我只是使用AVFoundation库的AVPlayer*myPlayer创建了自己的自定义视频播放器。现在我想通过在UIButton上运行播放视频来捕捉视频书签的图像，点击action。单击UIButton，它会拍摄播放器屏幕的快照并存储在文档目录中。请建议我怎么做？提前谢谢。
AVAsset *asset = [AVAsset assetWithURL:sourceURL];
AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc]initWithAsset:asset];
CMTime time = CMTimeMake(1, 1);
CGImageRef imageRef = [imageGenerator copyCGImageAtTime:time actualTime:NULL error:NULL];
UIImage *thumbnail = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef); // CGImageRef won't be released by ARC
参考链接：
在当前时间创建AVPlayer的缩略图或图像
AVAsset *asset = [AVAsset assetWithURL:videoURL];
AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc]initWithAsset:asset];
imageGenerator.appliesPreferredTrackTransform=TRUE;
CGImageRef imageRef = [imageGenerator copyCGImageAtTime:player.currentTime
actualTime:NULL error:NULL];