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;  

    @Override
    public 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() {
            @Override
            public 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 down
    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) {
                    // 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interval
                    while (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

    @Override
    public 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

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