我如何创建一个即使在Android中关闭应用程序也能正常工作的计时器

我写了下面的代码，它是一个计时器。但这有一个问题，问题是当我关闭应用程序时，计数操作也会停止。但是，即使关闭应用程序，我也想继续进行计数操作。

public class MainActivity extends Activity {
    private Button startButton;
    private Button pauseButton;
    private TextView timerValue;
    private long startTime = 0L;
    private Handler customHandler = new Handler();
    long timeInMilliseconds = 0L;
    long timeSwapBuff = 0L;
    long updatedTime = 0L;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        timerValue = (TextView) findViewById(R.id.timerValue);
        startButton = (Button) findViewById(R.id.startButton);
        startButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                startTime = SystemClock.uptimeMillis();
                customHandler.postDelayed(updateTimerThread, 0);
            }
        });pauseButton = (Button) findViewById(R.id.pauseButton);
        pauseButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                timeSwapBuff += timeInMilliseconds;
                customHandler.removeCallbacks(updateTimerThread);
            }
        });
    }
    private Runnable updateTimerThread = new Runnable() {



        public void run() {
            timeInMilliseconds = SystemClock.uptimeMillis() - startTime;
            updatedTime = timeSwapBuff + timeInMilliseconds;
            int secs = (int) (updatedTime / 1000);
            int mins = secs / 60;
            secs = secs % 60;
            int milliseconds = (int) (updatedTime % 1000);
            timerValue.setText("" + mins + ":"
                    + String.format("%02d", secs) + ":"
                    + String.format("%03d", milliseconds));
            customHandler.postDelayed(this, 0);
        }
    };
}