Clarify the relationship between PictureRecorder and Canvas (#19393)

lib/ui/painting.dart

@@ -3433,6 +3433,8 @@ class Canvas extends NativeFieldWrapperClass2 {
   Canvas(PictureRecorder recorder, [ Rect? cullRect ]) : assert(recorder != null) { // ignore: unnecessary_null_comparison
     if (recorder.isRecording)
       throw ArgumentError('"recorder" must not already be associated with another Canvas.');
+    _recorder = recorder;
+    _recorder!._canvas = this;
     cullRect ??= Rect.largest;
     _constructor(recorder, cullRect.left, cullRect.top, cullRect.right, cullRect.bottom);
   }
@@ -3442,6 +3444,11 @@ class Canvas extends NativeFieldWrapperClass2 {
                     double right,
                     double bottom) native 'Canvas_constructor';
 
+  // The underlying Skia SkCanvas is owned by the PictureRecorder used to create this Canvas.
+  // The Canvas holds a reference to the PictureRecorder to prevent the recorder from being
+  // garbage collected until PictureRecorder.endRecording is called.
+  PictureRecorder? _recorder;
+
   /// Saves a copy of the current transform and clip on the save stack.
   ///
   /// Call [restore] to pop the save stack.
@@ -4232,7 +4239,7 @@ class PictureRecorder extends NativeFieldWrapperClass2 {
   /// call to [endRecording], and false if either this
   /// [PictureRecorder] has not yet been associated with a [Canvas],
   /// or the [endRecording] method has already been called.
-  bool get isRecording native 'PictureRecorder_isRecording';
+  bool get isRecording => _canvas != null;
 
   /// Finishes recording graphical operations.
   ///
@@ -4240,12 +4247,18 @@ class PictureRecorder extends NativeFieldWrapperClass2 {
   /// recorded thus far. After calling this function, both the picture recorder
   /// and the canvas objects are invalid and cannot be used further.
   Picture endRecording() {
+    if (_canvas == null)
+      throw StateError('PictureRecorder did not start recording.');
     final Picture picture = Picture._();
     _endRecording(picture);
+    _canvas!._recorder = null;
+    _canvas = null;
     return picture;
   }
 
   void _endRecording(Picture outPicture) native 'PictureRecorder_endRecording';
+
+  Canvas? _canvas;
 }
 
 /// A single shadow.

lib/ui/painting/canvas.cc

@@ -79,7 +79,6 @@ fml::RefPtr<Canvas> Canvas::Create(PictureRecorder* recorder,
   if (!recorder)
     Dart_ThrowException(
         ToDart("Canvas constructor called with non-genuine PictureRecorder."));
-  FML_DCHECK(!recorder->isRecording());  // verified by Dart code
   fml::RefPtr<Canvas> canvas = fml::MakeRefCounted<Canvas>(
       recorder->BeginRecording(SkRect::MakeLTRB(left, top, right, bottom)));
   recorder->set_canvas(canvas);
@@ -432,12 +431,11 @@ void Canvas::drawShadow(const CanvasPath* path,
                                           elevation, transparentOccluder, dpr);
 }
 
-void Canvas::Clear() {
+void Canvas::Invalidate() {
+  if (dart_wrapper()) {
+    ClearDartWrapper();
+  }
   canvas_ = nullptr;
 }
 
-bool Canvas::IsRecording() const {
-  return !!canvas_;
-}
-
 }  // namespace flutter

lib/ui/painting/canvas.h

@@ -165,8 +165,7 @@ class Canvas : public RefCountedDartWrappable<Canvas> {
                   bool transparentOccluder);
 
   SkCanvas* canvas() const { return canvas_; }
-  void Clear();
-  bool IsRecording() const;
+  void Invalidate();
 
   static void RegisterNatives(tonic::DartLibraryNatives* natives);
 

lib/ui/painting/picture_recorder.cc

@@ -20,9 +20,7 @@ static void PictureRecorder_constructor(Dart_NativeArguments args) {
 
 IMPLEMENT_WRAPPERTYPEINFO(ui, PictureRecorder);
 
-#define FOR_EACH_BINDING(V)       \
-  V(PictureRecorder, isRecording) \
-  V(PictureRecorder, endRecording)
+#define FOR_EACH_BINDING(V) V(PictureRecorder, endRecording)
 
 FOR_EACH_BINDING(DART_NATIVE_CALLBACK)
 
@@ -40,16 +38,12 @@ PictureRecorder::PictureRecorder() {}
 
 PictureRecorder::~PictureRecorder() {}
 
-bool PictureRecorder::isRecording() {
-  return canvas_ && canvas_->IsRecording();
-}
-
 SkCanvas* PictureRecorder::BeginRecording(SkRect bounds) {
   return picture_recorder_.beginRecording(bounds, &rtree_factory_);
 }
 
 fml::RefPtr<Picture> PictureRecorder::endRecording(Dart_Handle dart_picture) {
-  if (!isRecording())
+  if (!canvas_)
     return nullptr;
 
   fml::RefPtr<Picture> picture =
@@ -58,8 +52,7 @@ fml::RefPtr<Picture> PictureRecorder::endRecording(Dart_Handle dart_picture) {
                           picture_recorder_.finishRecordingAsPicture()),
                       canvas_->external_allocation_size());
 
-  canvas_->Clear();
-  canvas_->ClearDartWrapper();
+  canvas_->Invalidate();
   canvas_ = nullptr;
   ClearDartWrapper();
   return picture;

lib/ui/painting/picture_recorder.h

@@ -27,7 +27,6 @@ class PictureRecorder : public RefCountedDartWrappable<PictureRecorder> {
 
   SkCanvas* BeginRecording(SkRect bounds);
   fml::RefPtr<Picture> endRecording(Dart_Handle dart_picture);
-  bool isRecording();
 
   void set_canvas(fml::RefPtr<Canvas> canvas) { canvas_ = std::move(canvas); }