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（七十六） CountDownTimer

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happyjiatai edited this page Jul 29, 2018 · 1 revision

demo:github.com/happyjiatai/demo_csdn/tree/master/demo_76_countdowntimer

1. CountDownTimer简介 ↑

源码上是这么解释的：Schedule a countdown until a time in the future, with regular notifications on intervals along the way.翻译一下就是说设一个定时器，每隔固定间隔提醒一下。看了api，其实这个类在定时的时间到了以后也会通知一下用户。

2. CountDownTimer demo ↑

demo(源码示例):

package com.example.demo_76_countdowntimer;

import android.os.Bundle; import android.os.CountDownTimer; import android.support.v7.app.AppCompatActivity; import android.util.Log; import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

private static final String TAG = "countdowntimer";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final TextView mTextField = findViewById(R.id.text);
    new CountDownTimer(15000, 1000) {

        public void onTick(long millisUntilFinished) {
            Log.d(TAG, "millisUntilFinished: " + millisUntilFinished);
            mTextField.setText("seconds remaining: " + millisUntilFinished / 1000);
        }

        public void onFinish() {
            Log.d(TAG, "onFinish() ");
            mTextField.setText("done!");
        }
    }.start();
}

} 效果：会从14依次减少，最后变为done!。

log:

07-27 19:59:29.346 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 14992 07-27 19:59:30.346 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 13992 07-27 19:59:31.346 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 12992 07-27 19:59:32.348 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 11990 07-27 19:59:33.350 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 10988 07-27 19:59:34.354 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 9984 07-27 19:59:35.358 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 8980 07-27 19:59:36.360 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 7978 07-27 19:59:37.364 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 6974 07-27 19:59:38.369 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 5969 07-27 19:59:39.371 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 4967 07-27 19:59:40.375 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 3963 07-27 19:59:41.379 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 2959 07-27 19:59:42.382 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 1956 07-27 19:59:43.385 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: millisUntilFinished: 953 07-27 19:59:44.341 19686-19686/com.example.demo_76_countdowntimer D/countdowntimer: onFinish()

3. 源码分析 ↑

/**

* Schedule a countdown until a time in the future, with
* regular notifications on intervals along the way.
*
* Example of showing a 30 second countdown in a text field:
*
* <pre class="prettyprint">
* new CountDownTimer(30000, 1000) {
*
*     public void onTick(long millisUntilFinished) {
*         mTextField.setText("seconds remaining: " + millisUntilFinished / 1000);
*     }
*
*     public void onFinish() {
*         mTextField.setText("done!");
*     }
*  }.start();
* </pre>
*
* 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 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 {
                long lastTickStart = SystemClock.elapsedRealtime();
                onTick(millisLeft);

                // take into account user's onTick taking time to execute
                long lastTickDuration = SystemClock.elapsedRealtime() - lastTickStart;
                long delay;

                if (millisLeft < mCountdownInterval) {
                    // just delay until done
                    delay = millisLeft - lastTickDuration;

                    // special case: user's onTick took more than interval to
                    // complete, trigger onFinish without delay
                    if (delay < 0) delay = 0;
                } else {
                    delay = mCountdownInterval - lastTickDuration;

                    // special case: user's onTick took more than interval to
                    // complete, skip to next interval
                    while (delay < 0) delay += mCountdownInterval;
                }

                sendMessageDelayed(obtainMessage(MSG), delay);
            }
        }
    }
};

} 源码实现其实一目了然，就是每隔某段时间发送一个消息回调onTick()方法，时间走完后回调onFinish()方法。

onTick()和onFinish()方法是抽象类，供调用方实现。

3.1 构造函数 ↑

指定定时时长和调用onTick时间间隔

/**
 * @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;
}

3.2 start() ↑

计算一下停止时间，然后开始发送消息供handler处理。

/**
 * 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;
}

3.3 cancel ↑

停止消息处理

/**
 * Cancel the countdown.
 */
public synchronized final void cancel() {
    mCancelled = true;
    mHandler.removeMessages(MSG);
}

3.4 handler ↑ 

// 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 {
                  long lastTickStart = SystemClock.elapsedRealtime();
                  onTick(millisLeft);

                  // take into account user's onTick taking time to execute
                  long lastTickDuration = SystemClock.elapsedRealtime() - lastTickStart;
                  long delay;

                  if (millisLeft < mCountdownInterval) {
                      // just delay until done
                      delay = millisLeft - lastTickDuration;

                      // special case: user's onTick took more than interval to
                      // complete, trigger onFinish without delay
                      if (delay < 0) delay = 0;
                  } else {
                      delay = mCountdownInterval - lastTickDuration;

                      // special case: user's onTick took more than interval to
                      // complete, skip to next interval
                      while (delay < 0) delay += mCountdownInterval;
                  }

                  sendMessageDelayed(obtainMessage(MSG), delay);
              }
          }
      }
  };

这个handler是直接new处理的，说明是运行在新建线程的，demo是运行在主线程中，这意味着

onTick和onFinish不能执行耗时操作mHandler持有Context的引用，如果退出应用时不调用cancel，容易造成内存泄露两个操作可以证实内存泄露问题：

1.打开demo然后点击返回键，发现log仍然继续打印

2.点击返回键后再点击demo 图标，发现会有两个计时器在一起跑。

07-27 20:15:00.475 451-29656/? W/PackageManager: Failure retrieving resources for com.example.demo_76_countdowntimer: Resource ID #0x7f0a0000 07-27 20:15:04.752 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 14981 07-27 20:15:05.753 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 13980 07-27 20:15:06.754 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 12979 07-27 20:15:07.758 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 11976 07-27 20:15:08.759 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 10975 07-27 20:15:09.760 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 9974 07-27 20:15:10.765 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 8969 07-27 20:15:11.766 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 7968 07-27 20:15:12.771 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 6963 07-27 20:15:13.773 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 5961 07-27 20:15:14.803 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 4930 07-27 20:15:15.149 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 14987 07-27 20:15:15.804 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 3929 07-27 20:15:16.149 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 13987 07-27 20:15:16.805 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 2928 07-27 20:15:17.149 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 12987 07-27 20:15:17.806 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 1927 07-27 20:15:18.151 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 11985 07-27 20:15:18.809 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 924 07-27 20:15:19.156 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 10980 07-27 20:15:19.737 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: onFinish() 07-27 20:15:20.160 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 9976 07-27 20:15:21.164 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 8973 07-27 20:15:22.166 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 7970 07-27 20:15:23.171 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 6966 07-27 20:15:24.175 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 5961 07-27 20:15:25.180 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 4957 07-27 20:15:26.181 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 3955 07-27 20:15:27.186 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 2951 07-27 20:15:28.190 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 1947 07-27 20:15:29.193 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread millisUntilFinished: 943 07-27 20:15:30.139 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: onFinish() 接下来看下handeMessage是如何处理的：

1）加锁，考虑到多线程情况

2）cancel方法不是说只通过移除消息肯定会取消下一次操作的，这里加了个保险，如果cancel为true，则不继续执行。

3）计算下剩余时间，到了就回调onFinish()方法。

final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();

if (millisLeft <= 0) {
    onFinish();
}

4）时间没到的话，以执行onTick方法的时长为依据设定消息发送延迟。

如果剩余时长小于时间间隔，那么delay为剩余时长减去onTick方法的执行时长，如果小于0，则delay为0。

如果剩余时长大于时间间隔，那么delay为时间间隔减去onTick方法的执行时长，如果小于0则补时间间隔，直到大于0。

举个例子：

总时长15s，间隔1s(mCountdownInterval)，执行onTick方法2.5s

millisLeft = 15s

先花费2.5s执行onTick方法，lastTickDuration=2.5s，剩余12.5s

走进if判断，由于millisLeft>mCountdownInterval，走进下面的else

1 - 2.5 = -1.5s,-1.5+1+1=0.5，下次消息将在0.5s后发出。

这样来看是3s发送一次消息。

例子举完总结一下，就是剩余时间不够一次间隔了，那么就从消息处理开始等剩余时间走完发送最后的消息；如果剩余时间够一次间隔，就从消息处理开始等n个间隔发送消息，n取决于onTick的执行时长是时间间隔的多少倍向上取整。

} else {
    long lastTickStart = SystemClock.elapsedRealtime();
    onTick(millisLeft);

    // take into account user's onTick taking time to execute
    long lastTickDuration = SystemClock.elapsedRealtime() - lastTickStart;
    long delay;

    if (millisLeft < mCountdownInterval) {
        // just delay until done
        delay = millisLeft - lastTickDuration;

        // special case: user's onTick took more than interval to
        // complete, trigger onFinish without delay
        if (delay < 0) delay = 0;
    } else {
        delay = mCountdownInterval - lastTickDuration;

        // special case: user's onTick took more than interval to
        // complete, skip to next interval
        while (delay < 0) delay += mCountdownInterval;
    }

    sendMessageDelayed(obtainMessage(MSG), delay);
}

4. 总结 ↑

使用CountDownTimer可以实现一些简单的固定间隔操作，定时结束后完成特定目标的需求（固定间隔定的是1s，但有可能是2s一刷新，取决于onTick执行时长），另外要注意其如果是在UI线程new出来的，那么onTick和onFinish不要执行耗时操作。CountDownTimer提前结束或者activity异常退出记得调用它的cancel方法，不然会有内存泄露。正常情况下使用handler请使用静态内部类加虚引用来规避内存泄露的风险。

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