可以动态修改时间的CountDownTimer

在做一个功能的时候，用到了CountDownTimer，做为一个倒计时功能类，用起来挺方便的。然而，我想动态修改倒计时时间，发现竟然没有这个接口！

经过一阵鼓捣，发现可以有两种方案来实现。

1，通过反射；
2，重写CountDownTimer，添加上修改倒计时时间的接口。

下面分别描述：

1，通过反射，动态修改倒计时时间

我们知道，在CountDownTimer的构造函数中有两个参数，如下：

public CountDownTimer(long millisInFuture, long countDownInterval)

通过查看源码，可以知道，这两个参数，是这样使用的：

    public CountDownTimer(long millisInFuture, long countDownInterval) {
        mMillisInFuture = millisInFuture;
        mCountdownInterval = countDownInterval;
    }

很显然，接收参数的，就是CountDownTimer类的成员变量了。查看定义如下：

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

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

描述也很简洁，mMillisInFuture是从启动到停止的毫秒数，也就是执行完成的总时间间隔，执行完成后会调用onFinish方法；
mCountdownInterval是用户接收回调的间隔时间，单位也是毫秒，执行的回调方法是onTick。
找到它们了，剩下的事情就是反射了。
我做了两个函数，来分别设置这两个参数：

//利用反射动态地改变CountDownTimer类的私有字段mCountdownInterval
    private void changeCountdownInterval(long time) {
        try {
            // 反射父类CountDownTimer的mCountdownInterval字段，动态改变回调频率
            Class clazz = Class.forName("android.os.CountDownTimer");
            Field field = clazz.getDeclaredField("mCountdownInterval");
            //从Toast对象中获得mTN变量
            field.setAccessible(true);
            field.set(timethis, time);          
        } catch (Exception e) {
            Log.e("Ye", "反射类android.os.CountDownTimer.mCountdownInterval失败：" + e);
        }
    }

//利用反射动态地改变CountDownTimer类的私有字段mMillisInFuture
    private void changeMillisInFuture(long time) {
        try {
            // 反射父类CountDownTimer的mMillisInFuture字段，动态改变定时总时间
            Class clazz = Class.forName("android.os.CountDownTimer");
            Field field = clazz.getDeclaredField("mMillisInFuture");
            field.setAccessible(true);
            field.set(timethis, time);
        } catch (Exception e) {
            Log.e("Ye", "反射类android.os.CountDownTimer.mMillisInFuture失败： "+ e);
        }
    }

说明下，这里的timethis，是一个CountDownTimer对象，

timethis = new CountDownTimer(365*24*3600 * 1000, 1000){...};

我这样定义，是避免定时器很快到时间了，导致异常，所以初始值定义到了一年这么长。其实，定义一个小间隔也没有问题，因为我们还没有启动定时器。

下面，就可以调用修改定时长度的函数了。

changeCountdownInterval(500);   
changeMillisInFuture(timeOutSeconds * 1000);    
timethis.start();   

2，重写CountDownTimer，添加上修改倒计时时间的接口

代码如下：

package cn.timer;

import android.os.Handler;
import android.os.Message;
import android.os.SystemClock;


/* 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.
        */

public abstract class CountDownTimerUtil {

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

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

    private long mStopTimeInFuture;

    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.
     */
    public CountDownTimerUtil(long millisInFuture, long countDownInterval) {
        mMillisInFuture = millisInFuture;
        mCountdownInterval = countDownInterval;
    }


    public final void setCountdownInterval(long countdownInterval) {
        mCountdownInterval = countdownInterval;
    }

    public final void setMillisInFuture(long millisInFuture) {
        mMillisInFuture = millisInFuture;       
    }

    /**
     * Cancel the countdown.
     *
     * Do not call it from inside CountDownTimer threads
     */
    public final void cancel() {
        mHandler.removeMessages(MSG);
        mCancelled = true;
    }

    /**
     * Start the countdown.
     */
    public synchronized final CountDownTimerUtil start() {
        if (mMillisInFuture <= 0) {
            onFinish();
            return this;
        }
        mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;
        mHandler.sendMessage(mHandler.obtainMessage(MSG));
        mCancelled = false;
        return this;
    }

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

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

    private static final int MSG = 1;

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

        @Override
        public void handleMessage(Message msg) {

            synchronized (CountDownTimerUtil.this) {
                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;

                    if (!mCancelled) {
                        sendMessageDelayed(obtainMessage(MSG), delay);
                    }
                }
            }
        }
    };
}

调用代码如下：

创建实例：

timethis = new CountDownTimerUtil(365*24*3600 * 1000, 1000){...};

在需要的时刻，进行动态修改：

    timethis.setCountdownInterval(500);
    timethis.setMillisInFuture(timeOutSeconds * 1000);
    timethis.start();   

如果是创建后就修改，这样顺序的执行，其实也没有什么特殊的效果。能显示动态修改价值的地方，是在某种时刻执行，例如，在按钮点击时执行，动态修改就很有必要了。

参考文章：
http://www.cnblogs.com/yexiubiao/archive/2013/01/02/2842026.html
http://blog.youkuaiyun.com/liuweiballack/article/details/46605787