最近在做一个项目，需要用到列表倒计时功能，捣鼓半天终于弄了出来，在安卓中实现这个效果需要用到Countdowntimer,通过这个类的使用，不仅可以实现倒计时的效果，还可以完美解决在实现倒计时过程中的两个bug。

1.内存问题
2.由于recyclerview的item复用导致不同条目的时间错乱

首先看下实现的最终效果

如何显示列表我相信大家都会，这里我只附上和倒计时功能实现的adapter类。

public class ClockAdapter extends RecyclerView.Adapter<ClockAdapter.ClockViewHolder> {
 private SparseArray<CountDownTimer> countDownMap = new SparseArray<>();

 @Override
 public ClockViewHolder onCreateViewHolder(ViewGroup parent,int viewType) {
  View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_rv,parent,false);

  return new ClockViewHolder(view);
 }
 /**
  * 清空资源
  */
 public void cancelAllTimers() {
  if (countDownMap == null) {
   return;
  }
  for (int i = 0,length = countDownMap.size(); i < length; i++) {
   CountDownTimer cdt = countDownMap.get(countDownMap.keyAt(i));
   if (cdt != null) {
    cdt.cancel();
   }
  }
 }

 @Override
 public void onBindViewHolder(final ClockViewHolder holder,int position) {
  long betweenDate;
  if (position == 0) {
   betweenDate= DateUtil.getLeftTime("2017-8-8 12:10:10");
  } else {
   betweenDate= DateUtil.getLeftTime("2017-8-9 15:10:10");
  }

  if (holder.countDownTimer != null) {
   holder.countDownTimer.cancel();
  }

  if (betweenDate > 0) {
   holder.countDownTimer = new CountDownTimer(betweenDate,1000) {
    public void onTick(long millisUntilFinished) {
     millisUntilFinished = millisUntilFinished / 1000;
     int hours = (int) (millisUntilFinished / (60 * 60));
     int leftSeconds = (int) (millisUntilFinished % (60 * 60));
     int minutes = leftSeconds / 60;
     int seconds = leftSeconds % 60;

     final StringBuffer sBuffer = new StringBuffer();
     sBuffer.append(addZeroPrefix(hours));
     sBuffer.append(":");
     sBuffer.append(addZeroPrefix(minutes));
     sBuffer.append(":");
     sBuffer.append(addZeroPrefix(seconds));
     holder.clock.setText(sBuffer.toString());
    }
    public void onFinish() {
//     时间结束后进行相应逻辑处理
    }
   }.start();
   countDownMap.put(holder.clock.hashCode(),holder.countDownTimer);
  } else {
//   时间结束 进行相应逻辑处理
  }

 }

 @Override
 public int getItemCount() {
  return 25;
 }

 class ClockViewHolder extends RecyclerView.ViewHolder {

  TextView clock;
  CountDownTimer countDownTimer;

  public ClockViewHolder(View itemView) {
   super(itemView);
   clock = (TextView) itemView.findViewById(R.id.clock);
  }
 }
}

其中cancelAllTimer()这个方法解决了内存的问题，通过这行代码，将item的hashcode作为key设入SparseArray中，这样在cancelAllTimer方法中可以一个一个取出来进行倒计时取消操作。

countDownMap.put(holder.clock.hashCode(),holder.countDownTimer);

接着通过下面这行代码新建一个CountDownTimer类

holder.countDownTimer = new CountDownTimer(betweenDate,1000) {
 public void onTick(long millisUntilFinished) {
 millisUntilFinished = millisUntilFinished / 1000;
 int hours = (int) (millisUntilFinished / (60 * 60));
 int leftSeconds = (int) (millisUntilFinished % (60 * 60));
 int minutes = leftSeconds / 60;
 int seconds = leftSeconds % 60;
 final StringBuffer sBuffer = new StringBuffer();
 sBuffer.append(addZeroPrefix(hours));
 sBuffer.append(":")   sBuffer.append(addZeroPrefix(minutes));
     sBuffer.append(":");
     sBuffer.append(addZeroPrefix(seconds));
     holder.clock.setText(sBuffer.toString());
}
public void onFinish() {
// 时间结束后进行相应逻辑处理
}
}.start();

分析它的源码

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

从中可以很清楚的看出，设置了两个值，第一个是倒计时结束时间，第二个是刷新时间的间隔时间。
然后通过start方法进行启动，接着看下start方法中进行的处理

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

源码中，当倒计时截止时间小于等0时也就是倒计时结束时，调用了onFinish方法，若时间还未结束，则通过handler的异步消息机制，将消息进行发出，通过一整个流程，最终方法会走到handler的handleMessage方法中，如果有不熟悉这个异步流程的伙伴，可以去看我以前写的一篇异步消息机制的文章 android异步消息机制，源码层面彻底解析。好了，接下来就来看看handler的handleMessage方法。

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);
    }
   }
  }
 };

相信这段源码还是很通熟易懂，首先计算出剩余时间，如果剩余时间小于刷新时间，就发送一条延时消息直到时间结束，如果剩余时间大于刷新时间就调用onTick(millisLeft)方法，这个方法在我们创建CountDownTimer类时就进行过重写，在里面就可以写我们倒计时展示的具体逻辑了。至此整个流程结束。

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