根据 URL 获取视频的第一帧图片

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

• (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;
  }