void _recordingButton_TouchCancel(object sender, EventArgs e)
        {
            if (_recorder == null)
            {
                // during playing, it can not record.
                stopPlayback();

                _playBackButton.Enabled = false;

                // setting audio session
                AudioSession.Category = AudioSessionCategory.RecordAudio;

                _recorder = new AudioQueueRecorder(_url);
                _recorder.Start();
                _recordingButton.SetTitle("Stop Recording", UIControlState.Normal);
            }
            else
            {
                _recorder.Dispose();
                _recorder = null;
                _recordingButton.SetTitle("Start Recording", UIControlState.Normal);
                _playBackButton.Enabled = true;

                // setting audio session
                AudioSession.Category = AudioSessionCategory.MediaPlayback;
            }
        }
        void _recordingButton_TouchCancel(object sender, EventArgs e)
        {
            if (_recorder == null)
            {
                // during playing, it can not record.
                stopPlayback();

                _playBackButton.Enabled = false;

                // setting audio session
                AudioSession.Category = AudioSessionCategory.RecordAudio;

                _recorder = new AudioQueueRecorder(_url);
                _recorder.Start();
                _recordingButton.SetTitle("Stop Recording", UIControlState.Normal);
            }
            else
            {
                _recorder.Dispose();
                _recorder = null;
                _recordingButton.SetTitle("Start Recording", UIControlState.Normal);
                _playBackButton.Enabled = true;

                // setting audio session
                AudioSession.Category = AudioSessionCategory.MediaPlayback;
            }
        }