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demo:https://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[main,5,main] millisUntilFinished: 14981
07-27 20:15:05.753 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 13980
07-27 20:15:06.754 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 12979
07-27 20:15:07.758 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 11976
07-27 20:15:08.759 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 10975
07-27 20:15:09.760 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 9974
07-27 20:15:10.765 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 8969
07-27 20:15:11.766 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 7968
07-27 20:15:12.771 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 6963
07-27 20:15:13.773 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 5961
07-27 20:15:14.803 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 4930
07-27 20:15:15.149 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 14987
07-27 20:15:15.804 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 3929
07-27 20:15:16.149 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 13987
07-27 20:15:16.805 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 2928
07-27 20:15:17.149 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 12987
07-27 20:15:17.806 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 1927
07-27 20:15:18.151 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 11985
07-27 20:15:18.809 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 924
07-27 20:15:19.156 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] 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[main,5,main] millisUntilFinished: 9976
07-27 20:15:21.164 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 8973
07-27 20:15:22.166 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 7970
07-27 20:15:23.171 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 6966
07-27 20:15:24.175 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 5961
07-27 20:15:25.180 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 4957
07-27 20:15:26.181 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 3955
07-27 20:15:27.186 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 2951
07-27 20:15:28.190 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] millisUntilFinished: 1947
07-27 20:15:29.193 20938-20938/com.example.demo_76_countdowntimer D/countdowntimer: Thread[main,5,main] 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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