windows: Implement keyboard_layout_change (#30624)

Part of #29144

Release Notes:

- N/A

crates/gpui/src/platform/windows/events.rs

@@ -93,6 +93,7 @@ pub(crate) fn handle_msg(
         WM_IME_COMPOSITION => handle_ime_composition(handle, lparam, state_ptr),
         WM_SETCURSOR => handle_set_cursor(lparam, state_ptr),
         WM_SETTINGCHANGE => handle_system_settings_changed(handle, lparam, state_ptr),
+        WM_INPUTLANGCHANGE => handle_input_language_changed(lparam, state_ptr),
         WM_GPUI_CURSOR_STYLE_CHANGED => handle_cursor_changed(lparam, state_ptr),
         _ => None,
     };
@@ -1279,6 +1280,18 @@ fn handle_system_theme_changed(
     Some(0)
 }
 
+fn handle_input_language_changed(
+    lparam: LPARAM,
+    state_ptr: Rc<WindowsWindowStatePtr>,
+) -> Option<isize> {
+    let thread = state_ptr.main_thread_id_win32;
+    let validation = state_ptr.validation_number;
+    unsafe {
+        PostThreadMessageW(thread, WM_INPUTLANGCHANGE, WPARAM(validation), lparam).log_err();
+    }
+    Some(0)
+}
+
 fn parse_syskeydown_msg_keystroke(wparam: WPARAM) -> Option<Keystroke> {
     let modifiers = current_modifiers();
     let vk_code = wparam.loword();

crates/gpui/src/platform/windows/platform.rs

@@ -33,8 +33,8 @@ use crate::{platform::blade::BladeContext, *};
 pub(crate) struct WindowsPlatform {
     state: RefCell<WindowsPlatformState>,
     raw_window_handles: RwLock<SmallVec<[HWND; 4]>>,
-    gpu_context: BladeContext,
     // The below members will never change throughout the entire lifecycle of the app.
+    gpu_context: BladeContext,
     icon: HICON,
     main_receiver: flume::Receiver<Runnable>,
     background_executor: BackgroundExecutor,
@@ -62,6 +62,7 @@ struct PlatformCallbacks {
     app_menu_action: Option<Box<dyn FnMut(&dyn Action)>>,
     will_open_app_menu: Option<Box<dyn FnMut()>>,
     validate_app_menu_command: Option<Box<dyn FnMut(&dyn Action) -> bool>>,
+    keyboard_layout_change: Option<Box<dyn FnMut()>>,
 }
 
 impl WindowsPlatformState {
@@ -201,6 +202,19 @@ impl WindowsPlatform {
         }
     }
 
+    fn handle_input_lang_change(&self) {
+        let mut lock = self.state.borrow_mut();
+        if let Some(mut callback) = lock.callbacks.keyboard_layout_change.take() {
+            drop(lock);
+            callback();
+            self.state
+                .borrow_mut()
+                .callbacks
+                .keyboard_layout_change
+                .get_or_insert(callback);
+        }
+    }
+
     // Returns true if the app should quit.
     fn handle_events(&self) -> bool {
         let mut msg = MSG::default();
@@ -208,7 +222,8 @@ impl WindowsPlatform {
             while PeekMessageW(&mut msg, None, 0, 0, PM_REMOVE).as_bool() {
                 match msg.message {
                     WM_QUIT => return true,
-                    WM_GPUI_CLOSE_ONE_WINDOW
+                    WM_INPUTLANGCHANGE
+                    | WM_GPUI_CLOSE_ONE_WINDOW
                     | WM_GPUI_TASK_DISPATCHED_ON_MAIN_THREAD
                     | WM_GPUI_DOCK_MENU_ACTION => {
                         if self.handle_gpui_evnets(msg.message, msg.wParam, msg.lParam, &msg) {
@@ -247,6 +262,7 @@ impl WindowsPlatform {
             }
             WM_GPUI_TASK_DISPATCHED_ON_MAIN_THREAD => self.run_foreground_task(),
             WM_GPUI_DOCK_MENU_ACTION => self.handle_dock_action_event(lparam.0 as _),
+            WM_INPUTLANGCHANGE => self.handle_input_lang_change(),
             _ => unreachable!(),
         }
         false
@@ -305,8 +321,8 @@ impl Platform for WindowsPlatform {
         )
     }
 
-    fn on_keyboard_layout_change(&self, _callback: Box<dyn FnMut()>) {
-        // todo(windows)
+    fn on_keyboard_layout_change(&self, callback: Box<dyn FnMut()>) {
+        self.state.borrow_mut().callbacks.keyboard_layout_change = Some(callback);
     }
 
     fn run(&self, on_finish_launching: Box<dyn 'static + FnOnce()>) {