编写多个视频会导致挂起

Posted 2023-03-08

【中文标题】编写多个视频会导致挂起

【英文标题】：Composing multiple videos causes hang

【发布时间】：2015-08-05 18:20:13

【问题描述】：

我正在开发一个应用程序，该应用程序可以组合用户拍摄的多个视频剪辑。剪辑被记录在相机上，并与另一个视频叠加，然后将录制的剪辑组合成一个长剪辑。每个剪辑的长度由叠加的视频文件决定。

我正在使用AVAssetExportSession 和exportAsynchronouslyWithCompletionHandler。奇怪的是，这适用于某些剪辑，而不适用于其他剪辑。真正的问题是导出器没有报告任何错误或失败，只是零进度并且从不调用完成处理程序。

我什至不知道从哪里开始寻找问题所在。这是我用来组合剪辑的函数

- (void) setupAndStitchVideos:(NSMutableArray*)videoData

    // Filepath to where the final generated video is stored
    NSURL                       *   exportUrl           = nil;
    // Contains information about a single asset/track
    NSDictionary                *   assetOptions        = nil;
    AVURLAsset                  *   currVideoAsset      = nil;
    AVURLAsset                  *   currAudioAsset      = nil;
    AVAssetTrack                *   currVideoTrack      = nil;
    AVAssetTrack                *   currAudioTrack      = nil;
    // Contains all tracks and time ranges used to build the final composition
    NSMutableArray              *   allVideoTracks      = nil;
    NSMutableArray              *   allVideoRanges      = nil;
    NSMutableArray              *   allAudioTracks      = nil;
    NSMutableArray              *   allAudioRanges      = nil;

    AVMutableCompositionTrack   *   videoTracks         = nil;
    AVMutableCompositionTrack   *   audioTracks         = nil;
    // Misc time values used when calculating a clips start time and total length
    float                           animationLength     = 0.0f;
    float                           clipLength          = 0.0f;
    float                           startTime           = 0.0f;
    CMTime                          clipStart           = kCMTimeZero;
    CMTime                          clipDuration        = kCMTimeZero;
    CMTimeRange                     currRange           = kCMTimeRangeZero;
    // The final composition to be generated and exported
    AVMutableComposition        *   finalComposition    = nil;

    // Cancel any already active exports
    if (m_activeExport)
    
        [m_activeExport cancelExport];
        m_activeExport = nil;
    

    // Initialize and setup all composition related member variables
    allVideoTracks      = [[NSMutableArray alloc] init];
    allAudioTracks      = [[NSMutableArray alloc] init];
    allVideoRanges      = [[NSMutableArray alloc] init];
    allAudioRanges      = [[NSMutableArray alloc] init];
    exportUrl           = [NSURL fileURLWithPath:[MobveoAnimation getMergeDestination]];
    finalComposition    = [AVMutableComposition composition];
    videoTracks         = [finalComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    audioTracks         = [finalComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    assetOptions        = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
    animationLength     = m_animation.videoDuration;

    // Define all of the audio and video tracks that will be used in the composition
    for (NSDictionary * currData in videoData)
    
        currVideoAsset  = [AVURLAsset URLAssetWithURL:[currData objectForKey:KEY_STITCH_VIDEO_URL] options:assetOptions];
        currAudioAsset  = [AVURLAsset URLAssetWithURL:[currData objectForKey:KEY_STITCH_AUDIO_URL] options:assetOptions];
        currVideoTrack  = [[currVideoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

        NSArray *audioTracks = [currAudioAsset tracksWithMediaType:AVMediaTypeAudio];
        if ( audioTracks != nil && audioTracks.count > 0 )
        
            currAudioTrack  = audioTracks[0];
        
        else
        
            currAudioTrack = nil;
        

        clipLength      = animationLength * [(NSNumber*)[currData objectForKey:KEY_STITCH_LENGTH_PERCENTAGE] floatValue];
        clipStart       = CMTimeMakeWithSeconds(startTime, currVideoAsset.duration.timescale);
        clipDuration    = CMTimeMakeWithSeconds(clipLength, currVideoAsset.duration.timescale);

        NSLog(@"Clip length: %.2f", clipLength);
        NSLog(@"Clip Start: %lld", clipStart.value );
        NSLog(@"Clip duration: %lld", clipDuration.value);

        currRange       = CMTimeRangeMake(clipStart, clipDuration);
        [allVideoTracks addObject:currVideoTrack];

        if ( currAudioTrack != nil )
        
            [allAudioTracks addObject:currAudioTrack];
            [allAudioRanges addObject:[NSValue valueWithCMTimeRange:currRange]];
        

        [allVideoRanges addObject:[NSValue valueWithCMTimeRange:currRange]];
        startTime       += clipLength;
    
    [videoTracks insertTimeRanges:allVideoRanges ofTracks:allVideoTracks atTime:kCMTimeZero error:nil];

    if ( allAudioTracks.count > 0 )
    
        [audioTracks insertTimeRanges:allAudioRanges ofTracks:allAudioTracks atTime:kCMTimeZero error:nil];
    
    for ( int i = 0; i < allVideoTracks.count - allAudioTracks.count; ++i )
    
        CMTimeRange curRange = [allVideoRanges[i] CMTimeRangeValue];
        [audioTracks insertEmptyTimeRange:curRange];
    

    // Delete any previous exported video files that may already exist
    [[NSFileManager defaultManager] removeItemAtURL:exportUrl error:nil];

    // Begin the composition generation and export process!
    m_activeExport = [[AVAssetExportSession alloc] initWithAsset:finalComposition presetName:AVAssetExportPreset1280x720];
    [m_activeExport setOutputFileType:AVFileTypeQuickTimeMovie];
    [m_activeExport setOutputURL:exportUrl];
    NSLog(@"Exporting async");
    [m_activeExport exportAsynchronouslyWithCompletionHandler:^(void)
     
         NSLog(@"Export complete");
         // Cancel the update timer
         [m_updateTimer invalidate];
         m_updateTimer = nil;

         // Dismiss the displayed dialog
         [m_displayedDialog hide:TRUE];
         m_displayedDialog = nil;

         // Re-enable touch events
         [[UIApplication sharedApplication] endIgnoringInteractionEvents];

         // Report the success/failure result
         switch (m_activeExport.status)
         
             case AVAssetExportSessionStatusFailed:
                 [self performSelectorOnMainThread:@selector(videoStitchingFailed:) withObject:m_activeExport.error waitUntilDone:FALSE];
                 break;
             case AVAssetExportSessionStatusCompleted:
                 [self performSelectorOnMainThread:@selector(videoStitchingComplete:) withObject:m_activeExport.outputURL waitUntilDone:FALSE];
                 break;
         

         // Clear our reference to the completed export
         m_activeExport = nil;
     ];

编辑：

感谢 cmets 中的 Josh，我注意到我没有使用错误参数。在现在失败的情况下，我在插入视频轨道的时间范围时遇到了非常有用的“无法完成操作”错误：

NSError *videoError = nil;
[videoTracks insertTimeRanges:allVideoRanges ofTracks:allVideoTracks atTime:kCMTimeZero error:&videoError];

if ( videoError != nil )

    NSLog(@"Error adding video track: %@", videoError);

输出：

Error adding video track: Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo=0x17426dd00 NSUnderlyingError=0x174040cc0 "The operation couldn’t be completed. (OSStatus error -12780.)", NSLocalizedFailureReason=An unknown error occurred (-12780), NSLocalizedDescription=The operation could not be completed

值得注意的是，在整个代码库中，没有任何地方使用urlWithString 代替fileUrlWithPath，所以这不是问题。

【问题讨论】：

@JoshCaswell 感谢您指出这一点，我不是这个 cod 的原作者，我倾向于不忽略这类函数的错误参数。您可以在我的编辑中看到我现在实际收到的错误

您是否正在尝试直播导出会话？

@ChrisHaze 不，该应用程序的工作方式是用户录制一系列剪辑，然后一旦完成，所有剪辑都会组合成一个长视频

我不这么认为，但我只是想确定一下。我知道问题是什么，我会尝试在我的回答中提供一些准确的细节

【参考方案1】：

从您对 videoData 数组的for in 枚举来看，在您初始化组合成员变量之后，看起来好像您正在阻塞调用线程。尽管允许访问每个 AVAssetTrack 实例，但键的值并不总是立即可用并同步运行..

请尝试使用AVSynchronousKeyValueLoading 协议注册更改通知。 Apple 的documentation 应该可以帮助您解决问题并让您顺利上路！

以下是我为 AVFoundation 汇总的一些 Apple 建议：

希望这能解决问题！祝您好运，如果您有任何其他问题/问题，请告诉我。

【讨论】：

我没看懂，你怎么看多线程安全的？

我不完全确定之前出了什么问题，但事实证明我弄错了一些剪辑不工作，而一些工作（不应该让客户自行决定）给我一份完整准确的质量保证报告）。我最终添加了对loadValuesAsynchronouslyForKeys 的调用，并且奇迹般地修复了它。

Apple 的 AVFoundation SDK 并不是最直接的框架，老实说，我也无法确切地告诉您遇到问题的原因。我在构建视频共享概念时遇到了类似的问题，不得不求助于 2011 年的 WWDC 视频，他们在其中提到了一些阻塞问题。很高兴它奏效了！ - 我会得到赏金吗？

我今天只想把修复程序通过绞拧机，然后一旦我确定它是正确的修复程序，我将奖励赏金

我已经构建了整个 AVFoundation 框架的思维导图。如果您熟悉 Xmind（或 theBrain）并想查看它们，希望能解释一下这些问题，请告诉我，我会通过电子邮件将它们发送给您。