Objective c 是否可以从视频URL下载前几帧，然后使用它创建缩略图？

我目前正在使用下面的代码从我的视频中生成一个缩略图，以填充tableview中的cell.image。然而，有些视频太长，等待下载整个视频对用户来说太不友好了。是否可以只下载视频的前1秒，并且仍然能够使用以下代码。不下载整个视频会破坏url吗

 -(void)addThumbNailImagesToMutableArray :(NSURL *)url
{
    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL: url options:nil];
    AVAssetImageGenerator *generator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
    NSError *err = NULL;
    CMTime requestedTime = CMTimeMake(1,30);     // To create thumbnail image
    CGImageRef imgRef = [generator copyCGImageAtTime:requestedTime actualTime:NULL error:&err];
       NSLog(@"err = %@, imageRef = %@", err, imgRef);

    if (imgRef != NULL) {
        _thumbnailImage = [[UIImage alloc] initWithCGImage:imgRef];
        CGImageRelease(imgRef);    // MUST release explicitly to avoid memory leak
        _thumbNailImageData = UIImagePNGRepresentation(_thumbnailImage);
        [_thumbNailArray addObject:_thumbNailImageData];

    } else {

        UIImage *img = [UIImage imageNamed:@"smallPlaceHolder.jpg"];
        _thumbNailImageData = UIImagePNGRepresentation(img);
        [_thumbNailArray addObject:_thumbNailImageData];

    }
}