【BroadcastReceiver】

class MainActivity : AppCompatActivity() {
		lateinit var timeChangeReceiver: TimeChangeReceiver
		override fun onCreate(savedInstanceState: Bundle?) { 
		super.onCreate(savedInstanceState) 
		setContentView(R.layout.activity_main)
		
		//首先，创建了一个IntentFilter的实例，并给它添加了一个值为android.intent.action.TIME_TICK的action
		val intentFilter = IntentFilter()
		intentFilter.addAction("android.intent.action.TIME_TICK") 
		
		timeChangeReceiver = TimeChangeReceiver() 
		registerReceiver(timeChangeReceiver, intentFilter)
	}
	override fun onDestroy() { super.onDestroy()
		unregisterReceiver(timeChangeReceiver) 
	}

	//内部类TimeChangeReceiver：继承BroadcastReceiver，并重写了父类的onReceive()
	inner class TimeChangeReceiver : BroadcastReceiver() {
		override fun onReceive(context: Context, intent: Intent) {
			Toast.makeText(context, "Time has changed", Toast.LENGTH_SHORT).show() 
		}
	} 
}

当系统时间发生变化时，系统发出的正是一条值为android.intent.action.TIME_TICK的广播。也就是说，我们的BroadcastReceiver想要监听什么广播，就添加相应的action.

最后要记得，动态注册的BroadcastReceiver一定要取消注册才行！比如，我们可以在 onDestroy()方法中通过调用unregisterReceiver()方法来实现的。