Iphone 使用AVAssetExportSession从3gp视频文件导出音频

我正在尝试从3gpp视频文件导出音频，但它不工作。。。有人知道我可能做错了什么吗？以下是我正在使用的代码：

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *filePath = [[paths objectAtIndex:0] stringByAppendingPathComponent:@"newFile.m4a"];
NSString *tempFile = [[paths objectAtIndex:0] stringByAppendingPathComponent:@"oldFile.3gp"];

AVURLAsset *asset = [AVURLAsset URLAssetWithURL:[NSURL URLWithString:tempFilePath] options:nil];
[asset loadValuesAsynchronouslyForKeys:[NSArray arrayWithObject:@"tracks"] completionHandler:^ {

    //HERE IS THE PROBLEM. THE ARRAY OF TRACKS IS EMPTY FOR SOME REASON.
    AVAssetTrack* audioTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

    AVMutableComposition* audioComposition = [AVMutableComposition composition];
    AVMutableCompositionTrack* audioCompositionTrack = [audioComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    [audioCompositionTrack insertTimeRange:[audioTrack timeRange] ofTrack:audioTrack atTime:CMTimeMake(0, 1) error:nil];


    AVAssetExportSession *exprortSession = [AVAssetExportSession exportSessionWithAsset:audioComposition presetName:AVAssetExportPresetAppleM4A];
    NSURL *toFileURL = [NSURL URLWithString:filePath];
    exprortSession.outputURL = toFileURL;
    exprortSession.outputFileType = @"com.apple.m4a-audio";

    NSLog(@"exportAsynchronouslyWithCompletionHandler will start");

    [exprortSession exportAsynchronouslyWithCompletionHandler: ^(void) {

        if (exprortSession.status == AVAssetExportSessionStatusCompleted) {
            NSLog(@"Export success");
        }
        else {
            NSLog(@"Export failed");
        }
    }];
}];

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尝试用

[AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:tempFilePath] options:nil];

而不是

[AVURLAsset URLAssetWithURL:[NSURL URLWithString:tempFilePath] options:nil];

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由于您的任务是直接操作音频样本缓冲区，因此您应该使用AVFoundation提供的第二种变体：成对Avassetrader和AVAssetWriter设置。您将在Apple developer source中的AVReaderWriterOSX中找到合适的示例代码。除了有不同的I/O格式设置外，这也适用于iOS。应给出将音频解压缩为PCM并写回未压缩的.wav或音频文件的可用性

是。我不敢相信我看得太多了。也在输出文件中。谢谢