<?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(jì)時(shí)標(biāo)記
   */
  public static final int COUNTDOWN_TIME_CODE = 99999;
  /**
   * 倒計(jì)時(shí)間隔
   */
  public static final int DELAY_MILLIS = 1000;
  /**
   * 倒計(jì)時(shí)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(chuàng)建一個(gè)handler
    CountdownTimeHandler handler = new CountdownTimeHandler(this);
    //新建一個(gè)message
    Message message = Message.obtain();
    message.what = COUNTDOWN_TIME_CODE;
    message.arg1 = MAX_COUNT;
    //第一次發(fā)送message
    handler.sendMessageDelayed(message, DELAY_MILLIS);
  }

  public static class CountdownTimeHandler extends Handler {
    /**
     * 倒計(jì)時(shí)最小值
     */
    public static final int MIN_COUNT = 0;
    //創(chuàng)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(duì)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(huán)發(fā)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;
      }
    }
  }
}