倒计时器 CountDownTimer

使用介绍

开发中经常会遇到一些和倒计时有关的场景，比如发送验证码的按钮，会在点击发送后，显示倒计时间，倒计时结束后才能够刷新按钮，再次允许点击。为了不阻塞软件的运行，又要实时刷新界面，我们通常会用到 Handler 或者 AsyncTask 等技术，自己写逻辑实现。其实 Android 中已经封装好了一套 CountDownTimer 来实现这个功能需求。

CountDownTimer(long millisInFuture, long countDownInterval)

CountDownTimer的两个参数分别表示倒计时的总时间 millisInFuture 和间隔时间 countDownInterval。

具体的调用如下：

TextView vertifyBtn;

CountDownTimer timer = new CountDownTimer(60000, 1000) {

    @Override

    public void onTick(long millisUntilFinished) {

        vertifyBtn.setText((millisUntilFinished / 1000) + " second");

    }

    @Override

    public void onFinish() {

        vertifyBtn.setEnabled(true);

        vertifyBtn.setText("Send");

    }

};

timer.start();

上面的调用举例表示总计 60 秒，每 1 秒都会执行一次 onTick 方法，其参数 millisUntilFinished 表示倒计时剩余时间毫秒数，最后倒计时结束执行 onFinish 方法。

实现原理

下面是 CountDownTimer 的源码，代码非常少，很好理解。从源代码中可以看出，其实 CountDownTimer 也是利用 Handler 的消息处理机制来实现效果的。初始化设定好起始和终止时间后，每隔一定的间隔时间通过 Handler 给主线程发送消息，然后再在消息处理中回调方法。好好利用官方封装好的工具类，可以避免我们重复的造轮子，当然了解轮子的原理就更好了！

package android.os;

public abstract class CountDownTimer {

    private final long mMillisInFuture;

    private final long mCountdownInterval;

    private long mStopTimeInFuture;

    private boolean mCancelled = false;

    public CountDownTimer(long millisInFuture, long countDownInterval) {

        mMillisInFuture = millisInFuture;

        mCountdownInterval = countDownInterval;

    }

    public synchronized final void cancel() {

        mCancelled = true;

        mHandler.removeMessages(MSG);

    }

    public synchronized final CountDownTimer start() {

        mCancelled = false;

        if (mMillisInFuture <= 0) {

            onFinish();

            return this;

        }

        mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;

        mHandler.sendMessage(mHandler.obtainMessage(MSG));

        return this;

    }

    public abstract void onTick(long millisUntilFinished);

    public abstract void onFinish();

    private static final int MSG = 1;

    private Handler mHandler = new Handler() {

        @Override

        public void handleMessage(Message msg) {

            synchronized (CountDownTimer.this) {

                if (mCancelled) {

                    return;

                }

                final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();

                if (millisLeft <= 0) {

                    onFinish();

                } else if (millisLeft < mCountdownInterval) {

                    // no tick, just delay until done

                    sendMessageDelayed(obtainMessage(MSG), millisLeft);

                } else {

                    long lastTickStart = SystemClock.elapsedRealtime();

                    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);

                }

            }

        }

    };

}