iOS 对视频抽帧。

这里有两种方法可以采用，

方法一：使用MPMoviePlayerController

MPMoviePlayerController *moviePlayer = [[MPMoviePlayerController alloc]  initWithContentURL:videoURL];

moviePlayer.shouldAutoplay = NO; 
UIImage *thumbnail = [moviePlayer thumbnailImageAtTime:time timeOption:MPMovieTimeOptionNearestKeyFrame];

优点：操作简单，省事

缺点：不够轻量，抓取图片耗时。

方法二：使用AVAssetImageGenerator

+ (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;
}