ios获取视频略缩图

NSURL *url = [info objectForKey:UIImagePickerControllerMediaURL];
_videoData = [NSData dataWithContentsOfURL:url];

AVAsset *asset = [AVAsset assetWithURL:url];
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];
[_videoChooseButton setBackgroundImage:thumbnail forState:UIControlStateNormal];