http://www.jianshu.com/p/aeb441816a7d

AV Foundation提供了直接處理媒體樣本的低階功能，其中需要使用的兩個重要的類，AVAssetReader和AVAssetWrite，AVAssetReader用於從AVAsset資源讀取媒體樣本，AVAssetWrite用於對媒體資源進行編碼並寫入到容器檔案中。下面簡單的使用一下：

初始化AVAssetReader

-(void)configAssetReader

{

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

       _asset = [AVAsset
 
 assetWithURL:videoUrl];

      //獲取資源的一個視訊軌道

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

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

//指定將讀取的樣本資料壓縮為BGRA格式

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

  //初始化輸出，指定從track軌道中讀取樣本資料
 


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

//新增輸出

   [_assetReader addOutput:_assetOutPut];

  //開始讀取過程

   [_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];

     //指定編碼格式，畫素寬高等資訊

     NSDictionary *setting = @{

          AVVideoCodecKey:AVVideoCodecH264,

        AVVideoWidthKey:@1280,

        AVVideoHeightKey:@720,

       AVVideoCompressionPropertiesKey:@{

                AVVideoMaxKeyFrameIntervalKey:@1,

               AVVideoAverageBitRateKey:@10500000,

               AVVideoProfileLevelKey:AVVideoProfileLevelH264Main31

         }

  };

    初始化寫入器，並制定了媒體格式

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

    //新增寫入器

    [_assetWrite addInput:_assetInput];

    [_assetWrite startWriting];

   }

}

將讀取的資料寫入到_assetInput寫入器中

-(void)assertReadToAssetInput

{

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

      if (_assetInput) {

             __block NSInteger count = 0; 

             __block BOOL isComplete = NO;

            //開啟寫入會話，並指定樣本的開始時間

            [_assetWrite startSessionAtSourceTime:kCMTimeZero];

            [_assetInput requestMediaDataWhenReadyOnQueue:queue usingBlock:^{

           if (!isComplete && _assetInput.readyForMoreMediaData)

           {

                   //樣本資料

                    CMSampleBufferRef buffer = [_assetOutPut copyNextSampleBuffer];

                   if (buffer) {

                            [_assetInput appendSampleBuffer:buffer];

                             count++;

                             // 展示第2000幀資料

                             if (count == 2000) {

                              CGImageRef imgref = [UIImage imageFromSampleBufferRef:buffer];

                             //讀取CMSampleBuffer中的資料，將其轉化為CGImageRef

                            參考程式碼見：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)

                 {

                         //關閉寫入會話 

                         [_assetWrite finishWritingWithCompletionHandler:^{

                                   AVAssetWriterStatus status = self.assetWrite.status;

                                   if (status == AVAssetWriterStatusCompleted) {

                                    NSLog(@"finsished");

                                   }               

                                   else

                                     {

                                            NSLog(@"failure");

                                     }

                                 }];

                   }

          } 

     }];

 }

}

執行結果生成了copy.mp4視訊檔案，點選播放，發現只有視訊沒有音訊資訊，因為我們只有讀取了視訊的樣本資料並寫入，並沒有讀取裡面的音訊資料。所以沒有音訊，AVAsset往往對應的是一個格式容器，裡面包含了很多格式的資料，音訊，視訊，字幕等。