如何根据环境光更改 Android 应用程序的主题？

Question

I am trying to develop an Android app (this is a school project and I do not have experience, but I would like to learn) and I need to change the theme based on the ambiental light.

About the project : I have an app which is used to keep the information about the books that the user is reading (open books, finished books, abandoned books, reading sessions, etc.). The idea is to change the theme based on the ambiental light.

This is the code used in order to detect the light value using the ambiental light sensor:

private Toast mToastToShow;
private SensorManager sensorManager;
private Sensor lightSensor;
private SensorEventListener lightSensorEventListener;
private float maxValue;


protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
    lightSensor = sensorManager.getDefaultSensor(TYPE_LIGHT);


    if(lightSensor == null){
        Toast.makeText(this,"Oh, snap... This device has not a light sensor", Toast.LENGTH_SHORT).show();
        finish();
    }

    maxValue = lightSensor.getMaximumRange();

    lightSensorEventListener = new SensorEventListener() {

        @Override
        public void onSensorChanged(SensorEvent event) {
            changeTheme(event);
        }

        @Override
        public void onAccuracyChanged(Sensor sensor, int accuracy) {
            Log.d(this.getClass().getName(), "The accuracy of the sensor has changed");
        }
    };
}

public void showToast(String message) {
    int toastDurationInMilliSeconds = 50000;
    mToastToShow = Toast.makeText(this, message, Toast.LENGTH_LONG);
    CountDownTimer toastCountDown;
    toastCountDown = new CountDownTimer(toastDurationInMilliSeconds, 1000 /*Tick duration*/) {
        public void onTick(long millisUntilFinished) {
            mToastToShow.show();
        }
        public void onFinish() {
            mToastToShow.cancel();
        }
    };
    mToastToShow.show();
    toastCountDown.start();
}

@Override
protected void onStart(){
    super.onStart();
    sensorManager.registerListener(lightSensorEventListener, lightSensor, SensorManager.SENSOR_DELAY_FASTEST);
}

@Override
protected void onResume(){
    super.onResume();
    sensorManager.registerListener(lightSensorEventListener, lightSensor, SensorManager.SENSOR_DELAY_FASTEST);
}

@Override
protected void onPause() {
    super.onPause();
    sensorManager.unregisterListener(lightSensorEventListener);
}


@Override
protected void onStop() {
    super.onStop();
    sensorManager.unregisterListener(lightSensorEventListener);
}

private void changeTheme(SensorEvent event){
    float value = event.values[0];
    if(value >= 14379) {
        AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO);
    }else{
        AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES);
    }
}

The only problem that I can not resolve is that when the app starts, I do not know how to get the light value without changing the ambiental light (using the virtual sensors from the emulator). The problem is that the light value is only retrieved when the on sensor changed event is triggered.

To be specific, the scenario is the following:

1. I need to have the dark theme on when the light value is not greater or equal than 14379
2. If the light value is greater or equal than 14379, then the light theme must be applied.
3. When the app starts, the theme is not updating because the on change event for the sensor is not triggered.
4. For example, when the app starts, if the light value is smaller than the limit, the dark theme must be applied, but because the on change event is not triggered, the light theme is applied.

Could you please tell me if there is a solution for this problem or if I have made a mistake (is this a normal behavior)?

Thank you for your time!

Answer 1

After some research, turns out that you are not able to do this thing because the light sensor is a hardware one and you cannot try to trigger the on change event. But on a physical device, it works (because the ambiental light is not always constant) and the only problem is when you try to simulate this behavior on the emulator.