[video_player] Avoid returning no frame on macOS

While the display-link-driven frame updater will only inform the engine
that a frame is available when the underlying player has a frame for the
current time, there's no guarantee that the player will have a frame for
the time that is current when the engine actually requests the frame.
To avoid flickering from returning no frame, if a frame isn't available
for the current time when it is requested, provide the frame that was
known to be available when the request was triggered in the first place.

Fixes flutter/flutter#135999

Author: stuartmorgan-g

packages/video_player/video_player_avfoundation/darwin/Classes/FVPVideoPlayerPlugin.m

@@ -20,6 +20,8 @@ @interface FVPFrameUpdater : NSObject
 @property(nonatomic, weak, readonly) NSObject<FlutterTextureRegistry> *registry;
 // The output that this updater is managing.
 @property(nonatomic, weak) AVPlayerItemVideoOutput *videoOutput;
+// The last time that has been validated as avaliable according to hasNewPixelBufferForItemTime:.
+@property(nonatomic, assign) CMTime lastKnownAvailableTime;
 #if TARGET_OS_IOS
 - (void)onDisplayLink:(CADisplayLink *)link;
 #endif
@@ -30,6 +32,7 @@ - (FVPFrameUpdater *)initWithRegistry:(NSObject<FlutterTextureRegistry> *)regist
   NSAssert(self, @"super init cannot be nil");
   if (self == nil) return nil;
   _registry = registry;
+  _lastKnownAvailableTime = kCMTimeInvalid;
   return self;
 }
 
@@ -45,6 +48,7 @@ - (void)displayLinkFired {
   // Only report a new frame if one is actually available.
   CMTime outputItemTime = [self.videoOutput itemTimeForHostTime:CACurrentMediaTime()];
   if ([self.videoOutput hasNewPixelBufferForItemTime:outputItemTime]) {
+    _lastKnownAvailableTime = outputItemTime;
     [_registry textureFrameAvailable:_textureId];
   }
 }
@@ -91,6 +95,7 @@ @interface FVPVideoPlayer : NSObject <FlutterTexture, FlutterStreamHandler>
 @property(nonatomic, readonly) BOOL isPlaying;
 @property(nonatomic) BOOL isLooping;
 @property(nonatomic, readonly) BOOL isInitialized;
+@property(nonatomic) FVPFrameUpdater *frameUpdater;
 // TODO(stuartmorgan): Extract and abstract the display link to remove all the display-link-related
 // ifdefs from this file.
 #if TARGET_OS_OSX
@@ -298,6 +303,7 @@ - (instancetype)initWithPlayerItem:(AVPlayerItem *)item
   NSAssert(self, @"super init cannot be nil");
 
   _registrar = registrar;
+  _frameUpdater = frameUpdater;
 
   AVAsset *asset = [item asset];
   void (^assetCompletionHandler)(void) = ^{
@@ -562,6 +568,12 @@ - (CVPixelBufferRef)copyPixelBuffer {
   if ([_videoOutput hasNewPixelBufferForItemTime:outputItemTime]) {
     return [_videoOutput copyPixelBufferForItemTime:outputItemTime itemTimeForDisplay:NULL];
   } else {
+    // If the current time isn't available yet, use the time that was checked when informing the
+    // engine that a frame was available (if any).
+    CMTime lastAvailableTime = self.frameUpdater.lastKnownAvailableTime;
+    if (CMTIME_IS_VALID(lastAvailableTime)) {
+      return [_videoOutput copyPixelBufferForItemTime:lastAvailableTime itemTimeForDisplay:NULL];
+    }
     return NULL;
   }
 }

packages/video_player/video_player_avfoundation/example/macos/Runner.xcodeproj/project.pbxproj

@@ -67,7 +67,7 @@
 		331C80D5294CF71000263BE5 /* RunnerTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = RunnerTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		333000ED22D3DE5D00554162 /* Warnings.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; path = Warnings.xcconfig; sourceTree = "<group>"; };
 		335BBD1A22A9A15E00E9071D /* GeneratedPluginRegistrant.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = GeneratedPluginRegistrant.swift; sourceTree = "<group>"; };
-		33683FF02ABCAC94007305E4 /* VideoPlayerTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; name = VideoPlayerTests.m; path = ../../../darwin/RunnerTests/VideoPlayerTests.m; sourceTree = "<group>"; };
+		33683FF02ABCAC94007305E4 /* VideoPlayerTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; name = VideoPlayerTests.m; path = ../../darwin/RunnerTests/VideoPlayerTests.m; sourceTree = SOURCE_ROOT; };
 		33CC10ED2044A3C60003C045 /* video_player_avfoundation_example.app */ = {isa = PBXFileReference; explicitFileType = wrapper.application; includeInIndex = 0; path = video_player_avfoundation_example.app; sourceTree = BUILT_PRODUCTS_DIR; };
 		33CC10F02044A3C60003C045 /* AppDelegate.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AppDelegate.swift; sourceTree = "<group>"; };
 		33CC10F22044A3C60003C045 /* Assets.xcassets */ = {isa = PBXFileReference; lastKnownFileType = folder.assetcatalog; name = Assets.xcassets; path = Runner/Assets.xcassets; sourceTree = "<group>"; };