Notify widgets of window focus change

CHANGELOG.md

@@ -61,6 +61,7 @@ You can find its changes [documented below](#070---2021-01-01).
 - `scroll_to_view` and `scroll_area_to_view` methods on `UpdateCtx`, `LifecycleCtx` and `EventCtx` ([#1976] by [@xarvic])
 - `Notification::route` ([#1978] by [@xarvic])
 - Build on OpenBSD ([#1993] by [@klemensn])
+- `Event::WindowFocus(bool)` to notify widgets of window focus changes ([#2016] by [@ForLoveOfCats])
 
 ### Changed
 
@@ -804,6 +805,7 @@ Last release without a changelog :(
 [#1978]: https://github.com/linebender/druid/pull/1978
 [#1993]: https://github.com/linebender/druid/pull/1993
 [#1996]: https://github.com/linebender/druid/pull/1996
+[#2016]: https://github.com/linebender/druid/pull/2016
 
 [Unreleased]: https://github.com/linebender/druid/compare/v0.7.0...master
 [0.7.0]: https://github.com/linebender/druid/compare/v0.6.0...v0.7.0

druid/src/core.rs

@@ -725,6 +725,7 @@ impl<T: Data, W: Widget<T>> WidgetPod<T, W> {
                 }
                 true
             }
+            Event::WindowFocus(_) => true,
             Event::WindowSize(_) => {
                 self.state.needs_layout = true;
                 ctx.is_root

druid/src/event.rs

@@ -74,6 +74,8 @@ pub enum Event {
     /// This event means the window *will* go away; it is safe to dispose of resources and
     /// do any other cleanup.
     WindowDisconnected,
+    /// Called when window focus changes.
+    WindowFocus(bool),
     /// Called on the root widget when the window size changes.
     ///
     /// Discussion: it's not obvious this should be propagated to user
@@ -431,6 +433,7 @@ impl Event {
             Event::WindowConnected
             | Event::WindowCloseRequested
             | Event::WindowDisconnected
+            | Event::WindowFocus(_)
             | Event::WindowSize(_)
             | Event::Timer(_)
             | Event::AnimFrame(_)

druid/src/win_handler.rs

@@ -977,6 +977,14 @@ impl<T: Data> WinHandler for DruidHandler<T> {
 
     fn got_focus(&mut self) {
         self.app_state.window_got_focus(self.window_id);
+
+        let event = Event::WindowFocus(true);
+        self.app_state.do_window_event(event, self.window_id);
+    }
+
+    fn lost_focus(&mut self) {
+        let event = Event::WindowFocus(false);
+        self.app_state.do_window_event(event, self.window_id);
     }
 
     fn timer(&mut self, token: TimerToken) {