var outProgressBarValue:Float = 0.0 {
    willSet {
        dispatch_async(dispatch_get_main_queue(), {
            println("New value received! -> (newValue)")
            self.progressBarOutlet.setProgress(newValue, animated: true)
        })
    }
}

@IBAction func saveButton(sender: AnyObject) {
    [...]
    let backgroundQueue = dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0)
    dispatch_async(backgroundQueue, {
        WriteOutputFile(url, self.musicSequence!, Sound.sharedInstance.musicPlayer, self.processingGraph,  &self.outProgressBarValue)

    dispatch_async(dispatch_get_main_queue(), {
            restoreOnlineRendering(processingGraph: self.processingGraph, mainMixerNode: self.mainMixerNode, ioNode: &(self.ioNode))
            self.saveButtonOutlet.setTitle("Done", forState: UIControlState.Normal)
    [...]
}

void WriteOutputFile(CFURLRef url, MusicSequence musicSequence, MusicPlayer musicPlayer, AUGraph processingGraph, float *outProgressBarValue) {
[…]
    CheckError(MusicPlayerStart(musicPlayer), "MusicPlayerStart");
    do {
        CheckError(AudioUnitRender (outputUnit, &actionFlags, &tStamp, 0, numFrames, bufferList), "AudioUnitRender");
        tStamp.mSampleTime += (Float64)numFrames;
        CheckError(ExtAudioFileWrite(outfile, numFrames, bufferList), "ExtAudioFileWrite");
        CheckError(MusicPlayerGetTime (musicPlayer, &currentTime), "MusicPlayerGetTime");
        *outProgressBarValue = currentTime / sequenceLength;
        printf("outProgressBarValue:t %fn", *outProgressBarValue);
    } while (currentTime < sequenceLength);

    CheckError(MusicPlayerStop(musicPlayer), "MusicPlayerStop");
[…]
}