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深入理解JavaScript系列(43):设计模式之状态模式

介绍状态模式(State)允许一个对象在其内部状态改变的时候改变它的行为,对象看起来似乎修改了它的类。正文举个例子,就比如我们平时在下载东西,通常就会有好几个状态,比如准备状态(ReadyState)、下载状态(DownloadingState)、暂停状态(DownloadPausedState)、下载完毕状态(DownloadedState)、失败状态(DownloadFailedState),也就是说在每个状态都只可以做当前状态才可以做的事情,而不能做其它状态能做的事儿。由于State模式描述了下载(Download)如何在每一种状态下表现出不同的行为。这一模式的关键思想就是引入了一个叫做State的抽象类(或JS里的函数)来表示下载状态,State函数(作为原型)为每个状态的子类(继承函数)声明了一些公共接口。其每个继承函数实现与特定状态相关的行为,比如DownloadingState和DownloadedState分别实现了正在下载和下载完毕的行为。这些行为可以通过Download来来维护。让我们来实现一把,首先定义作为其他基础函数的原型的State函数:var State = function () {};State.prototype.download = function () {    throw new Error("该方法必须被重载!");};State.prototype.pause = function () {    throw new Error("该方法必须被重载!");};State.prototype.fail = function () {    throw new Error("该方法必须被重载!");};State.prototype.finish = function () {    throw new Error("该方法必须被重载!");};我们为State的原型定义了4个方法接口,分别对应着下载(download)、暂停(pause)、失败(fail)、结束(finish)以便子函数可以重写。在编写子函数之前,我们先来编写一个ReadyState函数,以便可以将状态传递给第一个download状态:var ReadyState = function (oDownload) {    State.apply(this);    this.oDownload = oDownload;};ReadyState.prototype = new State();ReadyState.prototype.download = function () {    this.oDownload.setState(this.oDownload.getDownloadingState());    // Ready以后,可以开始下载,所以设置了Download函数里的状态获取方法 console.log("Start Download!");};ReadyState.prototype.pause = function () {    throw new Error("还没开始下载,不能暂停!");};ReadyState.prototype.fail = function () {    throw new Error("文件还没开始下载,怎么能说失败呢!");};ReadyState.prototype.finish = function () {    throw new Error("文件还没开始下载,当然也不能结束了!");};该函数接收了一个Download维护函数的实例作为参数,Download函数用于控制状态的改变和获取(类似于中央控制器,让外部调用),ReadyState重写了原型的download方法,以便开始进行下载。我们继续来看Download函数的主要功能:var Download = function () {    this.oState = new ReadyState(this);};Download.prototype.setState = function (oState) {    this.oState = oState;};// 对外暴露的四个公共方法,以便外部调用Download.prototype.download = function () {    this.oState.download();};Download.prototype.pause = function () {    this.oState.pause();};Download.prototype.fail = function () {    this.oState.fail();};Download.prototype.finish = function () {    this.oState.finish();};//获取各种状态,传入当前this对象Download.prototype.getReadyState = function () {    return new ReadyState(this);};Download.prototype.getDownloadingState = function () {    return new DownloadingState(this);};Download.prototype.getDownloadPausedState = function () {    return new DownloadPausedState(this);};Download.prototype.getDownloadedState = function () {    return new DownloadedState(this);};Download.prototype.getDownloadedFailedState = function () {    return new DownloadFailedState(this);};Download函数的原型提供了8个方法,4个是对用于下载状态的操作行为,另外4个是用于获取当前四个不同的状态,这4个方法都接收this作为参数,也就是将Download实例自身作为一个参数传递给处理该请求的状态对象(ReadyState 以及后面要实现的继承函数),这使得状态对象比必要的时候可以访问oDownlaod。接下来,继续定义4个相关状态的函数:var DownloadingState = function (oDownload) {    State.apply(this);    this.oDownload = oDownload;};DownloadingState.prototype = new State();DownloadingState.prototype.download = function () {    throw new Error("文件已经正在下载中了!");};DownloadingState.prototype.pause = function () { this.oDownload.setState(this.oDownload.getDownloadPausedState());    console.log("暂停下载!");};DownloadingState.prototype.fail = function () { this.oDownload.setState(this.oDownload.getDownloadedFailedState());    console.log("下载失败!");};DownloadingState.prototype.finish = function () {    this.oDownload.setState(this.oDownload.getDownloadedState());    console.log("下载完毕!");};DownloadingState的主要注意事项就是已经正在下载的文件,不能再次开始下载了,其它的状态都可以连续进行。var DownloadPausedState = function (oDownload) {    State.apply(this);    this.oDownload = oDownload;};DownloadPausedState.prototype = new State();DownloadPausedState.prototype.download = function () {    this.oDownload.setState(this.oDownload.getDownloadingState());    console.log("继续下载!");};DownloadPausedState.prototype.pause = function () {    throw new Error("已经暂停了,咋还要暂停呢!");};DownloadPausedState.prototype.fail = function () { this.oDownload.setState(this.oDownload.getDownloadedFailedState());    console.log("下载失败!");};DownloadPausedState.prototype.finish = function () {    this.oDownload.setState(this.oDownload.getDownloadedState());    console.log("下载完毕!");};DownloadPausedState函数里要注意的是,已经暂停的下载,不能再次暂停。var DownloadedState = function (oDownload) {    State.apply(this);    this.oDownload = oDownload;};DownloadedState.prototype = new State();DownloadedState.prototype.download = function () {    this.oDownload.setState(this.oDownload.getDownloadingState());    console.log("重新下载!");};DownloadedState.prototype.pause = function () {    throw new Error("对下载完了,还暂停啥?");};DownloadedState.prototype.fail = function () {    throw new Error("都下载成功了,咋会失败呢?");};DownloadedState.prototype.finish = function () {    throw new Error("下载成功了,不能再为成功了吧!");};DownloadedState函数,同理成功下载以后,不能再设置finish了,只能设置重新下载状态。var DownloadFailedState = function (oDownload) {    State.apply(this);    this.oDownload = oDownload;};DownloadFailedState.prototype = new State();DownloadFailedState.prototype.download = function () {    this.oDownload.setState(this.oDownload.getDownloadingState());    console.log("尝试重新下载!");};DownloadFailedState.prototype.pause = function () {    throw new Error("失败的下载,也不能暂停!");};DownloadFailedState.prototype.fail = function () {    throw new Error("都失败了,咋还失败呢!");};DownloadFailedState.prototype.finish = function () {    throw new Error("失败的下载,肯定也不会成功!");};同理,DownloadFailedState函数的失败状态,也不能再次失败,但可以和finished以后再次尝试重新下载。调用测试代码,就非常简单了,我们在HTML里演示吧,首先是要了jquery,然后有3个按钮分别代表:开始下载、暂停、重新下载。(注意在Firefox里用firebug查看结果,因为用了 console.log方法)。<html><head>    <link type="text/css" rel="stylesheet" href="http://www.cnblogs.com/css/style.css" />    <title>State Pattern</title>    <script type="text/javascript" src="http://www.mamicode.com/jquery.js"></script>    <script type="text/javascript" src="http://www.mamicode.com/Download.js"></script>    <script type="text/javascript" src="http://www.mamicode.com/states/State.js"></script>    <script type="text/javascript" src="http://www.mamicode.com/states/DownloadFailedState.js"></script>    <script type="text/javascript" src="http://www.mamicode.com/states/DownloadPausedState.js"></script>    <script type="text/javascript" src="http://www.mamicode.com/states/DownloadedState.js"></script>    <script type="text/javascript" src="http://www.mamicode.com/states/DownloadingState.js"></script>    <script type="text/javascript" src="http://www.mamicode.com/states/ReadyState.js"></script></head><body>    <input type="button" value="http://www.mamicode.com/开始下载" id="download_button" />    <input type="button" value="http://www.mamicode.com/暂停" id="pause_button" />    <input type="button" value="http://www.mamicode.com/重新下载" id="resume_button" />    <script type="text/javascript">        var oDownload = new Download();        $("#download_button").click(function () {            oDownload.download();        });        $("#pause_button").click(function () {            oDownload.pause();        });        $("#resume_button").click(function () {            oDownload.download();        });    </script></body></html>总结状态模式的使用场景也特别明确,有如下两点:一个对象的行为取决于它的状态,并且它必须在运行时刻根据状态改变它的行为。一个操作中含有大量的分支语句,而且这些分支语句依赖于该对象的状态。状态通常为一个或多个枚举常量的表示。参考:https://github.com/tcorral/Design-Patterns-in-Javascript/blob/master/State/1/index.html同步与推荐本文已同步至目录索引:深入理解JavaScript系列深入理解JavaScript系列文章,包括了原创,翻译,转载等各类型的文章,如果对你有用,请推荐支持一把,给大叔写作的动力。

 

深入理解JavaScript系列(43):设计模式之状态模式