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

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 shouldContinueValidatingAfterFindingInvalidValueForKey:(NSString *)key
{
    NSLog(@"%s===%@",__func__,key);
    return YES;
}

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

- (BOOL)videoComposition:(AVVideoComposition *)videoComposition shouldContinueValidatingAfterFindingInvalidTimeRangeInInstruction:(id<AVVideoCompositionInstruction>)videoCompositionInstruction
{
    NSLog(@"%s===%@",__func__,videoCompositionInstruction);
    return YES;
}

- (BOOL)videoComposition:(AVVideoComposition *)videoComposition shouldContinueValidatingAfterFindingInvalidTrackIDInInstruction:(id<AVVideoCompositionInstruction>)videoCompositionInstruction layerInstruction:(AVVideoCompositionLayerInstruction *)layerInstruction asset:(AVAsset *)asset
{
    NSLog(@"%s===%@===%@",__func__,layerInstruction,asset);
    return YES;
}

重新运行一下发现第三个方法打印输出，就是instruction的timeRange问题了，又重新看了一下两个instruction的设置问题,感觉又没什么问题，代码如下：

- (void)buildCompositionWithAssets:(NSArray *)assetsArray
{
    for (int i = 0; i < assetsArray.count; i++) {
        AVURLAsset *asset = assetsArray[i];
        NSArray *videoTracks = [asset tracksWithMediaType:AVMediaTypeVideo];
        NSArray *audioTracks = [asset tracksWithMediaType:AVMediaTypeAudio];

        AVAssetTrack *videoTrack = videoTracks[0];
        AVAssetTrack *audioTrack = audioTracks[0];
        NSError *error = nil;
        AVMutableCompositionTrack *videoT = [self.avcompostions.mutableComps addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
        AVMutableCompositionTrack *audioT = [self.avcompostions.mutableComps addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
        [videoT insertTimeRange:videoTrack.timeRange ofTrack:videoTrack atTime:self.offsetTime error:&error];
        [audioT insertTimeRange:audioTrack.timeRange ofTrack:audioTrack atTime:self.offsetTime error:nil];
        NSAssert(!error, @"insert error = %@",error);
        AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
        AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoT];
        instruction.layerInstructions = @[layerInstruction];
        instruction.timeRange = CMTimeRangeMake(self.offsetTime, asset.duration);
        [self.instrucionArray addObject:instruction];
        self.offsetTime = CMTimeAdd(self.offsetTime,asset.duration);
    }
}

这个数组里面会有两个加载好的AVAsset对象，加载AVAsset的代码如下：

- (void)loadAssetFromPath:(NSArray *)paths
{
    NSMutableArray *assetsArray = [NSMutableArray arrayWithCapacity:paths.count];
    dispatch_group_t dispatchGroup = dispatch_group_create();
    for (int i = 0; i < paths.count; i++) {
        NSString *path = paths[i];
        //first find from cache
        AVAsset *asset = [self.assetsCache objectForKey:path];
        if (!asset) {
            NSDictionary *inputOptions = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
            asset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:path] options:inputOptions];
            // cache asset
            NSAssert(asset != nil, @"Can't create asset from path", path);
            NSArray *loadKeys = @[@"tracks", @"duration", @"composable"];
            [self loadAsset:asset withKeys:loadKeys usingDispatchGroup:dispatchGroup];
            [self.assetsCache setObject:asset forKey:path];
        }else {
        }
        [assetsArray addObject:asset];
    }
    dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^{
        !self.assetLoadBlock?:self.assetLoadBlock(assetsArray);

    });
}

- (void)loadAsset:(AVAsset *)asset withKeys:(NSArray *)assetKeysToLoad usingDispatchGroup:(dispatch_group_t)dispatchGroup
{
    dispatch_group_enter(dispatchGroup);
    [asset loadValuesAsynchronouslyForKeys:assetKeysToLoad completionHandler:^(){
        for (NSString *key in assetKeysToLoad) {
            NSError *error;

            if ([asset statusOfValueForKey:key error:&error] == AVKeyValueStatusFailed) {
                NSLog(@"Key value loading failed for key:%@ with error: %@", key, error);
                goto bail;
            }
        }
        if (![asset isComposable]) {
            NSLog(@"Asset is not composable");
            goto bail;
        }
    bail:
        dispatch_group_leave(dispatchGroup);
    }];
}

按照正常来说，应该不会有什么问题的，但是问题还是来了。打印出创建的两个AVMutableVideoCompositionInstruction的信息，发现他们的timeRange确实有重合的地方。AVMutableVideoCompositionInstruction的timeRange必须对应AVMutableCompositionTrack里面的一段段插入的track的timeRange。开发文档是这么说的：

to report a video composition instruction with a timeRange that's invalid, that overlaps with the timeRange of a prior instruction, or that contains times earlier than the timeRange of a prior instruction

我在插入的时候使用的是videoTrack.timeRange，而设置instruction.timeRange = CMTimeRangeMake(self.offsetTime, asset.duration);使用的是asset.duration。这个两个竟然是不一样的。asset.duration是{59885/1000 = 59.885}，videoTrack.timeRange是{{0/1000 = 0.000}, {59867/1000 = 59.867}}

这两个时长有细微差别，到底哪个是比较准确一点的呢？

视频文件时长的计算

笔者在demo中使用的是mp4格式的文件，mp4文件是若干个不同的类型box组成的，box可以理解为装有数据的容器。其中有一种moov类型的box里面装有视频播放的元数据（metadata），这里面有视频的时长信息：timescale和duration。 duration / timescale = 可播放时长（s）。从mvhd中读到timescal为0x03e8，duration为0xe9ed，也就是59885 / 1000，为59.885。再看看tkhd box里面的内容，这里面是包含单一track的信息。如下图：

从上图可以看出videotrack和audiotrack两个的时长是不一样的，videotrack为0xe9db，也就是59.867，audiotrack为0xe9ed，就是59.885。所以我们在计算timeRange的时候最好统一使用相对精确一点的videotrack，而不要AVAsset的duration，尽量避免时间上的误差，视频精细化的编辑，对这些误差敏感。