Android Audio Focus Listener在服务中不工作

public class MyAudioFocusChangeListener implements AudioManager.OnAudioFocusChangeListener {
    @Override
    public void onAudioFocusChange(int focusChange) {
        switch (focusChange) {
            case AudioManager.AUDIOFOCUS_GAIN:
                // 获取到音频焦点，继续播放音频
                break;
            case AudioManager.AUDIOFOCUS_LOSS:
                // 失去音频焦点，停止播放音频
                break;
            case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
                // 暂时失去音频焦点，暂停播放音频
                break;
            case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
                // 暂时失去音频焦点，降低音量
                break;
        }
    }
}

public class MyService extends Service {
    private AudioManager audioManager;
    private MyAudioFocusChangeListener audioFocusChangeListener;

    @Override
    public void onCreate() {
        super.onCreate();
        audioManager = (AudioManager) getSystemService(Context.AUDIO_SERVICE);
        audioFocusChangeListener = new MyAudioFocusChangeListener();
        int result = audioManager.requestAudioFocus(audioFocusChangeListener, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN);
        if (result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
            // 成功获取到音频焦点，可以开始播放音频
        }
    }
}