第五十七篇、AVAssetReader和AVAssetWrite 对视频进行编码

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:@,
 
        AVVideoHeightKey:@,
 
       AVVideoCompressionPropertiesKey:@{
 
                AVVideoMaxKeyFrameIntervalKey:@,
 
               AVVideoAverageBitRateKey:@,
 
               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 = ; 
 
             __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 == ) {
 
                              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");
 
                                     }
 
                                 }];
 
                   }
 
          } 
 
     }];
 
 }
 
}

-(void)assertReadToAssetInput
 
{
 
        dispatch_queue_t queue = dispatch_queue_create("com.writequeue", DISPATCH_QUEUE_CONCURRENT);
 
      if (_assetInput) {
 
             __block NSInteger count = ; 
 
             __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 == ) {
 
                              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");
 
                                     }
 
                                 }];
 
                   }
 
          } 
 
     }];
 
 }
 
}