生成缩略图图像问题，hasNewPixelBufferForItemTime始终返回NO

Question

我正在尝试从mp4文件生成缩略图，这是Xcode项目的一部分。我已成功使用ThumbGenerator类从Mp4生成缩略图，但无法为m3u8文件生成缩略图。所以，我正在尝试一种稍微不同的方法来生成Thumbnail，其中我从AVPlayer获取像素缓冲区输出并将其转换为UIImage。我为示例项目创建了一个GitHub Repo：https://github.com/dep2k/VideoThumbnail.git

在ViewDidLoad中，我调用load方法来创建AVPlayer并加载视频。

-(void)load {

    NSURL * url = [[NSBundle bundleForClass:self.class] URLForResource:@"test" withExtension:@"mp4"];

    self.playerItem = [AVPlayerItem playerItemWithURL:url];
    self.player = [AVPlayer playerWithPlayerItem:self.playerItem];

    AVPlayerLayer *layer = [AVPlayerLayer playerLayerWithPlayer:self.player];
    layer.frame = self.view.frame;
    layer.backgroundColor = [UIColor redColor].CGColor;
    [self.view.layer addSublayer:layer];

    [self.player addObserver:self forKeyPath:@"currentItem.status"                      options:NSKeyValueObservingOptionNew context:kStatusDidChangeKVO];

    [self.player addObserver:self forKeyPath:@"currentItem.loadedTimeRanges"            options:NSKeyValueObservingOptionNew context:kTimeRangesKVO];

}

在玩家观察者方法中，我有生成缩略图的代码：

- (void) observeValueForKeyPath:(NSString*)inKeyPath ofObject:(id)inObject change:(NSDictionary*)inChange context:(void*)inContext
{
    if (inContext == kStatusDidChangeKVO){
        AVPlayerItemStatus status = self.player.currentItem.status;

        if (status == AVPlayerItemStatusReadyToPlay && self.player.status == AVPlayerStatusReadyToPlay) {

            if(!self.isPlaying){
                self.isPlaying = true;
                self.player.rate = 1.0;
                self.player.muted = YES;

                dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 4 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
                    [self generateThumb];
                });
            }

        }
    } else if (inContext == kTimeRangesKVO){
        NSArray *timeRanges = (NSArray *)[inChange objectForKey:NSKeyValueChangeNewKey];
        NSLog(@"Time Ranges");
    }

}

最后，为了生成缩略图，我有以下代码：

-(void)generateThumb {

    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){

        NSDictionary* settings = @{ (id)kCVPixelBufferPixelFormatTypeKey : [NSNumber numberWithInt:kCVPixelFormatType_32BGRA] };
        self.output = [[AVPlayerItemVideoOutput alloc] initWithPixelBufferAttributes:settings];
        [self.player.currentItem addOutput:self.output];

        CMTime vTime = CMTimeMakeWithSeconds(90000, 90000);
        BOOL foundFrame = [self.output hasNewPixelBufferForItemTime:vTime];
        if(foundFrame){

            CIContext *temporaryContext = [CIContext contextWithOptions:nil];
            CVPixelBufferRef pixelBuffer = [self.output copyPixelBufferForItemTime:vTime itemTimeForDisplay:nil];
            CIImage *ciImage = [CIImage imageWithCVPixelBuffer:pixelBuffer];

            CGImageRef videoImage = [temporaryContext
                                     createCGImage:ciImage
                                     fromRect:CGRectMake(0, 0,
                                                         288,
                                                         192)];

            UIImage *image = [UIImage imageWithCGImage:videoImage];
            CGImageRelease(videoImage);
            NSLog(@"FrameFound");
        }else{
            NSLog(@"FrameNotFound");
        }
    });

}

hasNewPixelBufferForItemTime方法始终返回NO，即使播放器成功播放视频也是如此。