创建前台服务并使用countDown计时器。然后每1秒钟发送一次广播以更改ui。 如果您希望应用在Android 8及更高版本上在后台运行，则应发送带有服务的通知以向用户显示您的应用正在执行的任务。

@Override
public int onStartCommand(Intent intent,int flags,int startId) {



    int milliseconds = 100* 1000;

    new CountDownTimer(milliseconds,1000) {

        @Override
        public void onTick(long millisUntilFinished) {

            Intent intent = new Intent("timer_tracking");
            intent.putExtra("timer",millisUntilFinished);
            LocalBroadcastManager.getInstance(getApplicationContext()).sendBroadcast(intent);

        }

        @Override
        public void onFinish() {

            stopSelf();
        }
    }.start();


    return START_STICKY;
}

并且在广播/接收器寄存器之后的活动/片段中使用此代码。

 public BroadcastReceiver mMessageReceiver = new BroadcastReceiver() {
    @SuppressLint("DefaultLocale")
    @Override
    public void onReceive(Context context,Intent intent) {

        Toast.makeText(MainActivity.this,"broad",Toast.LENGTH_LONG).show();

        long millisUntilFinished = intent.getLongExtra("timer",0);

        String counter = String.format("%02d:%02d:%02d",TimeUnit.MILLISECONDS.toHours(millisUntilFinished),TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished),TimeUnit.MILLISECONDS.toSeconds(millisUntilFinished) -
                        TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished)));


        textView.setText(counter);
    }
};