1. 创建一个 Service（服务）类并实现计时器功能。

class TimerService : Service() {
    private var startTime: Long = 0
    private var isRunning: Boolean = false
    private var elapsed: Long = 0

    // 计时器更新时间间隔，单位为毫秒
    private val TICK_INTERVAL = 1000L

    override fun onBind(intent: Intent): IBinder? {
        return null
    }

    override fun onStartCommand(intent: Intent, flags: Int, startId: Int): Int {
        if (!isRunning) {
            startTime = System.currentTimeMillis() - elapsed
            isRunning = true
            startTimer()
        }
        return START_STICKY
    }

    private fun startTimer() {
        val handler = Handler()
        handler.post(object : Runnable {
            override fun run() {
                elapsed = System.currentTimeMillis() - startTime
                val minutes = (elapsed / 1000) / 60
                val seconds = (elapsed / 1000) % 60
                val timerText = String.format("%02d:%02d", minutes, seconds)

                // 发送广播
                val sendIntent = Intent()
                sendIntent.putExtra("timer", timerText)
                sendIntent.action = "timer_update"
                sendBroadcast(sendIntent)

                handler.postDelayed(this, TICK_INTERVAL)
            }
        })
    }

    override fun onDestroy() {
        super.onDestroy()
        isRunning = false
    }
}

2. 在 MainActivity 中添加显示通知的函数。

private fun showNotification() {
    val intent = Intent(this, MainActivity::class.java)
    val pendingIntent = PendingIntent.getActivity(this, 0, intent, 0)

    // 创建通知并设置图标与标题
    val notificationBuilder = Notification.Builder(this)
            .setSmallIcon(R.drawable.ic_launcher_background)
            .setContentTitle("计时器已启动")
            .setContentIntent(pendingIntent)

    // 将 Service 绑定到 Activity
    val timerService = Intent(this, TimerService::class.java)
    bindService(timerService, serviceConnection, Context.BIND_AUTO_CREATE)

    // 显示通知
    startForeground(1, notificationBuilder.build())
}