CountDownTimer浅析

一，概述

在项目开发中经常会用到倒计时这个功能，而Android也帮我们封装好了一个类CountDownTimer，给我们的开发带来了很大的方便；

二，API

CountDownTimer (long millisInFuture, long countDownInterval)
参数1，设置倒计时的总时间（毫秒）
参数2，设置每次减去多少毫秒

三，基本用法

以App中获短信取验证码为例：

    private Button btn;private TextView vertifyView;  @Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);initView();}private void initView(){vertifyView =(TextView) findViewById(R.id.vertifyView);btn =(Button) findViewById(R.id.button);btn.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {//1,请求后台...//2,触发定时器刷新UI（启动代码最好放在请求后台回调成功之后）timer.start();}});}private CountDownTimer timer = new CountDownTimer(10000, 1000) {  @Override  public void onTick(long millisUntilFinished) {  vertifyView.setText((millisUntilFinished / 1000) + "秒后可重发");  }  @Override  public void onFinish() {  vertifyView.setEnabled(true);  vertifyView.setText("获取验证码");  }  };

ok~这样一个基本的CountDownTimer案例就完成了

四，存在的问题

CountDownTimer如果使用不当，常常会报空指针异常，甚至造成严重的内存泄漏
5.0源码：

public abstract class CountDownTimer {/*** Millis since epoch when alarm should stop.*/private final long mMillisInFuture;/*** The interval in millis that the user receives callbacks*/private final long mCountdownInterval;private long mStopTimeInFuture;/*** boolean representing if the timer was cancelled*/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 CountDownTimer(long millisInFuture, long countDownInterval) {mMillisInFuture = millisInFuture;mCountdownInterval = countDownInterval;}/*** Cancel the countdown.*/public synchronized final void cancel() {mCancelled = true;mHandler.removeMessages(MSG);}/*** Start the countdown.*/public synchronized final CountDownTimer start() {mCancelled = false;if (mMillisInFuture <= 0) {onFinish();return this;}mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;mHandler.sendMessage(mHandler.obtainMessage(MSG));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 downprivate Handler mHandler = new Handler() {@Overridepublic void handleMessage(Message msg) {synchronized (CountDownTimer.this) {if (mCancelled) {return;}final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();if (millisLeft <= 0) {onFinish();} else if (millisLeft < mCountdownInterval) {// 剩余时间小于一次时间间隔的时候，不再通知，只是延迟一下sendMessageDelayed(obtainMessage(MSG), millisLeft);} else {long lastTickStart = SystemClock.elapsedRealtime();onTick(millisLeft);// 处理用户onTick执行的时间long delay = lastTickStart + mCountdownInterval - SystemClock.elapsedRealtime();// 特殊情况：用户的onTick方法花费的时间比interval长，那么直接跳转到下一次intervalwhile (delay < 0) delay += mCountdownInterval;sendMessageDelayed(obtainMessage(MSG), delay);}}}};
}

从源码中我们可以看出，CountDownTimer的内部实现是采用Handler机制，通过sendMessageDelayed延迟发送一条message到主线程的looper中，然后在自身中收到之后判断剩余时间，并发出相关回调，然后再次发出message的方式。

这样的方式其实是有一定弊端的，那就是如果在Activity或者Fragment被回收时并未调用CountDownTimer的cancel()方法结束自己，这个时候CountDownTimer的Handler方法中如果判断到当前的时间未走完，那么会继续调用

sendMessageDelayed(obtainMessage(MSG), delay);

触发

onTick(millisLeft);

当回调了Activity或者fragment中CountDownTimer的onTick方法时，Activity或者Fragment已经被系统回收，从而里面的变量被设置为Null，再调用

vertifyView.setText((millisUntilFinished / 1000) + "秒后可重发");

vertifyView为空，也就空指针了~
同时，CountDownTimer中的Handler方法还在继续执行，这一块空间始终无法被系统回收也就造成了内存泄漏问题。

五，总结

1，在CountDownTimer的onTick方法中记得判空

activity中if(!activity.isFinishing()){//doing something...}fragment中if(getActivity()!=null){//doing something...}

2，在配合DialogFragment使用时，如果在onFinish()方法调用了 dismiss()方法让弹框消失，记得判断getFragmentManager是否为空

    @Overridepublic void onFinish() {if(getFragmentManager()!=null){dismiss();}}

3，在使用CountDownTimer时，在宿主Activity或fragment生命周期结束的时候，记得调用timer.cancle()方法

@Overridepublic void onDestroy() {if(timer!=null){timer.cancel();timer = null;}super.onDestroy();}

遇到问题还是尽量先从控件的源码中寻找答案~相信源码是最好的老师O(∩_∩)O哈哈~