AV Foundation系列（四）AVAssetReader和

AV Foundation提供了直接处理媒体样本的低级功能，其中需要使用的两个重要的类，AVAssetReader和AVAssetWrite，AVAssetReader用于从AVAsset资源读取媒体样本，AVAssetWrite用于对媒体资源进行编码并写入到容器文件中。下面简单的使用一下：

初始化AVAssetReader

-(void)configAssetReader

{

       NSURL *videoUrl = [NSURL fileURLWithPath:[self resoucePath]];

       _asset = [AVAsset assetWithURL:videoUrl];

      //获取资源的一个视频轨道

      AVAssetTrack *track = [[_asset tracksWithMediaType:AVMediaTypeVideo] firstObject];

   _assetReader = [[AVAssetReader alloc] initWithAsset:_asset error:nil];

//指定将读取的样本数据压缩为BGRA格式

    NSDictionary *setting =   @{(id)kCVPixelBufferPixelFormatTypeKey:@(kCVPixelFormatType_32BGRA)};

  //初始化输出，指定从track轨道中读取样本数据

   _assetOutPut = [[AVAssetReaderTrackOutput alloc] initWithTrack:track outputSettings:setting];

//添加输出

   [_assetReader addOutput:_assetOutPut];

  //开始读取过程

   [_assetReader startReading];

}

初始化AVAssetWrite

-(void)configWriteInput

{

     NSString *storePath = nil;

     NSString *path  = [self resoucePath];

    NSRange range = [path rangeOfString:@"/" options:NSBackwardsSearch];

   if (range.location != NSNotFound) {

          NSString *pathRoot = [path substringToIndex:range.location];

         storePath = [pathRoot stringByAppendingPathComponent:@"copy.mp4"];

   }

if (storePath) {

       _assetWrite = [[AVAssetWriter alloc] initWithURL:[NSURL           fileURLWithPath:storePath] fileType:AVFileTypeQuickTimeMovie error:nil];

     //指定编码格式，像素宽高等信息

     NSDictionary *setting = @{

          AVVideoCodecKey:AVVideoCodecH264,

        AVVideoWidthKey:@1280,

        AVVideoHeightKey:@720,

       AVVideoCompressionPropertiesKey:@{

                AVVideoMaxKeyFrameIntervalKey:@1,

               AVVideoAverageBitRateKey:@10500000,

               AVVideoProfileLevelKey:AVVideoProfileLevelH264Main31

         }

  };

    初始化写入器，并制定了媒体格式

    _assetInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:setting];

    //添加写入器

    [_assetWrite addInput:_assetInput];

    [_assetWrite startWriting];

   }

}

将读取的数据写入到_assetInput写入器中

-(void)assertReadToAssetInput

{

        dispatch_queue_t queue = dispatch_queue_create("com.writequeue", DISPATCH_QUEUE_CONCURRENT);

      if (_assetInput) {

             __block NSInteger count = 0; 

             __block BOOL isComplete = NO;

            //开启写入会话，并指定样本的开始时间

            [_assetWrite startSessionAtSourceTime:kCMTimeZero];

            [_assetInput requestMediaDataWhenReadyOnQueue:queue usingBlock:^{

           if (!isComplete && _assetInput.readyForMoreMediaData)

           {

                   //样本数据

                    CMSampleBufferRef buffer = [_assetOutPut copyNextSampleBuffer];

                   if (buffer) {

                            [_assetInput appendSampleBuffer:buffer];

                             count++;

                             // 展示第2000帧数据

                             if (count == 2000) {

                              CGImageRef imgref = [UIImage imageFromSampleBufferRef:buffer];

                             //读取CMSampleBuffer中的数据，将其转化为CGImageRef

                            参考代码见：http://www.jianshu.com/p/3d5ccbde0de1

                             UIImage *img = [UIImage imageWithCGImage:imgref];

                             dispatch_sync(dispatch_get_main_queue(), ^{

                             _imageView.image = img;

                                  });

                            }

                     }

                    else

                   {

                          isComplete = YES;

                   }

                 if(isComplete)

                 {

                         //关闭写入会话 

                         [_assetWrite finishWritingWithCompletionHandler:^{

                                   AVAssetWriterStatus status = self.assetWrite.status;

                                   if (status == AVAssetWriterStatusCompleted) {

                                    NSLog(@"finsished");

                                   }               

                                   else

                                     {

                                            NSLog(@"failure");

                                     }

                                 }];

                   }

          } 

     }];

 }

}

运行结果生成了copy.mp4视频文件，点击播放，发现只有视频没有音频信息，因为我们只有读取了视频的样本数据并写入，并没有读取里面的音频数据。所以没有音频，AVAsset往往对应的是一个格式容器，里面包含了很多格式的数据，音频，视频，字幕等。

这是第一次使用markdown格式编辑，之前一直不回使用，后来百度才知道，需要在设置中指定markdwon编辑，而非富文本编辑。