一次“Error Domain=AVFoundationErrorDomain Code=-11841”的调试

起因

最近在重构视频输出模块的时候，调试碰到AVAssetReader 调用开始方法总是返回NO而失败，代码如下：

if ([reader startReading] == NO) 
    {
        NSLog(@"Error reading from file at URL: %@", self.url);
        return;
    }

reader的创建代码如下，主要用的是GPUImageMovieCompostion.

- (AVAssetReader*)createAssetReader
{
    NSError *error = nil;
    AVAssetReader *assetReader = [AVAssetReader assetReaderWithAsset:self.compositon error:&error];
    
    NSDictionary *outputSettings = @{(id)kCVPixelBufferPixelFormatTypeKey: @(kCVPixelFormatType_420YpCbCr8BiPlanarFullRange)};
    AVAssetReaderVideoCompositionOutput *readerVideoOutput = [AVAssetReaderVideoCompositionOutput assetReaderVideoCompositionOutputWithVideoTracks:[_compositon tracksWithMediaType:AVMediaTypeVideo]
                                                                                                                                     videoSettings:outputSettings];
    readerVideoOutput.videoComposition = self.videoComposition;
    readerVideoOutput.alwaysCopiesSampleData = NO;
    if ([assetReader canAddOutput:readerVideoOutput]) {
        [assetReader addOutput:readerVideoOutput];
    }
    
    NSArray *audioTracks = [_compositon tracksWithMediaType:AVMediaTypeAudio];
    BOOL shouldRecordAudioTrack = (([audioTracks count] > 0) && (self.audioEncodingTarget != nil) );
    AVAssetReaderAudioMixOutput *readerAudioOutput = nil;
    
    if (shouldRecordAudioTrack)
    {
        [self.audioEncodingTarget setShouldInvalidateAudioSampleWhenDone:YES];
        NSDictionary *audioReaderSetting = @{AVFormatIDKey: @(kAudioFormatLinearPCM),
                                             AVLinearPCMIsBigEndianKey: @(NO),
                                             AVLinearPCMIsFloatKey: @(NO),
                                             AVLinearPCMBitDepthKey: @(16)};
        readerAudioOutput = [AVAssetReaderAudioMixOutput assetReaderAudioMixOutputWithAudioTracks:audioTracks audioSettings:audioReaderSetting];
        readerAudioOutput.audioMix = self.audioMix;
        readerAudioOutput.alwaysCopiesSampleData = NO;
        [assetReader addOutput:readerAudioOutput];
    }
    
    return assetReader;
}

然后在该处断点，查看reader的status和error，显示Error Domain=AVFoundationErrorDomain Code=-11841。查了一下文档发现如下：

AVErrorInvalidVideoComposition = -11841

You attempted to perform a video composition operation that is not supported.

应该就是readerVideoOutput.videoComposition = self.videoComposition; 这个videoCompostion有问题了。AVMutableVideoComposition这个类在视频编辑里面非常重要，包含对AVComposition中的各个视频track如何融合的信息，我做的是两个视频的融合小demo，就需要用到它，设置这个类稍微有点复杂，比较容易出错，特别是设置AVMutableVideoCompositionInstruction这个类的timeRange属性，我的这次错误就是因为这个。

开始调试

马上调用AVMutableVideoComposition的如下方法：

BOOL isValid = [self.videoComposition isValidForAsset:self.compositon timeRange:CMTimeRangeMake(kCMTimeZero, self.compositon.duration) validationDelegate:self];

这个时候isValid为No，确定就是这个videoCompostion问题了，添加了AVVideoCompositionValidationHandling协议四个方法打印一下，这个协议太贴心了，估计知道这个地方出错概率比较高，所以特地弄的吧。代码如下：

- (BOOL)videoComposition:(AVVideoComposition *)videoComposition shouldContinueva lidatingAfterFindingInvalidValueForKey:(NSString *)key 
{
    NSLog(@"%s===%@",__func__,key);
    return YES;
}

- (BOOL)videoComposition:(AVVideoComposition *)videoComposition shouldContinueva lidatingAfterFindingEmptyTimeRange:(CMTimeRange)timeRange 
{
    NSLog(@"%s===%@",__func__,CFBridgingRelease(CMTimeRangeCopyDescription(kCFAllocatorDefault, timeRange)));
    return YES;
}

- (BOOL)videoComposition:(AVVi