想要寫一個倒計時函數(shù)，思路也就是獲取系統(tǒng)時間礼殊，與倒計時時間數(shù)計算來時間倒計才菠。沒想到Google官方有CountDownTimer API茸时，代碼比較短，小白也讀的懂赋访。

CountDownTimer簡單思路：

• 構造函數(shù)初始化倒計時總時間與間隔時間
• 虛擬函數(shù)onTick()與onFinish()交互倒計時結果
• 利用handler處理倒計時可都，根據(jù)系統(tǒng)時間獲取倒計時還剩余的時間，判斷反饋onTick（比如一秒tick一下）／繼續(xù)后臺倒計時／結束倒計時

但源碼也因為太簡單蚓耽，就與官方文檔的例子一樣渠牲，只能指定倒計時總時間與時間間隔。
本身自己的倒計時要求也不多步悠，就是想增加暫停與恢復倒計時功能签杈。但重載該class不太可行，主要是還需要修改handler鼎兽，干脆就直接粘貼復制該代碼答姥，添加了自己需要的幾行代碼。如此自定義后的countdownTimer代碼如下：（注釋中“!add”就是自己添加的代碼）

import android.os.Handler;
import android.os.Message;
import android.os.SystemClock;
/**
 * Schedule a countdown until a time in the future, with
 * regular notifications on intervals along the way.
 *官方文檔中的使用例子：
 * Example of showing a 30 second countdown in a text field:
 *
 * <pre class="prettyprint">
 * new CountDownTimer(30000, 1000) {
 *
 *     public void onTick(long millisUntilFinished) {
 *         mTextField.setText("seconds remaining: " + millisUntilFinished / 1000);
 *     }
 *
 *     public void onFinish() {
 *         mTextField.setText("done!");
 *     }
 *  }.start();
 * </pre>
 *
 * The calls to {@link #onTick(long)} are synchronized to this object so that
 * one call to {@link #onTick(long)} won't ever occur before the previous
 * callback is complete.  This is only relevant when the implementation of
 * {@link #onTick(long)} takes an amount of time to execute that is significant
 * compared to the countdown interval.
 */

/**
 * customize from CountDownTimer
 * Created by zhubingning on 16/09/16.
 */
public abstract class CustomCountDownTimer {


    /**
     * Millis since epoch when alarm should stop.
     */
    private final long mMillisInFuture;

    //!add,為了暫停時保存當前還剩下的毫秒數(shù)
    private long mCurrentMillisLeft;

    /**
     * The interval in millis that the user receives callbacks
     */
    private final long mCountdownInterval;

    private long mStopTimeInFuture;

    /**
     * boolean representing if the timer was cancelled
     */
    private boolean mCancelled = false;

    /**
     * @param millisInFuture The number of millis in the future from the call
     *   to {@link #start()} until the countdown is done and {@link #onFinish()}
     *   is called.
     * @param countDownInterval The interval along the way to receive
     *   {@link #onTick(long)} callbacks.
     */
    //構造函數(shù)谚咬，（總倒計時毫秒為單位踢涌，倒計時間隔）
    public CustomCountDownTimer(long millisInFuture, long countDownInterval) {
        mMillisInFuture = millisInFuture;
        mCountdownInterval = countDownInterval;
    }

    //!add, 獲取此時倒計時的總時間
    public long getCountTimes(){
        return mMillisInFuture;
    }

    /**
     * Cancel the countdown.
     */
    //取消倒計時，handler從消息隊列里取出message
    public synchronized final void cancel() {
        mCancelled = true;
        mHandler.removeMessages(MSG);
    }

    /**
     * Pause the countdown.
     */
    //序宦！add, 暫停，調用cancel（）函數(shù)背苦， mCurrentMillisLeft為全局變量自動保存
    public synchronized final void pause() {
       cancel();
    }

    /**
     * Resume the countdown.
     */
    //互捌！add, 恢復函數(shù)，根據(jù)mCurrentMillisLeft的值重新添加message開始倒計時
    public synchronized final void resume() {
        mCancelled=false;
        if (mCurrentMillisLeft <= 0) {
            onFinish();
            return ;
        }
        mStopTimeInFuture = SystemClock.elapsedRealtime() + mCurrentMillisLeft;
        mHandler.sendMessage(mHandler.obtainMessage(MSG));
        return ;
    }

    /**
     * Start the countdown.
     */
    //開始倒計時行剂，handler發(fā)送消息到隊列
    public synchronized final CustomCountDownTimer start() {
        mCancelled = false;
        if (mMillisInFuture <= 0) {
            onFinish();
            return this;
        }
        mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;
        mHandler.sendMessage(mHandler.obtainMessage(MSG));
        return this;
    }


    /**
     * Callback fired on regular interval.
     * @param millisUntilFinished The amount of time until finished.
     */
    //虛擬函數(shù)
    public abstract void onTick(long millisUntilFinished);

    /**
     * Callback fired when the time is up.
     */
    //虛擬函數(shù)
    public abstract void onFinish();


    private static final int MSG = 1;


    // handles counting down
    //handler
    private Handler mHandler = new Handler() {

        @Override
        public void handleMessage(Message msg) {

            //同步線程
            synchronized (CustomCountDownTimer.this) {
                //判斷倒計時是否已取消
                if (mCancelled) {
                    return;
                }

                //計算當前剩余毫秒數(shù)
                final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();


                //根據(jù)剩余毫秒數(shù)秕噪，或者結束倒計時，或者只延時厚宰，或者調用onTick并延時
                if (millisLeft <= 0) {
                    onFinish();
                } else if (millisLeft < mCountdownInterval) {
                    // no tick, just delay until done
                    onTick(0);//腌巾！add
                    sendMessageDelayed(obtainMessage(MSG), millisLeft);
                } else {
                    long lastTickStart = SystemClock.elapsedRealtime();
                    mCurrentMillisLeft=millisLeft;//！add
                    onTick(millisLeft);

                    // take into account user's onTick taking time to execute
                    long delay = lastTickStart + mCountdownInterval - SystemClock.elapsedRealtime();

                    // special case: user's onTick took more than interval to
                    // complete, skip to next interval
                    while (delay < 0) delay += mCountdownInterval;

                    sendMessageDelayed(obtainMessage(MSG), delay);
                }
            }
        }
    };
}

p.s. 附注知識：關于handler
“異步處理大師－handler“可以關聯(lián)任意線程并添加消息（即task）到消息隊列中铲觉，當相關線程的looper循環(huán)到自己的消息時澈蝙，能夠自動執(zhí)行該消息（異步），執(zhí)行完畢后再回到looper撵幽。
特別是上面的例子中handler中有delay處理灯荧，所以單獨用handler處理比較合適。

1.handler發(fā)送消息到消息隊列中

2.handler異步處理消息