Android Handler 即使在 activity 被销毁后也会保留 运行 一段时间

Question

我读到 handler.postDelayed 方法有助于将任务暂停任意时间。所以我把它写进了光线传感器，让传感器每2秒检测一次光线，以节省电池寿命：

public class MainActivity extends Activity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    SensorManager sm = (SensorManager) getSystemService(SENSOR_SERVICE);
    Sensor light = sm.getDefaultSensor(Sensor.TYPE_LIGHT);
    SensorEventListener listener;

    if (light != null) {

        final Handler handler = new Handler();
        final Runnable task = new Runnable() {
            @Override
            public void run() {
                sm.registerListener(listener, light, SensorManager.SENSOR_DELAY_NORMAL);
            }
        };

        listener = new SensorEventListener() {

            @Override
            public void onAccuracyChanged(Sensor sensor, int accuracy) {
                Toast.makeText(MainActivity.this, "accuracy changed!", Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onSensorChanged(SensorEvent event) {

                sm.unregisterListener(listener, light);

                // Do the stuff here with the received sensor light value, which is event.values[0]
                
                handler.postDelayed(task, 2000);
            }
        };

    }

    @Override
    protected void onPause() {
        super.onPause();
        sm.unregisterListener(listener, light);
    }

    @Override
    protected void onResume() {
        super.onResume();
        sm.registerListener(listener, light, SensorManager.SENSOR_DELAY_NORMAL);
    }

    @Override
    protected void onStop() {
        super.onStop();
        sm.unregisterListener(listener, light);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        sm.unregisterListener(listener, light);
    }
}

起初它似乎工作正常。但我发现，每次我退出应用程序时，它都会检测到光线大约 20 到 30 秒，就好像 activity 仍然打开一样。我究竟做错了什么？我什至添加了 onStop 和 onDestroy 方法只是为了确保它正确关闭。

Answer 1

在 activity 的 onDestroy() 中清除你的处理程序，如

handler.removeCallbacksAndMessages(null);

而且会马上停止！