ios - + 导出 session 与 Assets :presetName: returning nil on iOS7

Question

我使用以下代码:

- (AVAssetExportSession *)testRecording
{
    AVMutableComposition *composition = [[AVMutableComposition alloc] init];

    NSURL *url = [[NSBundle mainBundle] URLForResource:kTestSongName withExtension:@"mp3"];
    AVAsset *asset = [AVURLAsset URLAssetWithURL:url options:nil];
    CMTimeRange range = CMTimeRangeMake(CMTimeMake(0, 600), asset.duration);
    NSError *editError;

    BOOL result = [composition insertTimeRange:range ofAsset:asset atTime:kCMTimeZero error:&editError];
    if (!result) {
        NSLog(@"Error inserting timerange");
    }
    AVAssetExportSession *exportSession = [AVAssetExportSession
                                           exportSessionWithAsset:composition
                                           presetName:AVAssetExportPresetAppleM4A];
    return exportSession;

}

这在 iOS8 上完成了它应该做的事情，但在 iOS7 上，exportSession 在方法末尾始终为 nil。我尝试使用不同格式的资源(mp3、caf)，但结果是相同的。

如果我播放乐曲而不是导出它，声音就很好:

AVPlayerItem * item = [[AVPlayerItem alloc] initWithAsset:composition];
AVPlayer * player = [AVPlayer playerWithPlayerItem:item];
[player play];

知道可能出了什么问题吗？

Answer 1

    exportSession = [[AVAssetExportSession alloc]
                                              initWithAsset:songAsset
                                              presetName:AVAssetExportPresetAppleM4A];
        exportSession.outputFileType = @"com.apple.m4a-audio";
    filePath = [[docDir stringByAppendingPathComponent:fileName] stringByAppendingPathExtension:@"m4a.mov"]
        exportSession.outputURL = [NSURL fileURLWithPath:filePath];
        [exportSession exportAsynchronouslyWithCompletionHandler:^{
            if (exportSession.status == AVAssetExportSessionStatusCompleted) {
                NSString *newPath = [NSString stringWithFormat:@"%@/%@", documentDirectory, [[[filePath lastPathComponent] substringToIndex:[[filePath lastPathComponent] length]-5] stringByAppendingString:@"a"]];
                [[NSFileManager defaultManager] removeItemAtPath:newPath error:nil];
                NSError *error = nil;
                [[NSFileManager defaultManager] moveItemAtPath:filePath toPath:newPath error:&error];
            }
            if (exportSession.status == AVAssetExportSessionStatusCancelled) {
            }
            if (exportSession.status == AVAssetExportSessionStatusExporting) {
            }
            if (exportSession.status == AVAssetExportSessionStatusFailed) {
                          NSLog(@"erros:%@",exportSession.error);
            }
            if (exportSession.status == AVAssetExportSessionStatusUnknown) {
            }
            if (exportSession.status == AVAssetExportSessionStatusWaiting) {
            }
        }];

如果要保存为mp3格式，请使用ffmpeg .