AVPlayer 播放 m3u8 流时如何捕获图像？

I use AVPlayer玩一个m3u8文件，我想在这些代码中捕获图像：

AVAssetImageGenerator *gen = [[AVAssetImageGenerator alloc] initWithAsset:self.player.currentItem.asset];
gen.appliesPreferredTrackTransform = YES;
NSError *error = nil;
CMTime actualTime;
CMTime now = self.player.currentTime;
[gen setRequestedTimeToleranceAfter:kCMTimeZero];
[gen setRequestedTimeToleranceBefore:kCMTimeZero];
CGImageRef image = [gen copyCGImageAtTime:now actualTime:&actualTime error:&error];
UIImage *thumb = [[UIImage alloc] initWithCGImage:image];
NSLog(@"%f , %f",CMTimeGetSeconds(now),CMTimeGetSeconds(actualTime));

NSLog(@"%@",error);
if (image) {
    CFRelease(image);
}

但它不起作用。错误是：

Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo=0x7fadf25f59f0 {NSUnderlyingError=0x7fadf25f1670 "The operation couldn’t be completed. (OSStatus error -12782.)", NSLocalizedFailureReason=An unknown error occurred (-12782), NSLocalizedDescription=The operation could not be completed}

我该如何解决？
多谢。

AVAssetImageGenerator可能需要本地资产。也许你会更幸运添加一个AVPlayerItemVideoOutput给你的AVPlayer，寻找所需地点并呼叫copyPixelBufferForItemTime:itemTimeForDisplay:在视频输出上。