在做一個功能的時候，用到了CountDownTimer，做爲一個倒計時功能類，用起來挺方便的。然而，我想動態修改倒計時時間，發現竟然沒有這個接口！
經過一陣鼓搗，發現可以有兩種方案來實現。
1，通過反射；
2，重寫CountDownTimer，添加上修改倒計時時間的接口。
下面分別描述：
1，通過反射，動態修改倒計時時間
我們知道，在CountDownTimer的構造函數中有兩個參數，如下：
public CountDownTimer(long millisInFuture, long countDownInterval)通過查看源碼，可以知道，這兩個參數，是這樣使用的：
public CountDownTimer(long millisInFuture, long countDownInterval) {
mMillisInFuture = millisInFuture;
mCountdownInterval = countDownInterval;
}很顯然，接收參數的，就是CountDownTimer類的成員變量了。查看定義如下：
/**
* Millis since epoch when alarm should stop.
*/
private final long mMillisInFuture;
/**
* The interval in millis that the user receives callbacks
*/
private final long mCountdownInterval;描述也很簡潔，mMillisInFuture是從啓動到停止的毫秒數，也就是執行完成的總時間間隔，執行完成後會調用onFinish方法；
mCountdownInterval是用戶接收回調的間隔時間，單位也是毫秒，執行的回調方法是onTick。
找到它們了，剩下的事情就是反射了。
我做了兩個函數，來分別設置這兩個參數：
//利用反射動態地改變CountDownTimer類的私有字段mCountdownInterval
private void changeCountdownInterval(long time) {
try {
// 反射父類CountDownTimer的mCountdownInterval字段，動態改變回調頻率
Class clazz = Class.forName("android.os.CountDownTimer");
Field field = clazz.getDeclaredField("mCountdownInterval");
//從Toast對象中獲得mTN變量
field.setAccessible(true);
field.set(timethis, time);
} catch (Exception e) {
Log.e("Ye", "反射類android.os.CountDownTimer.mCountdownInterval失敗：" + e);
}
}
//利用反射動態地改變CountDownTimer類的私有字段mMillisInFuture
private void changeMillisInFuture(long time) {
try {
// 反射父類CountDownTimer的mMillisInFuture字段，動態改變定時總時間
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", "反射類android.os.CountDownTimer.mMillisInFuture失敗： "+ e);
}
}說明下，這裏的timethis，是一個CountDownTimer對象，
timethis = new CountDownTimer(365*24*3600 * 1000, 1000){...};我這樣定義，是避免定時器很快到時間了，導致異常，所以初始值定義到了一年這麼長。其實，定義一個小間隔也沒有問題，因爲我們還沒有啓動定時器。
下面，就可以調用修改定時長度的函數了。
changeCountdownInterval(500);
changeMillisInFuture(timeOutSeconds * 1000);
timethis.start(); 2，重寫CountDownTimer，添加上修改倒計時時間的接口
代碼如下：
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);
}
}
}
}
};
}調用代碼如下：
創建實例：
timethis = new CountDownTimerUtil(365*24*3600 * 1000, 1000){...};在需要的時刻，進行動態修改：
timethis.setCountdownInterval(500);
timethis.setMillisInFuture(timeOutSeconds * 1000);
timethis.start(); 如果是創建後就修改，這樣順序的執行，其實也沒有什麼特殊的效果。能顯示動態修改價值的地方，是在某種時刻執行，例如，在按鈕點擊時執行，動態修改就很有必要了。
參考文章：
http://www.cnblogs.com/yexiubiao/archive/2013/01/02/2842026.html
http://blog.csdn.net/liuweiballack/article/details/46605787