屏幕锁定时处理音量按钮

Question

当前，我正在使用简单的android studio脚本，在锁定移动屏幕时必须降低音量。

请看一下我的脚本：

public class MainActivity extends AppCompatActivity {

    public MediaPlayer mediaPlayer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final BroadcastReceiver vReceiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                initAudio(getApplicationContext(), "https://www.w3schools.com/html/horse.mp3");
            }
        };
        registerReceiver(vReceiver, new IntentFilter("android.media.VOLUME_CHANGED_ACTION"));
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_VOLUME_DOWN) {
            event.startTracking();
            initAudio(getApplicationContext(), "https://www.w3schools.com/html/horse.mp3");
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }

    public void initAudio(final Context context, String url) {
        if (mediaPlayer == null) {
            try {
                mediaPlayer = new MediaPlayer();
                mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
                mediaPlayer = MediaPlayer.create(context, Uri.parse(url));

                mediaPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
                    @Override
                    public void onCompletion(MediaPlayer mp) {
                        killMediaPlayer();
                    }
                });

                mediaPlayer.start();
            } catch (Exception e) {
                e.printStackTrace();
                Toast.makeText(context, "Fail", Toast.LENGTH_LONG).show();
                killMediaPlayer();
            }
        }
    }

    public void pauseAudio() {
        if (!(mediaPlayer == null)) {
            mediaPlayer.pause();
        }
    }

    public void startAudio() {
        if (!(mediaPlayer == null)) {
            mediaPlayer.start();
        }
    }

    public void killMediaPlayer() {
        if (mediaPlayer != null) {
            try {
                mediaPlayer.reset();
                mediaPlayer.release();
                mediaPlayer = null;
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

我该怎么做才能解决此问题？没有错误，该脚本仅在屏幕解锁时起作用。 你能帮我吗？

非常感谢您。