从线程中的视频网址中提取缩略图

Question

我在另一个线程中的 MPMoviePlayer 出现问题，使用 (Grand Central Dispatch)。我想通过 url 吸引视频，剪切第 5 帧并返回图片。在模拟器中工作正常，但在真实设备上 - 下降。这里有什么问题？也许它只适用于主线程？

NSURL* url = [NSURL URLWithString:filePath];

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^(void)
{
  MPMoviePlayerController* moviePlayer = [[MPMoviePlayerController alloc] initWithContentURL:url];
  moviePlayer.shouldAutoplay = NO;
  UIImage* thumbnail = [moviePlayer thumbnailImageAtTime:5 timeOption:MPMovieTimeOptionNearestKeyFrame];

  dispatch_async(dispatch_get_main_queue(), ^(void)
  {
     [[Singleton sharedSingleton].imageCache setValue:thumbnail forKey:filePath];
     [cell.presentationViewOutlet setImage:thumbnail];;
  });
});

现在，我尝试使用另一种方法，但它也在模拟器上运行良好，并且在真实设备上不显示。有时返回imageRef = (null)，有时返回imageRef = <CGImage 0x1766dd20>。

编辑：

NSURL* url = [NSURL URLWithString:filePath];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^(void)
{
    AVAsset *asset = [AVAsset assetWithURL:videoURL];
    AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc]initWithAsset:asset];
    imageGenerator.appliesPreferredTrackTransform = TRUE;
    CMTime time = CMTimeMakeWithSeconds(1, 1);
    NSError *err = NULL;
    CGImageRef imageRef = [imageGenerator copyCGImageAtTime:time actualTime:NULL error:&err];
    dispatch_async(dispatch_get_main_queue(), ^(void)
    {
        NSLog(@"imageRef = %@", imageRef);
        UIImage* image = [UIImage imageWithCGImage:imageRef];
        if (image != nil)
        {
            NSLog(@"image != nil");
            [view setImage:image];
            NSString* videoURLString = [videoURL absoluteString];
            [[Singleton sharedSingleton].imageCache setValue:image forKey:videoURLString];
            CGImageRelease(imageRef);
        }
        else
        {
            NSLog(@"err = %@", err);
        }
    });
});

错误：

如果 imageRef = (null) 显示 err = Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo=0x156c1a50 {NSLocalizedDescription=The operation could not be completed, NSUnderlyingError=0x155b29a0 "The operation couldn’t be completed. (OSStatus error -12792.)", NSLocalizedFailureReason=An unknown error occurred (-12792)}

Answer 1

MPMoviePlayerController 是一个用户界面控制器。因此，几乎可以肯定它不是线程安全的。文档没有具体说明，但我敢打赌它不是。

Answer 2

如果将 imageref 移到 async 部分内会更好吗？喜欢：

NSURL* url = [NSURL URLWithString:filePath];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^(void)
{
    AVAsset *asset = [AVAsset assetWithURL:videoURL];
    AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc]initWithAsset:asset];
    imageGenerator.appliesPreferredTrackTransform = TRUE;
    CMTime time = CMTimeMakeWithSeconds(1, 1);
    dispatch_async(dispatch_get_main_queue(), ^(void)
    {
        NSError *err = NULL;
        CGImageRef imageRef = [imageGenerator copyCGImageAtTime:time actualTime:NULL error:&err];
        NSLog(@"imageRef = %@", imageRef);
        UIImage* image = [UIImage imageWithCGImage:imageRef];
        if (image != nil)
        {
            NSLog(@"image != nil");
            [view setImage:image];
            NSString* videoURLString = [videoURL absoluteString];
            [[Singleton sharedSingleton].imageCache setValue:image forKey:videoURLString];
            CGImageRelease(imageRef);
        }
        else
        {
            NSLog(@"err = %@", err);
        }
    });
});