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);
}
}
}
};
}