Remove chunk event subscription when disposing a MultiFrameImageStreamCompleter (#94902)

* Remove chunk event subscription when disposing a MultiFrameImageStreamCompleter

* Add impl..

* stray prints

packages/flutter/lib/src/painting/image_stream.dart

@@ -571,6 +571,8 @@ abstract class ImageStreamCompleter with Diagnosticable {
   }
 
   bool _disposed = false;
+
+  @mustCallSuper
   void _maybeDispose() {
     if (!_hadAtLeastOneListener || _disposed || _listeners.isNotEmpty || _keepAliveHandles != 0) {
       return;
@@ -851,7 +853,7 @@ class MultiFrameImageStreamCompleter extends ImageStreamCompleter {
       );
     });
     if (chunkEvents != null) {
-      chunkEvents.listen(reportImageChunkEvent,
+      _chunkSubscription = chunkEvents.listen(reportImageChunkEvent,
         onError: (Object error, StackTrace stack) {
           reportError(
             context: ErrorDescription('loading an image'),
@@ -865,6 +867,7 @@ class MultiFrameImageStreamCompleter extends ImageStreamCompleter {
     }
   }
 
+  StreamSubscription<ImageChunkEvent>? _chunkSubscription;
   ui.Codec? _codec;
   final double _scale;
   final InformationCollector? _informationCollector;
@@ -990,4 +993,14 @@ class MultiFrameImageStreamCompleter extends ImageStreamCompleter {
       _timer = null;
     }
   }
+
+  @override
+  void _maybeDispose() {
+    super._maybeDispose();
+    if (_disposed) {
+      _chunkSubscription?.onData(null);
+      _chunkSubscription?.cancel();
+      _chunkSubscription = null;
+    }
+  }
 }

packages/flutter/test/painting/image_stream_test.dart

@@ -811,44 +811,31 @@ void main() {
     expect(oneFrameCodec.numFramesAsked, 1);
   });  // https://github.com/flutter/flutter/issues/82532
 
-  // TODO(amirh): enable this once WidgetTester supports flushTimers.
-  // https://github.com/flutter/flutter/issues/30344
-  // testWidgets('remove and add listener before a delayed frame is scheduled', (WidgetTester tester) async {
-  //   final MockCodec mockCodec = MockCodec();
-  //   mockCodec.frameCount = 3;
-  //   mockCodec.repetitionCount = 0;
-  //   final Completer<Codec> codecCompleter = Completer<Codec>();
-  //
-  //   final ImageStreamCompleter imageStream = MultiFrameImageStreamCompleter(
-  //     codec: codecCompleter.future,
-  //     scale: 1.0,
-  //   );
-  //
-  //   final ImageListener listener = (ImageInfo image, bool synchronousCall) { };
-  //   imageStream.addListener(ImageLoadingListener(listener));
-  //
-  //   codecCompleter.complete(mockCodec);
-  //   await tester.idle();
-  //
-  //   final FrameInfo frame1 = FakeFrameInfo(20, 10, const Duration(milliseconds: 200));
-  //   final FrameInfo frame2 = FakeFrameInfo(200, 100, const Duration(milliseconds: 400));
-  //   final FrameInfo frame3 = FakeFrameInfo(200, 100, Duration.zero);
-  //
-  //   mockCodec.completeNextFrame(frame1);
-  //   await tester.idle(); // let nextFrameFuture complete
-  //   await tester.pump(); // first animation frame shows on first app frame.
-  //
-  //   mockCodec.completeNextFrame(frame2);
-  //   await tester.pump(const Duration(milliseconds: 100)); // emit 2nd frame.
-  //
-  //   tester.flushTimers();
-  //
-  //   imageStream.removeListener(listener);
-  //   imageStream.addListener(ImageLoadingListener(listener));
-  //
-  //   mockCodec.completeNextFrame(frame3);
-  //   await tester.idle(); // let nextFrameFuture complete
-  //
-  //   await tester.pump();
-  // });
+  test('Multi-frame complete unsubscribes to chunk events when disposed', () async {
+    final FakeCodec codec = await FakeCodec.fromData(Uint8List.fromList(kTransparentImage));
+    final StreamController<ImageChunkEvent> chunkStream = StreamController<ImageChunkEvent>();
+
+    final MultiFrameImageStreamCompleter completer = MultiFrameImageStreamCompleter(
+      codec: Future<Codec>.value(codec),
+      scale: 1.0,
+      chunkEvents: chunkStream.stream,
+    );
+
+    expect(chunkStream.hasListener, true);
+
+    chunkStream.add(const ImageChunkEvent(cumulativeBytesLoaded: 1, expectedTotalBytes: 3));
+
+    final ImageStreamListener listener = ImageStreamListener((ImageInfo info, bool syncCall) {});
+    // Cause the completer to dispose.
+    completer.addListener(listener);
+    completer.removeListener(listener);
+
+    expect(chunkStream.hasListener, false);
+
+    // The above expectation should cover this, but the point of this test is to
+    // make sure the completer does not assert that it's disposed and still
+    // receiving chunk events. Streams from the network can keep sending data
+    // even after evicting an image from the cache, for example.
+    chunkStream.add(const ImageChunkEvent(cumulativeBytesLoaded: 2, expectedTotalBytes: 3));
+  });
 }