CountDownTimer

Android API自带的"计时"类，简单分析源码

1、CountDownTimer的启动：

    /**
     * Start the countdown.
     */
    public synchronized final CountDownTimer start() {
//开关，每次start必须确保开关是打开状态
        mCancelled = false;
//总时间不能小于0
        if (mMillisInFuture <= 0) {
//结束的回调函数
            onFinish();
            return this;
        }
//结束时间为当前时间+总时间
        mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;
//发送消息
        mHandler.sendMessage(mHandler.obtainMessage(MSG));
        return this;
    }

2、Handler 接收消息

    private Handler mHandler = new Handler() {

        @Override
        public void handleMessage(Message msg) {
//线程锁，多线程操作可能会导致millisLeft 的差异
            synchronized (CountDownTimer.this) {
//开关为关闭状态
                if (mCancelled) {
                    return;
                }

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

                if (millisLeft <= 0) {
                    onFinish();
//mCountdownInterval为计时的时间间隔
                } else if (millisLeft < mCountdownInterval) {
                    // no tick, just delay until done  剩余时间小于间隔时间，直接发送延迟剩余时间
                    sendMessageDelayed(obtainMessage(MSG), millisLeft);
                }
剩余时间大于间隔时间，直接发送延迟间隔时间
 else {
                    long lastTickStart = SystemClock.elapsedRealtime();
//每次计时回调该函数，同时传递剩余时间
                    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  最后回调一次onTick
                    while (delay < 0) delay += mCountdownInterval;

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

3、取消计时：

    public synchronized final void cancel() {
        mCancelled = true;
        mHandler.removeMessages(MSG);
    }

为何取消的时候handleMessage不执行onFinish()呢？
理当加上啊..
4、关于SystemClock
常用的API；

        SystemClock.currentThreadTimeMillis();//1970 00:00:00到现在过去的毫秒数 
        SystemClock.elapsedRealtime();//手机从启动到现在的运行时间，且包括系统sleep(CPU关闭)的时间
        SystemClock.uptimeMillis();//手机从启动到现在的运行时间，且不包括系统sleep(CPU关闭)的时间

5、小结
完全使用的Handler机制，未另外开辟新的线程或者Timer，延迟间隔时间后sendMessages，handler收到消息后继续sendMessages，简单的递归，很巧妙，用法自定义View里面很常见。