为UIImage 添加拓展方法 转自：http://blog.csdn.net/zhonggaorong/article/details/51578655 感谢提供
//根据URL 获取 视频的 第一帧图片
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(UIImage*) thumbnailImageForVideo:(NSURL *)videoURL atTime:(NSTimeInterval)time {
AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:videoURL options:nil];
NSParameterAssert(asset);
AVAssetImageGenerator *assetImageGenerator =[[AVAssetImageGenerator alloc] initWithAsset:asset];
assetImageGenerator.appliesPreferredTrackTransform = YES;
assetImageGenerator.apertureMode = AVAssetImageGeneratorApertureModeEncodedPixels;CGImageRef thumbnailImageRef = NULL;
CFTimeInterval thumbnailImageTime = time;
NSError *thumbnailImageGenerationError = nil;
thumbnailImageRef = [assetImageGenerator copyCGImageAtTime:CMTimeMake(thumbnailImageTime, 60)actualTime:NULL error:&thumbnailImageGenerationError];if(!thumbnailImageRef)
NSLog(@"thumbnailImageGenerationError %@",thumbnailImageGenerationError);UIImage*thumbnailImage = thumbnailImageRef ? [[UIImage alloc]initWithCGImage: thumbnailImageRef] : nil;
return thumbnailImage;
}
