egui-winit: Automatically detect and apply dark or light mode (#1045)

egui-winit/CHANGELOG.md

@@ -5,6 +5,7 @@ All notable changes to the `egui-winit` integration will be noted in this file.
 
 ## Unreleased
 * Fixed horizontal scrolling direction on Linux.
+* Automatically detect and apply dark or light mode from system ([#1045](https://github.com/emilk/egui/pull/1045)).
 * Replaced `std::time::Instant` with `instant::Instant` for WebAssembly compatability ([#1023](https://github.com/emilk/egui/pull/1023))
 * Shift-scroll will now result in horizontal scrolling on all platforms ([#1136](https://github.com/emilk/egui/pull/1136)).
 * Require knowledge about max texture side (e.g. `GL_MAX_TEXTURE_SIZE`)) ([#1154](https://github.com/emilk/egui/pull/1154)).

egui-winit/Cargo.toml

@@ -30,6 +30,7 @@ winit = "0.26.1"
 epi = { version = "0.16.0", path = "../epi", optional = true }
 
 copypasta = { version = "0.7", optional = true }
+dark-light = { version = "0.2.1", optional = true } # detect dark mode system preference
 serde = { version = "1.0", optional = true, features = ["derive"] }
 webbrowser = { version = "0.5", optional = true }
 
@@ -37,7 +38,7 @@ webbrowser = { version = "0.5", optional = true }
 tts = { version = "0.19", optional = true }
 
 [features]
-default = ["clipboard", "links"]
+default = ["clipboard", "dark-light", "links"]
 
 # enable cut/copy/paste to OS clipboard.
 # if disabled a clipboard will be simulated so you can still copy/paste within the egui app.

egui-winit/src/epi.rs

@@ -223,11 +223,13 @@ impl EpiIntegration {
 
         *egui_ctx.memory() = persistence.load_memory().unwrap_or_default();
 
+        let prefer_dark_mode = prefer_dark_mode();
+
         let frame = epi::Frame::new(epi::backend::FrameData {
             info: epi::IntegrationInfo {
                 name: integration_name,
                 web_info: None,
-                prefer_dark_mode: None, // TODO: figure out system default
+                prefer_dark_mode,
                 cpu_usage: None,
                 native_pixels_per_point: Some(crate::native_pixels_per_point(window)),
             },
@@ -235,6 +237,12 @@ impl EpiIntegration {
             repaint_signal,
         });
 
+        if prefer_dark_mode == Some(true) {
+            egui_ctx.set_visuals(egui::Visuals::dark());
+        } else {
+            egui_ctx.set_visuals(egui::Visuals::light());
+        }
+
         let mut slf = Self {
             frame,
             persistence,
@@ -340,3 +348,16 @@ impl EpiIntegration {
             .save(&mut *self.app, &self.egui_ctx, window);
     }
 }
+
+#[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
+}