Fix getNextFrame (flutter#21422)

lib/ui/painting.dart

@@ -1905,8 +1905,6 @@ class Codec extends NativeFieldWrapperClass2 {
   int get repetitionCount => _cachedRepetitionCount ??= _repetitionCount;
   int get _repetitionCount native 'Codec_repetitionCount';
 
-  FrameInfo? _cachedFrame;
-
   /// Fetches the next animation frame.
   ///
   /// Wraps back to the first frame after returning the last frame.
@@ -1916,24 +1914,20 @@ class Codec extends NativeFieldWrapperClass2 {
   /// The caller of this method is responsible for disposing the
   /// [FrameInfo.image] on the returned object.
   Future<FrameInfo> getNextFrame() async {
-    if (_cachedFrame == null || frameCount != 1) {
-      final Completer<void> completer = Completer<void>.sync();
-      final String? error = _getNextFrame((_Image? image, int durationMilliseconds) {
-        if (image == null) {
-          throw Exception('Codec failed to produce an image, possibly due to invalid image data.');
-        }
-        _cachedFrame = FrameInfo._(
-          image: Image._(image),
-          duration: Duration(milliseconds: durationMilliseconds),
-        );
-        completer.complete();
-      });
-      if (error != null) {
-        throw Exception(error);
+    final Completer<FrameInfo> completer = Completer<FrameInfo>.sync();
+    final String? error = _getNextFrame((_Image? image, int durationMilliseconds) {
+      if (image == null) {
+        throw Exception('Codec failed to produce an image, possibly due to invalid image data.');
       }
-      await completer.future;
+      completer.complete(FrameInfo._(
+        image: Image._(image),
+        duration: Duration(milliseconds: durationMilliseconds),
+      ));
+    });
+    if (error != null) {
+      throw Exception(error);
     }
-    return _cachedFrame!;
+    return await completer.future;
   }
 
   /// Returns an error message on failure, null on success.

testing/dart/image_dispose_test.dart

@@ -114,6 +114,18 @@ void main() {
 
     expect(frame2.image.isCloneOf(frame.image), false);
   });
+
+  test('getNextFrame does not return a disposed image', () async {
+    final Uint8List bytes = await readFile('2x2.png');
+    final Codec codec = await instantiateImageCodec(bytes);
+    final FrameInfo frame = await codec.getNextFrame();
+
+    frame.image.dispose();
+
+    final FrameInfo frame2 = await codec.getNextFrame();
+    expect(frame2.image.clone()..dispose(), isNotNull);
+    frame2.image.dispose();
+  });
 }
 
 Future<Uint8List> readFile(String fileName) async {