在AVPlayer中截取一帧HLS(m3u8)格式图片在AVPlayer中截取一帧HLS(m3u8)格式

AVPlayer的API真心不友好，一个截屏问题搞得我花了6个小时才彻底解决。在网上搜索会找到两种截屏方案: 使用AVPlayerItemVideoOutput
和AVAssetImageGenerator。
其中AVAssetImageGenerator用于处理非HLS(切片视频流)的截屏，比如MPEG-4格式。但一开始我因为没有限制精度导致截出来的帧总是慢了1-2秒:

AVURLAsset *asset = self.playerItem.asset; self.imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:asset]; self.imageGenerator.appliesPreferredTrackTransform = YES; // 注意：设置为高精度截屏，这和seekToTime的精度其实是一样的 self.imageGenerator.requestedTimeToleranceBefore = kCMTimeZero; self.imageGenerator.requestedTimeToleranceAfter = kCMTimeZero; // 截屏操作 int attemptNumber = 0; BOOL success = NO; // 截屏时间点可能不准，会反复重试 while (attemptNumber < 3 && !success) { CMTime actualTime; NSError *error; videoImage = [self.imageGenerator copyCGImageAtTime:time actualTime:&actualTime error:&error]; captureImage = [UIImage imageWithCGImage:videoImage]; CGImageRelease(videoImage); if (error) { return nil; } float actual =CMTimeGetSeconds(actualTime); if (isFloatEqual(timestamp, actual)) { success = YES; } else { attemptNumber++; } }// 你也可以使用generateCGImagesAsynchronouslyForTimes方法，异步获取多个时间点的帧图片

而AVPlayerItemVideoOutput则用于处理HLS(m3u8)的截屏，我一开始是先把视频暂停了再截屏没问题，但是在播放中时截屏就出现了这篇文章中出现的buffer返回null的问题
。然后各种查询之后发现这个回答中说：

1. Create player & player item, dispatch queue and display link 2. Register observer for AVPlayerItem status key 3. On status AVPlayerStatusReadyToPlay, create AVPlayerItemVideoOutput and start display link

然后我也将AVPlayerItemVideoOutput的构造和注册放到了AVPlayerStatusReadyToPlay之后发现还是不行，最后在这篇早年的objcio的文章中演示了对CADisplayLink的使用，才理解了前面那句create AVPlayerItemVideoOutput and start display link的含义

// 在AVPlayerItemStatusReadyToPlay时创建实例 if (self.config.isVideoHLSFormat && self.videoOutput == nil && self.playerItem.status == AVPlayerItemStatusReadyToPlay) { NSDictionary *settings = @{(id)kCVPixelBufferPixelFormatTypeKey: @(kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange) }; self.videoOutput = [[AVPlayerItemVideoOutput alloc] initWithPixelBufferAttributes:settings]; [self.playerItem addOutput:self.videoOutput]; self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(displayLinkCallback:)]; [self.displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes]; }// 在runloop的间隙获取当前videoOutput的buffer - (void)displayLinkCallback:(CADisplayLink *)sender { CMTime time = [self.videoOutput itemTimeForHostTime:CACurrentMediaTime()]; if ([self.videoOutput hasNewPixelBufferForItemTime:time]) { self.lastSnapshotPixelBuffer = [self.videoOutput copyPixelBufferForItemTime:time itemTimeForDisplay:NULL]; } }// 截屏处理 CVPixelBufferRef buffer = [self.videoOutput copyPixelBufferForItemTime:time itemTimeForDisplay:nil]; // 再取一次buffer是为了防止在暂停时截屏不准确 if (buffer == NULL && self.lastSnapshotPixelBuffer) { buffer = self.lastSnapshotPixelBuffer; } if (buffer) { //生成CIImage CIImage *ciImage = [CIImage imageWithCVPixelBuffer:buffer]; // 转化为CGImage CIContext *context = [CIContext contextWithOptions:nil]; size_t width = CVPixelBufferGetWidth(buffer); size_t height = CVPixelBufferGetHeight(buffer); videoImage = [context createCGImage:ciImage fromRect:CGRectMake(0, 0, width, height)]; // 生成UIImage captureImage = [UIImage imageWithCGImage:videoImage]; CGImageRelease(videoImage); }

我对截屏的图片处理比较简单就直接用UIImageJPEGRepresentation(image, 0.5)压缩了，也可以更复杂的处理。最后在重建playerItem时别忘了清理资源:

[self.playerItem removeOutput:self.videoOutput]; self.videoOutput = nil; [self.displayLink invalidate]; self.displayLink = nil; self.lastSnapshotPixelBuffer = NULL;

【在AVPlayer中截取一帧HLS(m3u8)格式图片】最后还有一个问题没有答案，如果要实现HLS视频的缓冲进度的图片预览不知道用AVPlayer的原生API怎么做。