dispatch_get_main_queue not working

Question

My iOS base SDK is 8.1. dispatch_get_main_queue works fine when I'm running on 8.1 simulator. However when I run it on 7.1 simulator, it does not get called. I noticed that dispatch_get_main_queue has been reimplemented in iOS 8.0 and later.

How can I solve this problem? change base SDK or what?

my code

    AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];

    // audio track
    AVMutableCompositionTrack *audioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];
    //
    NSError *error;

    AVAsset *videoAsset = [AVAsset assetWithURL:videoURL];
    [audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                        ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeAudio] firstObject]
                         atTime:kCMTimeZero
                          error:&error];
    if (error) {
        NSLog(@"extract audio error!");
        return;
    }
    error = nil;

    // audio path
    NSString *path = [NSString stringWithFormat:@"%@newAudio.m4a", NSTemporaryDirectory()];
    if ([[NSFileManager defaultManager] fileExistsAtPath:path]) {
        if (![[NSFileManager defaultManager] removeItemAtPath:path error:&error]) {
            NSLog(@"audio cannot be saved!");
        }
    }
    // exporter
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                                                      presetName:AVAssetExportPresetAppleM4A];
    exporter.outputURL = [NSURL fileURLWithPath:path];
    exporter.outputFileType = AVFileTypeAppleM4A;
    exporter.shouldOptimizeForNetworkUse = YES;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        //NSLog(@"export status: %ld", exporter.status);
        dispatch_async(dispatch_get_main_queue(), ^{
            [self exportDidFinish:exporter];
        });
    }];
}

Answer 1

Finally i figured it out. The presetName need to be AVAssetExportPresetPassthrough so that it works fine on iOS 7 simulator. I don't really know why, but thank you @Kai and @Rob for your replies.