Introduce auto-detect-theme feature

Cargo.toml

@@ -18,7 +18,7 @@ all-features = true
 maintenance = { status = "actively-developed" }
 
 [features]
-default = ["wgpu", "fira-sans"]
+default = ["wgpu", "fira-sans", "auto-detect-theme"]
 # Enable the `wgpu` GPU-accelerated renderer backend
 wgpu = ["iced_renderer/wgpu", "iced_widget/wgpu"]
 # Enables the `Image` widget
@@ -53,8 +53,11 @@ multi-window = ["iced_winit/multi-window"]
 advanced = []
 # Enables embedding Fira Sans as the default font on Wasm builds
 fira-sans = ["iced_renderer/fira-sans"]
+# Enables auto-detecting light/dark mode for the built-in theme
+auto-detect-theme = ["iced_core/auto-detect-theme"]
 
 [dependencies]
+iced_core.workspace = true
 iced_futures.workspace = true
 iced_renderer.workspace = true
 iced_widget.workspace = true
@@ -121,6 +124,7 @@ async-std = "1.0"
 bitflags = "2.0"
 bytemuck = { version = "1.0", features = ["derive"] }
 cosmic-text = "0.10"
+dark-light = "1.0"
 futures = "0.3"
 glam = "0.25"
 glyphon = "0.5"

core/Cargo.toml

@@ -10,6 +10,9 @@ homepage.workspace = true
 categories.workspace = true
 keywords.workspace = true
 
+[features]
+auto-detect-theme = ["dep:dark-light"]
+
 [dependencies]
 bitflags.workspace = true
 glam.workspace = true
@@ -22,6 +25,9 @@ thiserror.workspace = true
 web-time.workspace = true
 xxhash-rust.workspace = true
 
+dark-light.workspace = true
+dark-light.optional = true
+
 [target.'cfg(windows)'.dependencies]
 raw-window-handle.workspace = true
 

core/src/theme.rs

@@ -7,10 +7,9 @@ use std::fmt;
 use std::sync::Arc;
 
 /// A built-in theme.
-#[derive(Debug, Clone, PartialEq, Default)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum Theme {
     /// The built-in light variant.
-    #[default]
     Light,
     /// The built-in dark variant.
     Dark,
@@ -161,6 +160,28 @@ impl Theme {
     }
 }
 
+impl Default for Theme {
+    fn default() -> Self {
+        #[cfg(feature = "auto-detect-theme")]
+        {
+            use once_cell::sync::Lazy;
+
+            static DEFAULT: Lazy<Theme> =
+                Lazy::new(|| match dark_light::detect() {
+                    dark_light::Mode::Dark => Theme::Dark,
+                    dark_light::Mode::Light | dark_light::Mode::Default => {
+                        Theme::Light
+                    }
+                });
+
+            DEFAULT.clone()
+        }
+
+        #[cfg(not(feature = "auto-detect-theme"))]
+        Theme::Light
+    }
+}
+
 impl fmt::Display for Theme {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {