Add API to force redraw of flutter view (#24186)

Currently when creating hidden window and showing it later the content does not get repainted automatically (unless something else triggers the repaint). This adds public `FlutterDesktopViewControllerForceRedraw()` to trigger repaint. The current implementation triggers this via a call to `SendWindowMetrics` but this should eventually be replaced by an embedder API call.

See: https://github.com/flutter/flutter/issues/75319
See: https://github.com/flutter/flutter/issues/69716

shell/platform/windows/client_wrapper/testing/stub_flutter_windows_api.cc

@@ -64,6 +64,9 @@ FlutterDesktopViewRef FlutterDesktopViewControllerGetView(
   return reinterpret_cast<FlutterDesktopViewRef>(1);
 }
 
+void FlutterDesktopViewControllerForceRedraw(
+    FlutterDesktopViewControllerRef controller) {}
+
 bool FlutterDesktopViewControllerHandleTopLevelWindowProc(
     FlutterDesktopViewControllerRef controller,
     HWND hwnd,

shell/platform/windows/flutter_windows.cc

@@ -76,6 +76,11 @@ FlutterDesktopViewRef FlutterDesktopViewControllerGetView(
   return HandleForView(controller->view.get());
 }
 
+void FlutterDesktopViewControllerForceRedraw(
+    FlutterDesktopViewControllerRef controller) {
+  controller->view->ForceRedraw();
+}
+
 FlutterDesktopEngineRef FlutterDesktopEngineCreate(
     const FlutterDesktopEngineProperties& engine_properties) {
   flutter::FlutterProjectBundle project(engine_properties);

shell/platform/windows/flutter_windows_view.cc

@@ -95,6 +95,16 @@ uint32_t FlutterWindowsView::GetFrameBufferId(size_t width, size_t height) {
   return kWindowFrameBufferID;
 }
 
+void FlutterWindowsView::ForceRedraw() {
+  if (resize_status_ == ResizeState::kDone) {
+    // Request new frame
+    // TODO(knopp): Replace with more specific call once there is API for it
+    // https://github.com/flutter/flutter/issues/69716
+    SendWindowMetrics(resize_target_width_, resize_target_height_,
+                      binding_handler_->GetDpiScale());
+  }
+}
+
 void FlutterWindowsView::OnWindowSizeChanged(size_t width, size_t height) {
   // Called on the platform thread.
   std::unique_lock<std::mutex> lock(resize_mutex_);

shell/platform/windows/flutter_windows_view.h

@@ -64,6 +64,9 @@ class FlutterWindowsView : public WindowBindingHandlerDelegate,
   // Returns the engine backing this view.
   FlutterWindowsEngine* GetEngine();
 
+  // Tells the engine to generate a new frame
+  void ForceRedraw();
+
   // Callbacks for clearing context, settings context and swapping buffers,
   // these are typically called on an engine-controlled (non-platform) thread.
   bool ClearContext();

shell/platform/windows/public/flutter_windows.h

@@ -107,6 +107,10 @@ FLUTTER_EXPORT FlutterDesktopEngineRef FlutterDesktopViewControllerGetEngine(
 FLUTTER_EXPORT FlutterDesktopViewRef
 FlutterDesktopViewControllerGetView(FlutterDesktopViewControllerRef controller);
 
+// Requests new frame from engine and repaints the view
+FLUTTER_EXPORT void FlutterDesktopViewControllerForceRedraw(
+    FlutterDesktopViewControllerRef controller);
+
 #ifndef WINUWP
 // Allows the Flutter engine and any interested plugins an opportunity to
 // handle the given message.