ios - iOS7 中的 AVAssetImageGenerator 随机失败

Question

我有一个 AVComposition 和一个 AVVideoComposition，我会为视频的每半秒左右生成一个缩略图。我使用动画工具进行一些叠加。我对操作队列执行调用，它在某些类型的编辑后触发。操作队列是串行的而不是异步的，所以进程永远不会并发运行，我已经验证了这一点。下面的代码在串行队列中的另一个 NSOperation 的主线程上执行。

现在我的问题是，拇指随机失败，没有模式。我可以运行一次并让它全部成功。我可以使用完全相同的参数再次运行它，并且有一些失败。我可以再次运行它并让不同的拇指失败。

这似乎与其他线程中正在进行的其他事件的数量有关，例如从 ALAssetLibrary 或 MediaFramework 中查询大量项目。

我还要提到这在 iOS6 中没有这个问题。

在生成缩略图之前，我确实创建了 AVComposition 和 AVVideoComposition 的副本。

有人给我指点吗？

我很感激。

//this must be called on the main thread to correctly generate overlays
[[NSOperationQueue mainQueue] addOperationWithBlock:^{

    AVAsset *asset = avComposition;

    AVAssetImageGenerator *generator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
    generator.appliesPreferredTrackTransform = TRUE;

    AVMutableVideoComposition *mutableVideoComposition = (AVMutableVideoComposition *) avVideoComposition.mutableCopy;
    CALayer *baseSyncLayer = [CALayer layer];
    baseSyncLayer.frame = CGRectMake(0, 0, resolution.width, resolution.height);
    CALayer *videoLayer = [CALayer layer];
    videoLayer.frame = CGRectMake(0, 0, resolution.width, resolution.height);
    [baseSyncLayer addSublayer:videoLayer];
    __unused NSArray *imageLayers = [MovieCompositionService applyImageAnimations:avComposition andLayer:baseSyncLayer andProject:project fileRender:YES addTextOverlays:addTextOverlays andResolution:resolution];
    mutableVideoComposition.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:baseSyncLayer];
    generator.videoComposition = mutableVideoComposition;

    NSMutableArray *images = [[NSMutableArray alloc] init];

    __block BOOL hasErrors = NO;

    AVAssetImageGeneratorCompletionHandler handler = ^(CMTime requestedTime, CGImageRef im, CMTime actualTime, AVAssetImageGeneratorResult result, NSError *error) {
        if (!hasErrors) {

            DDLogVerbose(@"ACTUAL GENERATION TIME %f", CMTimeGetSeconds(actualTime));

            if (result != AVAssetImageGeneratorSucceeded) {
                DDLogError(@"Failed to generate thumb: %@", error.localizedDescription);
                hasErrors = YES;
                [generator cancelAllCGImageGeneration];
                finishBlock(images, YES);
            } else {
                [images addObject:[UIImage imageWithCGImage:im]];

                if (CMTIME_COMPARE_INLINE(requestedTime, ==, ((NSValue *) timeIntervals.lastObject).CMTimeValue)) {
                    DDLogVerbose(@"Movie composition thumb generation complete");
                    finishBlock(images, NO);
                }
            }
        }
    };

    generator.maximumSize = thumbSize;
    [generator generateCGImagesAsynchronouslyForTimes:timeIntervals completionHandler:handler];
}];

Answer 1

我使用以下内容生成缩略图。

- (void)getThumbnailImageForTime:(float)instance_time withCompletion:(void(^)(UIImage *))completion
{
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0ul);
    dispatch_async(queue, ^{
        __block UIImage *thumb ;
        AVPlayerItem *playerItem = [AVPlayerItem playerItemWithURL:self.videoUrl];
        AVAssetImageGenerator *_generator;
        _generator = [AVAssetImageGenerator assetImageGeneratorWithAsset:playerItem.asset];

        AVAssetImageGeneratorCompletionHandler handler = ^(CMTime requestedTime, CGImageRef image, CMTime actualTime, AVAssetImageGeneratorResult result, NSError *error) {
            if (result == AVAssetImageGeneratorSucceeded) {
                thumb = [UIImage imageWithCGImage:image];
                NSLog(@"Succesfully generater the thumbnail!!!");
            } else {
                NSLog(@"Failed to generater the thumbnail!!!");
                NSLog(@"Error : %@",error.localizedDescription);
                [self performSelector:@selector(retryingThumbnialGeneration) withObject:nil afterDelay:1.0f];
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                 completion(thumb);
            });
        };

        [_generator generateCGImagesAsynchronouslyForTimes:[NSArray arrayWithObject:[NSValue valueWithCMTime:CMTimeMakeWithSeconds(2,10)]] completionHandler:handler];

    });

}