在 Android 裡定時更新 UI，通常使用的是 java.util.Timer, java.util.TimerTask, android.os.Handler 組合，這裡有相關的討論。但實際上 Handler 自身已經提供了定時的功能。
參考 android.os.Handler 的文檔
引用
There are two main uses for a Handler: (1) to schedule messages and runnables to be executed as some point in the future; and (2) to enqueue an action to be performed on a different thread than your own.
Scheduling messages is accomplished with the post(Runnable), postAtTime(Runnable, long), postDelayed(Runnable, long), sendEmptyMessage(int), sendMessage(Message), sendMessageAtTime(Message, long), and sendMessageDelayed(Message, long) methods. The post versions allow you to enqueue Runnable objects to be called by the message queue when they are received; the sendMessage versions allow you to enqueue a Message object containing a bundle of data that will be processed by the Handler's handleMessage(Message) method (requiring that you implement a subclass of Handler).
When posting or sending to a Handler, you can either allow the item to be processed as soon as the message queue is ready to do so, or specify a delay before it gets processed or absolute time for it to be processed. The latter two allow you to implement timeouts, ticks, and other timing-based behavior。
下面是一個簡單的計數器程序，每隔一秒遞增計數器
代碼
main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:layout_height="fill_parent">
<TextView android:id="@+id/counter" android:layout_width="fill_parent"
android:layout_height="wrap_content" android:text="Count: 0" />
<LinearLayout android:orientation="horizontal"
android:layout_width="fill_parent" android:layout_height="wrap_content">
<Button android:text="start" android:id="@+id/Button01"
android:layout_width="fill_parent" android:layout_height="wrap_content" android:layout_weight="1.0"></Button>
<Button android:text="stop" android:id="@+id/Button02"
android:layout_width="fill_parent" android:layout_height="wrap_content" android:layout_weight="1.0" android:enabled="false"></Button>
<Button android:text="reset" android:id="@+id/Button03"
android:layout_width="fill_parent" android:layout_height="wrap_content" android:layout_weight="1.0"></Button>
</LinearLayout>
</LinearLayout>
