IOS解码MP4播放

From: http://hawk0620.github.io/blog/2015/11/17/ios-play-video/

从体验说起

  对比微信和Instagram可以发现播放视频的两个思路：微信的处理是把视频加载好后播放，这样确保了视频是完整的，用户很直观视频是否下载完成，不影响用户观看视频的体验；而Instagram的做法是边加载边播，当网络不给力的时候，视频就卡在那里，给用户增加了观看视频的焦虑，并且用户还得自己判断下视频是不是加载完成了，最不幸的是，当视频的网络请求不可达时，不能给出加载失败的提示引导用户重新加载，只能滑动列表触发刷新。

播放视频的实现

1、通过实践，我发现Instagram采用的应该是AVPlayer实现的。

AVPlayerItem可以通过远程URL创建出来，并且支持流的形式播放，还可以添加视频播放卡住和视频播放完成的两个观察者：

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(itemDidBufferPlaying:) name:AVPlayerItemPlaybackStalledNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(itemDidFinishPlaying:) name:AVPlayerItemDidPlayToEndTimeNotification object:nil];

但遗憾的是，我没有找到视频加载失败的观察者。

2、结合微信团队的技术分享[链接][1]，得知微信的视频播放是采用AVAssetReader+AVAssetReaderTrackOutput，根据微信的思路，自己也尝试实现了一番： buffer的转换：

[1]: http://mp.weixin.qq.com/s?__biz=MzAwNDY1ODY2OQ==&mid=207686973&idx=1&sn=1883a6c9fa0462dd5596b8890b6fccf6&scene=0#wechat_redirect

- (CGImageRef) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer {
       CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
       // Lock the base address of the pixel buffer
       CVPixelBufferLockBaseAddress(imageBuffer, 0);
       // Get the number of bytes per row for the pixel buffer
       size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
       // Get the pixel buffer width and height
       size_t width = CVPixelBufferGetWidth(imageBuffer);
       size_t height = CVPixelBufferGetHeight(imageBuffer);
       //Generate image to edit
       unsigned char* pixel = (unsigned char *)CVPixelBufferGetBaseAddress(imageBuffer);
       CGColorSpaceRef colorSpace=CGColorSpaceCreateDeviceRGB();
       CGContextRef context=CGBitmapContextCreate(pixel, width, height, 8, bytesPerRow, colorSpace, kCGBitmapByteOrder32Little|kCGImageAlphaPremultipliedFirst);
       CGImageRef image = CGBitmapContextCreateImage(context);
       CGContextRelease(context);
       CGColorSpaceRelease(colorSpace);
       return image;
    }

视频的解码：

AVURLAsset *asset = [AVURLAsset URLAssetWithURL:[[NSURL alloc] initFileURLWithPath:path] options:nil];
NSError *error;
AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
NSArray* videoTracks = [asset tracksWithMediaType:AVMediaTypeVideo];
AVAssetTrack* videoTrack = [videoTracks objectAtIndex:0];

int m_pixelFormatType = kCVPixelFormatType_32BGRA;
NSDictionary* options = [NSDictionary dictionaryWithObject:[NSNumber numberWithInt: (int)m_pixelFormatType] forKey:(id)kCVPixelBufferPixelFormatTypeKey];
AVAssetReaderTrackOutput* videoReaderOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:videoTrack outputSettings:options];
[reader addOutput:videoReaderOutput];
[reader startReading];

// 读取视频每一个buffer转换成CGImageRef
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   CMSampleBufferRef audioSampleBuffer = NULL;
   while ([reader status] == AVAssetReaderStatusReading && videoTrack.nominalFrameRate > 0) {
   CMSampleBufferRef sampleBuffer = [videoReaderOutput copyNextSampleBuffer];
   CGImageRef image = [self imageFromSampleBuffer:sampleBuffer];
   if (self.delegate && [self.delegate respondsToSelector:@selector(mMovieDecoder:onNewVideoFrameReady:)]) {
        [self.delegate mMovieDecoder:self onNewVideoFrameReady:image];
    }
   if(sampleBuffer) {
       if(audioSampleBuffer) { // release old buffer.
            CFRelease(audioSampleBuffer);
            audioSampleBuffer = nil;
       }
       audioSampleBuffer = sampleBuffer;
   } else {
       break;
   }

// 休眠的间隙刚好是每一帧的间隔
   [NSThread sleepForTimeInterval:CMTimeGetSeconds(videoTrack.minFrameDuration)];
 }
 // decode finish
 float durationInSeconds = CMTimeGetSeconds(asset.duration);
  if (self.delegate && [self.delegate respondsToSelector:@selector(mMovieDecoderOnDecodeFinished:duration:)]) {
     [self.delegate mMovieDecoderOnDecodeFinished:self duration:durationInSeconds];
   }
});

处理每一帧CGImageRef的回调：

- (void)mMovieDecoder:(VideoDecoder *)decoder onNewVideoFrameReady:(CGImageRef)imgRef {
    __weak PlayBackView *weakView = self;
    dispatch_async(dispatch_get_main_queue(), ^{
        weakView.layer.contents = (__bridge id _Nullable)(imgRef);
    });
}

处理视频解码完成的回调：

images即每一帧传上来的CGImageRef的数组
- (void)mMovieDecoderOnDecodeFinished:(VideoDecoder *)decoder images:(NSArray *)imgs duration:(float)duration {
    __weak PlayBackView *weakView = self;
    dispatch_async(dispatch_get_main_queue(), ^{
        weakView.layer.contents = nil;

        CAKeyframeAnimation *animation = [CAKeyframeAnimation animationWithKeyPath: @"contents"];
        animation.calculationMode = kCAAnimationDiscrete;
        animation.duration = duration;
        animation.repeatCount = HUGE; //循环播放
        animation.values = images; // NSArray of CGImageRefs
        [weakView.layer addAnimation:animation forKey: @"contents"];
    });
}

//写在最后: 看到多种这样的方式,基本出处差不多, 然后发现,10秒的视频还行 超过的话基本上都会爆内存 , 30fps 算的话 10秒300张图片, 我现在的处理方式是间隔取图 ,想当于减少fps