Reland 2 (part 2): Enforce the rule of calling FlutterView.Render

lib/ui/platform_dispatcher.dart

@@ -308,6 +308,28 @@ class PlatformDispatcher {
     _invoke(onMetricsChanged, _onMetricsChangedZone);
   }
 
+  // A debug-only variable that stores the [FlutterView]s for which
+  // [FlutterView.render] has already been called during the current
+  // [onBeginFrame]/[onDrawFrame] callback sequence.
+  //
+  // It 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.
+  //
+  // In release build, this variable is null, and therefore the calling rule is
+  // not enforced. This is because the check might hurt cold startup delay;
+  // see https://github.com/flutter/engine/pull/46919.
+  Set<FlutterView>? _debugRenderedViews;
+  // A debug-only variable that temporarily stores the `_renderedViews` value
+  // between `_beginFrame` and `_drawFrame`.
+  //
+  // In release build, this variable is null.
+  Set<FlutterView>? _debugRenderedViewsBetweenCallbacks;
 
   /// A callback invoked when any view begins a frame.
   ///
@@ -329,11 +351,26 @@ class PlatformDispatcher {
 
   // Called from the engine, via hooks.dart
   void _beginFrame(int microseconds) {
+    assert(_debugRenderedViews == null);
+    assert(_debugRenderedViewsBetweenCallbacks == null);
+    assert(() {
+      _debugRenderedViews = <FlutterView>{};
+      return true;
+    }());
+
     _invoke1<Duration>(
       onBeginFrame,
       _onBeginFrameZone,
       Duration(microseconds: microseconds),
     );
+
+    assert(_debugRenderedViews != null);
+    assert(_debugRenderedViewsBetweenCallbacks == null);
+    assert(() {
+      _debugRenderedViewsBetweenCallbacks = _debugRenderedViews;
+      _debugRenderedViews = null;
+      return true;
+    }());
   }
 
   /// A callback that is invoked for each frame after [onBeginFrame] has
@@ -351,7 +388,22 @@ class PlatformDispatcher {
 
   // Called from the engine, via hooks.dart
   void _drawFrame() {
+    assert(_debugRenderedViews == null);
+    assert(_debugRenderedViewsBetweenCallbacks != null);
+    assert(() {
+      _debugRenderedViews = _debugRenderedViewsBetweenCallbacks;
+      _debugRenderedViewsBetweenCallbacks = null;
+      return true;
+    }());
+
     _invoke(onDrawFrame, _onDrawFrameZone);
+
+    assert(_debugRenderedViews != null);
+    assert(_debugRenderedViewsBetweenCallbacks == null);
+    assert(() {
+      _debugRenderedViews = null;
+      return true;
+    }());
   }
 
   /// A callback that is invoked when pointer data is available.

lib/ui/window.dart

@@ -353,7 +353,18 @@ 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) {
+    // Duplicated calls or calls outside of onBeginFrame/onDrawFrame (indicated
+    // by _debugRenderedViews being null) are ignored. See _debugRenderedViews.
+    bool validRender = true;
+    assert(() {
+      validRender = platformDispatcher._debugRenderedViews?.add(this) ?? false;
+      return true;
+    }());
+    if (validRender) {
+      _render(scene as _NativeScene);
+    }
+  }
 
   @Native<Void Function(Pointer<Void>)>(symbol: 'PlatformConfigurationNativeApi::Render')
   external static void _render(_NativeScene scene);