你好,今天我要和大家分享一些东西,举例来说这个在JavaScript中用的很多。我要讲讲回调(callbacks)。你知道什么时候用,怎么用这个吗?你真的理解了它在java环境中的用法了吗?当我也问我自己这些问题,这也是我开始研究这些的原因。这个背后的思想是控制反转( PS:*的解释是控制反转(Inversion of Control,缩写为IoC),是面向对象编程中的一种设计原则,可以用来减低计算机代码之间的耦合度。)这个范例描述了框架(framework)的工作方式,也以“好莱坞原则:不要打电话给我们,我们会打给你("Hollywood principle - Don't call me, we will call you)”为人们所熟知。
简单的Java里的回调模式来理解它,具体的例子在下面:
1 interface CallBack {
2 void methodToCallBack();
3 }
4
5 class CallBackImpl implements CallBack {
6 public void methodToCallBack() {
7 System.out.println("I've been called back");
8 }
9 }
10
11 class Caller {
12
13 public void register(CallBack callback) {
14 callback.methodToCallBack();
15 }
16
17 public static void main(String[] args) {
18 Caller caller = new Caller();
19 CallBack callBack = new CallBackImpl();
20 caller.register(callBack);
21 }
22 }
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你可能要问我,什么时候用这个或者会问直接调用和回调机制有什么不同呢?
答案是:好吧,这个例子仅仅向你展示了怎样在java环境中构造这样的回调函数。当然用那种方式使用它毫无意义。让我们现在更加深入具体地研究它。
在它之中的思想是控制反转。让我们用定时器作为现实中的例子。假设你知道,有一个特别的定时器支持每小时回调的功能。准确地说意思是,每小时,定时器会调用你注册的调用方法。
具体的例子:
我们想要每小时更新一次网站的时间,下面是例子的UML模型:
回调接口:
让我们首先定义回调接口:
1 import java.util.ArrayList;
2 import java.util.List;
3
4 // For example: Let's assume that this interface is offered from your OS to be implemented
5 interface TimeUpdaterCallBack {
6 void updateTime(long time);
7 }
8
9 // this is your implementation.
10 // for example: You want to update your website time every hour
11 class WebSiteTimeUpdaterCallBack implements TimeUpdaterCallBack {
12
13 @Override
14 public void updateTime(long time) {
15 // print the updated time anywhere in your website's example
16 System.out.println(time);
17 }
18 }
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在我们的例子中系统定时器支持回调方法:
1 // This is the SystemTimer implemented by your Operating System (OS)
2 // You don't know how this timer was implemented. This example just
3 // show to you how it could looks like. How you could implement a
4 // callback by yourself if you want to.
5 class SystemTimer {
6
7 List<TimeUpdaterCallBack> callbacks = new ArrayList<TimeUpdaterCallBack>();
8
9 public void registerCallBackForUpdatesEveryHour(TimeUpdaterCallBack timerCallBack) {
10 callbacks.add(timerCallBack);
11 }
12
13 // ... This SystemTimer may have more logic here we don't know ...
14
15 // At some point of the implementaion of this SystemTimer (you don't know)
16 // this method will be called and every registered timerCallBack
17 // will be called. Every registered timerCallBack may have a totally
18 // different implementation of the method updateTime() and my be
19 // used in different ways by different clients.
20 public void oneHourHasBeenExprired() {
21
22 for (TimeUpdaterCallBack timerCallBack : callbacks) {
23 timerCallBack.updateTime(System.currentTimeMillis());
24 }
25 }
26 }
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最后是我们虚拟简单的例子中的网站时间更新器:
1 // This is our client. It will be used in our WebSite example. It shall update
2 // the website's time every hour.
3 class WebSiteTimeUpdater {
4
5 public static void main(String[] args) {
6 SystemTimer SystemTimer = new SystemTimer();
7 TimeUpdaterCallBack webSiteCallBackUpdater = new WebSiteTimeUpdaterCallBack();
8 SystemTimer.registerCallBackForUpdatesEveryHour(webSiteCallBackUpdater);
9 }
10 }
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最新内容请见作者的GitHub页:http://qaseven.github.io/