<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  xmlns:tools="http://schemas.android.com/tools"  android:layout_width="match_parent"  android:layout_height="match_parent"  tools:context=".MainActivity">  <TextView    android:id="@+id/countdownTimeTextView"    android:layout_width="wrap_content"    android:layout_height="wrap_content"    android:layout_centerInParent="true"    android:text="@string/maxTime"    android:textSize="60sp" /></RelativeLayout>

public class MainActivity extends AppCompatActivity {  /**   * 倒計時標記   */  public static final int COUNTDOWN_TIME_CODE = 99999;  /**   * 倒計時間隔   */  public static final int DELAY_MILLIS = 1000;  /**   * 倒計時最大值   */  public static final int MAX_COUNT = 10;  /**   * 文本控件   */  private TextView countdownTimeTextView;  @Override  protected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    //初始化文本控件    countdownTimeTextView = findViewById(R.id.countdownTimeTextView);    //創建一個handler    CountdownTimeHandler handler = new CountdownTimeHandler(this);    //新建一個message    Message message = Message.obtain();    message.what = COUNTDOWN_TIME_CODE;    message.arg1 = MAX_COUNT;    //第一次發送message    handler.sendMessageDelayed(message, DELAY_MILLIS);  }  public static class CountdownTimeHandler extends Handler {    /**     * 倒計時最小值     */    public static final int MIN_COUNT = 0;    //創建MainActivity弱引用    final WeakReference<MainActivity> mWeakReference;    public CountdownTimeHandler(MainActivity activity) {      this.mWeakReference = new WeakReference<>(activity);    }    @Override    public void handleMessage(Message msg) {      super.handleMessage(msg);      //獲取對MainActivity的弱引用      MainActivity activity = mWeakReference.get();      switch (msg.what) {        case COUNTDOWN_TIME_CODE:          int value = msg.arg1;          activity.countdownTimeTextView.setText(String.valueOf(value--));          //循環發送消息的控制          if (value >= MIN_COUNT) {            Message message = Message.obtain();            message.what = COUNTDOWN_TIME_CODE;            message.arg1 = value;            sendMessageDelayed(message, DELAY_MILLIS);          }          break;      }    }  }}