Report theme changes to debug API

core/src/theme.rs

@@ -252,7 +252,7 @@ impl fmt::Display for Custom {
     }
 }
 
-/// The base style of a [`Theme`].
+/// The base style of a theme.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Style {
     /// The background [`Color`] of the application.
@@ -262,16 +262,27 @@ pub struct Style {
     pub text_color: Color,
 }
 
-/// The default blank style of a [`Theme`].
+/// The default blank style of a theme.
 pub trait Base {
-    /// Returns the default base [`Style`] of a [`Theme`].
+    /// Returns the default base [`Style`] of a theme.
     fn base(&self) -> Style;
+
+    /// Returns the color [`Palette`] of the theme.
+    ///
+    /// This [`Palette`] may be used by the runtime for
+    /// debugging purposes; like displaying performance
+    /// metrics or devtools.
+    fn palette(&self) -> Option<Palette>;
 }
 
 impl Base for Theme {
     fn base(&self) -> Style {
         default(self)
     }
+
+    fn palette(&self) -> Option<Palette> {
+        Some(self.palette())
+    }
 }
 
 /// The default [`Style`] of a built-in [`Theme`].

winit/src/lib.rs

@@ -597,6 +597,14 @@ async fn run_instance<P>(
                     continue;
                 }
 
+                debug::theme_changed(|| {
+                    if window_manager.is_empty() {
+                        theme::Base::palette(&program.theme(id))
+                    } else {
+                        None
+                    }
+                });
+
                 let window = window_manager.insert(
                     id,
                     window,
@@ -1026,6 +1034,12 @@ async fn run_instance<P>(
                                 window.raw.request_redraw();
                             }
 
+                            debug::theme_changed(|| {
+                                window_manager.first().and_then(|window| {
+                                    theme::Base::palette(window.state.theme())
+                                })
+                            });
+
                             user_interfaces =
                                 ManuallyDrop::new(build_user_interfaces(
                                     &program,