage: Option<&dyn epi::Storage>) -> Option<egui::Mem
     #[cfg(not(feature = "persistence"))]
     None
 }
-
-#[cfg(feature = "dark-light")]
-fn prefer_dark_mode() -> Option<bool> {
-    match dark_light::detect() {
-        dark_light::Mode::Dark => Some(true),
-        dark_light::Mode::Light => Some(false),
-    }
-}
-
-#[cfg(not(feature = "dark-light"))]
-fn prefer_dark_mode() -> Option<bool> {
-    None
-}

eframe/src/native/run.rs

@@ -66,15 +66,22 @@ pub fn run_glow(
     let mut painter = egui_glow::Painter::new(gl.clone(), None, "")
         .unwrap_or_else(|error| panic!("some OpenGL error occurred {}\n", error));
 
+    let system_theme = native_options.system_theme();
     let mut integration = epi_integration::EpiIntegration::new(
         &event_loop,
         painter.max_texture_side(),
         gl_window.window(),
+        system_theme,
         storage,
         Some(gl.clone()),
         #[cfg(feature = "wgpu")]
         None,
     );
+    integration.egui_ctx.set_visuals(
+        system_theme
+            .unwrap_or(native_options.default_theme)
+            .egui_visuals(),
+    );
 
     {
         let event_loop_proxy = egui::mutex::Mutex::new(event_loop.create_proxy());
@@ -248,15 +255,22 @@ pub fn run_wgpu(
 
     let render_state = painter.get_render_state().expect("Uninitialized");
 
+    let system_theme = native_options.system_theme();
     let mut integration = epi_integration::EpiIntegration::new(
         &event_loop,
         painter.max_texture_side().unwrap_or(2048),
         &window,
+        system_theme,
         storage,
         #[cfg(feature = "glow")]
         None,
         Some(render_state.clone()),
     );
+    integration.egui_ctx.set_visuals(
+        system_theme
+            .unwrap_or(native_options.default_theme)
+            .egui_visuals(),
+    );
 
     {
         let event_loop_proxy = egui::mutex::Mutex::new(event_loop.create_proxy());

egui_demo_app/src/wrap_app.rs

@@ -112,12 +112,6 @@ impl WrapApp {
             }
         }
 
-        if cc.integration_info.prefer_dark_mode == Some(false) {
-            cc.egui_ctx.set_visuals(egui::Visuals::light()); // use light mode if explicitly asked for
-        } else {
-            cc.egui_ctx.set_visuals(egui::Visuals::dark()); // use dark mode if there is no preference, or the preference is dark mode
-        }
-
         slf
     }