Android開發：完美實現倒計時功能

一、實現原理

倒計時功能的實現原理很簡單，就是通過Handler不斷地向主線程發送Message，用Message中的arg1來存放倒計時的時間，通過TextView顯示出來。當倒計時結束時，停止向主線程發送Message。

二、實現步驟

1、首先在布局文件中添加一個TextView來顯示倒計時的時間：

<TextView
    android:id="@+id/tv_countdown"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textSize="20sp"
    android:textColor="@android:color/black" />

2、在Activity中獲取TextView的實例，並使用Handler實現倒計時：

public class MainActivity extends AppCompatActivity {
    
    private TextView mCountdownView;
    private static final int MSG_UPDATE_COUNTDOWN = 1;
    private static final int COUNTDOWN_TIME = 60;//倒計時60秒
    private int mCountdown = COUNTDOWN_TIME;//用於記錄倒計時的剩餘時間
    
    private Handler mHandler = new Handler() {
        @Override
        public void handleMessage(@NonNull Message msg) {
            super.handleMessage(msg);
            switch (msg.what) {
                case MSG_UPDATE_COUNTDOWN:
                    mCountdownView.setText(msg.arg1 + "秒");
                    if (msg.arg1 == 0) {
                        stopCountdown();
                    }
                    break;
            }
        }
    };
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        mCountdownView = findViewById(R.id.tv_countdown);
        
        startCountdown();
    }

    //開始倒計時
    private void startCountdown() {
        mCountdown = COUNTDOWN_TIME;
        mHandler.post(mCountdownRunnable);
    }

    //停止倒計時
    private void stopCountdown() {
        mHandler.removeCallbacksAndMessages(null);
    }

    private Runnable mCountdownRunnable = new Runnable() {
        @Override
        public void run() {
            mCountdown--;
            Message msg = mHandler.obtainMessage(MSG_UPDATE_COUNTDOWN);
            msg.arg1 = mCountdown;
            mHandler.sendMessage(msg);
            if (mCountdown > 0) {
                mHandler.postDelayed(this, 1000);//每隔1秒向主線程發送一次Message
            }
        }
    };
}

三、實現效果

啟動應用後，TextView會顯示60秒，然後每秒鐘減少1秒，直到倒計時結束：

60秒
59秒
58秒
57秒
56秒
……
2秒
1秒
0秒

完整代碼如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    android:gravity="center_horizontal"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv_countdown"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="20sp"
        android:textColor="@android:color/black" />

</LinearLayout>

public class MainActivity extends AppCompatActivity {

    private TextView mCountdownView;
    private static final int MSG_UPDATE_COUNTDOWN = 1;
    private static final int COUNTDOWN_TIME = 60;//倒計時60秒
    private int mCountdown = COUNTDOWN_TIME;//用於記錄倒計時的剩餘時間

    private Handler mHandler = new Handler() {
        @Override
        public void handleMessage(@NonNull Message msg) {
            super.handleMessage(msg);
            switch (msg.what) {
                case MSG_UPDATE_COUNTDOWN:
                    mCountdownView.setText(msg.arg1 + "秒");
                    if (msg.arg1 == 0) {
                        stopCountdown();
                    }
                    break;
            }
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mCountdownView = findViewById(R.id.tv_countdown);

        startCountdown();
    }

    //開始倒計時
    private void startCountdown() {
        mCountdown = COUNTDOWN_TIME;
        mHandler.post(mCountdownRunnable);
    }

    //停止倒計時
    private void stopCountdown() {
        mHandler.removeCallbacksAndMessages(null);
    }

    private Runnable mCountdownRunnable = new Runnable() {
        @Override
        public void run() {
            mCountdown--;
            Message msg = mHandler.obtainMessage(MSG_UPDATE_COUNTDOWN);
            msg.arg1 = mCountdown;
            mHandler.sendMessage(msg);
            if (mCountdown > 0) {
                mHandler.postDelayed(this, 1000);//每隔1秒向主線程發送一次Message
            }
        }
    };
}