Android 处理程序在活动被销毁后仍会继续运行一段时间

Question

I read that the handler.postDelayed method helps pause a task for any amount of time. So I wrote it into the light sensor so that the sensor only detects light every 2 seconds, in order to save battery life:

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);
    }
}

At first it seemed to work fine. But I found that every time I exit the app it kept detecting the light for like 20 to 30 seconds as if the activity was still open. What am I doing wrong? I even added both the onStop and onDestroy methods just to be sure it was closing correctly.

Answer 1

Inside the onDestroy() of your activity clear your handler like

handler.removeCallbacksAndMessages(null);

And it will stop right away!