gpui: Add support for observing window appearance (#7294)

This PR adds support to GPUI for observing when the appearance of a
window changes.

Based on the initial work done in
https://github.com/zed-industries/zed/pull/6881.

Release Notes:

- N/A

crates/gpui/src/platform/test/window.rs

@@ -171,7 +171,7 @@ impl PlatformWindow for TestWindow {
     }
 
     fn appearance(&self) -> WindowAppearance {
-        unimplemented!()
+        WindowAppearance::Light
     }
 
     fn display(&self) -> std::rc::Rc<dyn crate::PlatformDisplay> {
@@ -276,9 +276,7 @@ impl PlatformWindow for TestWindow {
         unimplemented!()
     }
 
-    fn on_appearance_changed(&self, _callback: Box<dyn FnMut()>) {
+    fn on_appearance_changed(&self, _callback: Box<dyn FnMut()>) {}
-        unimplemented!()
-    }
 
     fn is_topmost_for_position(&self, _position: crate::Point<Pixels>) -> bool {
         unimplemented!()

crates/gpui/src/window.rs

@@ -6,8 +6,8 @@ use crate::{
     KeymatchResult, Keystroke, KeystrokeEvent, Model, ModelContext, Modifiers, MouseButton,
     MouseMoveEvent, MouseUpEvent, Pixels, PlatformAtlas, PlatformDisplay, PlatformInput,
     PlatformWindow, Point, PromptLevel, Render, ScaledPixels, SharedString, Size, SubscriberSet,
-    Subscription, TaffyLayoutEngine, Task, View, VisualContext, WeakView, WindowBounds,
+    Subscription, TaffyLayoutEngine, Task, View, VisualContext, WeakView, WindowAppearance,
-    WindowOptions, WindowTextSystem,
+    WindowBounds, WindowOptions, WindowTextSystem,
 };
 use anyhow::{anyhow, Context as _, Result};
 use collections::FxHashSet;
@@ -269,6 +269,8 @@ pub struct Window {
     scale_factor: f32,
     bounds: WindowBounds,
     bounds_observers: SubscriberSet<(), AnyObserver>,
+    appearance: WindowAppearance,
+    appearance_observers: SubscriberSet<(), AnyObserver>,
     active: bool,
     pub(crate) dirty: bool,
     pub(crate) refreshing: bool,
@@ -338,6 +340,7 @@ impl Window {
         let content_size = platform_window.content_size();
         let scale_factor = platform_window.scale_factor();
         let bounds = platform_window.bounds();
+        let appearance = platform_window.appearance();
         let text_system = Arc::new(WindowTextSystem::new(cx.text_system().clone()));
 
         platform_window.on_request_frame(Box::new({
@@ -364,6 +367,14 @@ impl Window {
                     .log_err();
             }
         }));
+        platform_window.on_appearance_changed(Box::new({
+            let mut cx = cx.to_async();
+            move || {
+                handle
+                    .update(&mut cx, |_, cx| cx.appearance_changed())
+                    .log_err();
+            }
+        }));
         platform_window.on_active_status_change(Box::new({
             let mut cx = cx.to_async();
             move |active| {
@@ -413,6 +424,8 @@ impl Window {
             scale_factor,
             bounds,
             bounds_observers: SubscriberSet::new(),
+            appearance,
+            appearance_observers: SubscriberSet::new(),
             active: false,
             dirty: false,
             refreshing: false,
@@ -742,6 +755,20 @@ impl<'a> WindowContext<'a> {
         self.window.bounds
     }
 
+    fn appearance_changed(&mut self) {
+        self.window.appearance = self.window.platform_window.appearance();
+
+        self.window
+            .appearance_observers
+            .clone()
+            .retain(&(), |callback| callback(self));
+    }
+
+    /// Returns the appearance of the current window.
+    pub fn appearance(&self) -> WindowAppearance {
+        self.window.appearance
+    }
+
     /// Returns the size of the drawable area within the window.
     pub fn viewport_size(&self) -> Size<Pixels> {
         self.window.viewport_size
@@ -2066,6 +2093,20 @@ impl<'a, V: 'static> ViewContext<'a, V> {
         subscription
     }
 
+    /// Registers a callback to be invoked when the window appearance changes.
+    pub fn observe_window_appearance(
+        &mut self,
+        mut callback: impl FnMut(&mut V, &mut ViewContext<V>) + 'static,
+    ) -> Subscription {
+        let view = self.view.downgrade();
+        let (subscription, activate) = self.window.appearance_observers.insert(
+            (),
+            Box::new(move |cx| view.update(cx, |view, cx| callback(view, cx)).is_ok()),
+        );
+        activate();
+        subscription
+    }
+
     /// Register a listener to be called when the given focus handle receives focus.
     /// Returns a subscription and persists until the subscription is dropped.
     pub fn on_focus(