当我最小化应用程序时，该应用程序无法在前台运行

Question

我正在尝试向Android屏幕对脸距离测量https://github.com/philiiiiiipp/Android-Screen-to-Face-Distance-Measurement添加一些功能 例如使其在前景或背景下工作，当人脸离屏幕太近时，会弹出通知。现在，该应用程序可以在前台运行，并可以在通知中更新距离测量值。 但是，一旦按下主屏幕按钮以最小化该应用程序，距离测量值就会在该通知中停止更新。

我尝试过前台和后台。当我最小化应用程序时，该应用程序仍无法在后台或前景中工作。

MainActivity.java

public class MainActivity extends Activity implements MessageListener{

public static final String CAM_SIZE_WIDTH = "intent_cam_size_width";
public static final String CAM_SIZE_HEIGHT = "intent_cam_size_height";
public static final String AVG_NUM = "intent_avg_num";
public static final String PROBANT_NAME = "intent_probant_name";
private NotificationManagerCompat notificationManager;
private CameraSurfaceView _mySurfaceView;
Camera _cam;

private final static DecimalFormat _decimalFormater = new DecimalFormat(
        "0.0");

private float _currentDevicePosition;

private int _cameraHeight;
private int _cameraWidth;
private int _avgNum;

//new added
private static final String TAG = "MainActivity";

TextView _currentDistanceView;
Button _calibrateButton;

//for floating
public static final int SYSTEM_ALERT_WINDOW_PERMISSION = 7;
Button button ;

/**
 * Abusing the media controls to create a remote control
 */
// ComponentName _headSetButtonReceiver;
// AudioManager _audioManager;

@Override
public void onCreate(final Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.measurement_activity);

    //for notification
    notificationManager = NotificationManagerCompat.from(this);


    _mySurfaceView = (CameraSurfaceView) findViewById(R.id.surface_camera);

    RelativeLayout.LayoutParams layout = new RelativeLayout.LayoutParams(
            (int) (0.95 * this.getResources().getDisplayMetrics().widthPixels),
            (int) (0.6 * this.getResources().getDisplayMetrics().heightPixels));

    layout.setMargins(0, (int) (0.05 * this.getResources()
            .getDisplayMetrics().heightPixels), 0, 0);

    _mySurfaceView.setLayoutParams(layout);
    _currentDistanceView = (TextView) findViewById(R.id.currentDistance);
    _calibrateButton = (Button) findViewById(R.id.calibrateButton);

   }



@Override
protected void onResume() {
    super.onResume();

    MessageHUB.get().registerListener(this);
    // _audioManager.registerMediaButtonEventReceiver(_headSetButtonReceiver);

    // 1 for front cam. No front cam ? Not my fault!
    _cam = Camera.open(1);
    Camera.Parameters param = _cam.getParameters();

    // Find the best suitable camera picture size for your device. Competent
    // research has shown that a smaller size gets better results up to a
    // certain point.
    // http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6825217&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel7%2F6816619%2F6825201%2F06825217.pdf%3Farnumber%3D6825217
    List<Size> pSize = param.getSupportedPictureSizes();
    double deviceRatio = (double) this.getResources().getDisplayMetrics().widthPixels
            / (double) this.getResources().getDisplayMetrics().heightPixels;

    Size bestSize = pSize.get(0);
    double bestRation = (double) bestSize.width / (double) bestSize.height;

    for (Size size : pSize) {
        double sizeRatio = (double) size.width / (double) size.height;

        if (Math.abs(deviceRatio - bestRation) > Math.abs(deviceRatio
                - sizeRatio)) {
            bestSize = size;
            bestRation = sizeRatio;
        }
    }
    _cameraHeight = bestSize.height;
    _cameraWidth = bestSize.width;

    Log.d("PInfo", _cameraWidth + " x " + _cameraHeight);

    param.setPreviewSize(_cameraWidth, _cameraHeight);
    _cam.setParameters(param);

    _mySurfaceView.setCamera(_cam);
}

@Override
protected void onPause() {
    super.onPause();

    MessageHUB.get().unregisterListener(this);

    // _audioManager
    // .unregisterMediaButtonEventReceiver(_headSetButtonReceiver);

    resetCam();
}

/**
 * Sets the current eye distance to the calibration point.
 *
 * @param v
 */
public void pressedCalibrate(final View v) {

    if (!_mySurfaceView.isCalibrated()) {

        _calibrateButton.setBackgroundResource(R.drawable.yellow_button);
        _mySurfaceView.calibrate();
    }
}

public void pressedReset(final View v) {

    if (_mySurfaceView.isCalibrated()) {

        _calibrateButton.setBackgroundResource(R.drawable.red_button);
        _mySurfaceView.reset();
    }
}

public void onShowMiddlePoint(final View view) {
    // Is the toggle on?
    boolean on = ((Switch) view).isChecked();
    _mySurfaceView.showMiddleEye(on);
}

public void onShowEyePoints(final View view) {
    // Is the toggle on?
    boolean on = ((Switch) view).isChecked();
    _mySurfaceView.showEyePoints(on);
}

/**
 * Update the UI values.
 *
 * @param eyeDist
 * @param frameTime
 */
void updateUI(final MeasurementStepMessage message) {

    _currentDistanceView.setText(_decimalFormater.format(message
            .getDistToFace()) + " cm");

    float fontRatio = message.getDistToFace() / 29.7f;

    _currentDistanceView.setTextSize(fontRatio * 20);

    //Foreground Services
    //String input = editTextInput.getText().toString();
    float number = message.getDistToFace();
    String input = Float.toString(number);
    Intent serviceIntent = new Intent(this, ExampleService.class);
    serviceIntent.putExtra("inputExtra",input);

   ContextCompat.startForegroundService(this, serviceIntent);

    //my new code added here(working)
    if(message.getDistToFace()<25)
    {
        Notification notification = new NotificationCompat.Builder(this, CHANNEL_1_ID)
                .setSmallIcon(R.drawable.notification_icon)
                .setContentTitle("ALERT")
                .setContentText("You are too near to the screen!")
                .setPriority(NotificationCompat.PRIORITY_HIGH)
                .setCategory(NotificationCompat.CATEGORY_ALARM)
                .setVibrate(new long[] { 1000, 1000, 1000, 1000, 1000 })
                .build();

        notificationManager.notify(3, notification);



    }


}

private void resetCam() {
    _mySurfaceView.reset();

    _cam.stopPreview();
    _cam.setPreviewCallback(null);
    _cam.release();
}

@Override
public void onMessage(final int messageID, final Object message) {

    switch (messageID) {

        case MessageHUB.MEASUREMENT_STEP:
            updateUI((MeasurementStepMessage) message);
            break;

        case MessageHUB.DONE_CALIBRATION:

            _calibrateButton.setBackgroundResource(R.drawable.green_button);

            break;
        default:
            break;
    }

}

ExampleService.java

  public class ExampleService extends Service {

    @Override
    public void onCreate() {
        super.onCreate();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        String input = intent.getStringExtra("inputExtra");

        Intent notificationIntent = new Intent(this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(this,
                0, notificationIntent, 0);

        Notification notification = new NotificationCompat.Builder(this, CHANNEL_ID)
                .setContentTitle("Example Service")
                .setContentText(input)
                .setSmallIcon(R.drawable.ic_android)
                .setContentIntent(pendingIntent)
                .build();

        startForeground(1, notification);

        //do heavy work on a background thread
        //stopSelf();

        return START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
      }
    }