AV Foundation系列（四）AVAssetReader和AVAssetWrite

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

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。

文／jiangamh（简书作者）
原文链接：http://www.jianshu.com/p/aeb441816a7d
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