Reland: Enforce the rule of calling FlutterView.Render (#45300)

lib/ui/fixtures/ui_test.dart

@@ -1076,3 +1076,45 @@ external void _callHook(
   Object? arg20,
   Object? arg21,
 ]);
+
+Scene _createRedBoxScene(Size size) {
+  final SceneBuilder builder = SceneBuilder();
+  builder.pushOffset(0.0, 0.0);
+  final Paint paint = Paint()
+    ..color = Color.fromARGB(255, 255, 0, 0)
+    ..style = PaintingStyle.fill;
+  final PictureRecorder baseRecorder = PictureRecorder();
+  final Canvas canvas = Canvas(baseRecorder);
+  canvas.drawRect(Rect.fromLTRB(0.0, 0.0, size.width, size.height), paint);
+  final Picture picture = baseRecorder.endRecording();
+  builder.addPicture(Offset(0.0, 0.0), picture);
+  builder.pop();
+  return builder.build();
+}
+
+@pragma('vm:entry-point')
+void incorrectImmediateRender() {
+  PlatformDispatcher.instance.views.first.render(_createRedBoxScene(Size(2, 2)));
+  _finish();
+  // Don't schedule a frame here. This test only checks if the
+  // [FlutterView.render] call is propagated to PlatformConfiguration.render
+  // and thus doesn't need anything from `Animator` or `Engine`, which,
+  // besides, are not even created in the native side at all.
+}
+
+@pragma('vm:entry-point')
+void incorrectDoubleRender() {
+  PlatformDispatcher.instance.onBeginFrame = (Duration value) {
+    PlatformDispatcher.instance.views.first.render(_createRedBoxScene(Size(2, 2)));
+    PlatformDispatcher.instance.views.first.render(_createRedBoxScene(Size(3, 3)));
+  };
+  PlatformDispatcher.instance.onDrawFrame = () {
+    PlatformDispatcher.instance.views.first.render(_createRedBoxScene(Size(4, 4)));
+    PlatformDispatcher.instance.views.first.render(_createRedBoxScene(Size(5, 5)));
+  };
+  _finish();
+  // Don't schedule a frame here. This test only checks if the
+  // [FlutterView.render] call is propagated to PlatformConfiguration.render
+  // and thus doesn't need anything from `Animator` or `Engine`, which,
+  // besides, are not even created in the native side at all.
+}

lib/ui/platform_dispatcher.dart

@@ -308,6 +308,21 @@ class PlatformDispatcher {
     _invoke(onMetricsChanged, _onMetricsChangedZone);
   }
 
+  // [FlutterView]s for which [FlutterView.render] has already been called
+  // during the current [onBeginFrame]/[onDrawFrame] callback sequence.
+  //
+  // The field is null outside the scope of those callbacks indicating that
+  // calls to [FlutterView.render] must be ignored. Furthermore, if a given
+  // [FlutterView] is already present in this set when its [FlutterView.render]
+  // is called again, that call must be ignored as a duplicate.
+  //
+  // Between [onBeginFrame] and [onDrawFrame] the properties value is
+  // temporarily stored in `_renderedViewsBetweenCallbacks` so that it survives
+  // the gap between the two callbacks.
+  Set<FlutterView>? _renderedViews;
+  // Temporary storage of the `_renderedViews` value between `_beginFrame` and
+  // `_drawFrame`.
+  Set<FlutterView>? _renderedViewsBetweenCallbacks;
 
   /// A callback invoked when any view begins a frame.
   ///
@@ -329,11 +344,20 @@ class PlatformDispatcher {
 
   // Called from the engine, via hooks.dart
   void _beginFrame(int microseconds) {
+    assert(_renderedViews == null);
+    assert(_renderedViewsBetweenCallbacks == null);
+
+    _renderedViews = <FlutterView>{};
     _invoke1<Duration>(
       onBeginFrame,
       _onBeginFrameZone,
       Duration(microseconds: microseconds),
     );
+    _renderedViewsBetweenCallbacks = _renderedViews;
+    _renderedViews = null;
+
+    assert(_renderedViews == null);
+    assert(_renderedViewsBetweenCallbacks != null);
   }
 
   /// A callback that is invoked for each frame after [onBeginFrame] has
@@ -351,7 +375,16 @@ class PlatformDispatcher {
 
   // Called from the engine, via hooks.dart
   void _drawFrame() {
+    assert(_renderedViews == null);
+    assert(_renderedViewsBetweenCallbacks != null);
+
+    _renderedViews = _renderedViewsBetweenCallbacks;
+    _renderedViewsBetweenCallbacks = null;
     _invoke(onDrawFrame, _onDrawFrameZone);
+    _renderedViews = null;
+
+    assert(_renderedViews == null);
+    assert(_renderedViewsBetweenCallbacks == null);
   }
 
   /// A callback that is invoked when pointer data is available.

lib/ui/window.dart

@@ -327,14 +327,21 @@ class FlutterView {
 
   /// Updates the view's rendering on the GPU with the newly provided [Scene].
   ///
-  /// This function must be called within the scope of the
+  /// ## Requirement for calling this method
+  ///
+  /// This method must be called within the synchronous scope of the
   /// [PlatformDispatcher.onBeginFrame] or [PlatformDispatcher.onDrawFrame]
-  /// callbacks being invoked.
+  /// callbacks. Calls out of this scope will be ignored. To use this method,
+  /// create a callback that calls this method instead, and assign it to either
+  /// of the fields above; then schedule a frame, which is done typically with
+  /// [PlatformDispatcher.scheduleFrame]. Also, make sure the callback does not
+  /// have `await` before the `FlutterWindow.render` call.
+  ///
+  /// Additionally, this method can only be called once for each view during a
+  /// single [PlatformDispatcher.onBeginFrame]/[PlatformDispatcher.onDrawFrame]
+  /// callback sequence. Duplicate calls will be ignored in production.
   ///
-  /// If this function is called a second time during a single
-  /// [PlatformDispatcher.onBeginFrame]/[PlatformDispatcher.onDrawFrame]
-  /// callback sequence or called outside the scope of those callbacks, the call
-  /// will be ignored.
+  /// ## How to record a scene
   ///
   /// To record graphical operations, first create a [PictureRecorder], then
   /// construct a [Canvas], passing that [PictureRecorder] to its constructor.
@@ -353,7 +360,14 @@ class FlutterView {
   ///   scheduling of frames.
   /// * [RendererBinding], the Flutter framework class which manages layout and
   ///   painting.
-  void render(Scene scene) => _render(scene as _NativeScene);
+  void render(Scene scene) {
+    if (platformDispatcher._renderedViews?.add(this) != true) {
+      // Duplicated calls or calls outside of onBeginFrame/onDrawFrame
+      // (indicated by _renderedViews being null) are ignored, as documented.
+      return;
+    }
+    _render(scene as _NativeScene);
+  }
 
   @Native<Void Function(Pointer<Void>)>(symbol: 'PlatformConfigurationNativeApi::Render')
   external static void _render(_NativeScene scene);