提及发布订阅，我们都知道是一种比较经典的设计模式。比如像redux等比较流行的一些库或者一些前端框架底层都会用来作为通讯机制，那么我们今天就来封装一个基于发布订阅的组件。

设计一个发布订阅的类库jpslib

jpslib这个名字代表了简单的发布订阅模式的类库，是实现我们目的工具。
先打造好工具，然后我们打算使用这个工具从’猿’进化为’员’。

API设计及实现

• 首先，我们需要抽象一个订阅者，just show me the code:

  /*** base class of subscriber*/jps.ISubscriber = function () {};/*** must be implemented* interface to observe subscriber's interested notification*/jps.ISubscriber.prototype.listNotificationInterested = function () { return [];};/*** must be implemented* interface to execute a notification interested by the subscriber*/jps.ISubscriber.prototype.executeNotification = function (notification) { };

订阅者的职责第一便是列出他感兴趣的主题listNotificationInterested，第二是对于收到订阅的主题消息的处理executeNotification

• 其次，我们抽象一个消息模型，show me the code:

  /*** base class of notification*/ jps.INotification = function (name, body, type) { if(name === null || name === undefined) { throw new Error('notification must have a name'); } if(typeof name !== 'string') { throw new TypeError('notification name must be string type'); } this._name = name; this._body = body; this._type = type;};/*** interface to get notification's name*/jps.INotification.prototype.getName = function () { return this._name;};/*** interface to get notification's body*/jps.INotification.prototype.getBody = function () { return this._body;};/*** interface to get notification's type*/jps.INotification.prototype.getType = function () { return this._type;};

  消息对象需要有他的具体名字、消息体、消息类型，其实一般我们只需要只知道消息名便可，消息类型只是用来以后做扩展使用。

• 最后，是一个管理者的实现

  /** * interface to subscribe notification */ jps.subscribe = function (subscriber) { if(subscriber.listNotificationInterested && typeof subscriber.listNotificationInterested === 'function' && subscriber.executeNotification && typeof subscriber.executeNotification === 'function') { var names = subscriber.listNotificationInterested(); if(names instanceof Array) { //check names type names.forEach(function (name) { if(typeof name === 'string') { //do nothing } else { throw new Error('interested notification name must be String type'); } }); //clear jps._removeSubscribersFromMap(subscriber); //add names.forEach(function (name) { jps._addSubscriberToMap(name, subscriber); }); } else { throw new Error('interface listNotificationInterested of subscriber must return Array type'); } } else { throw new Error('subscriber must implement ISubscriber'); } }; /** * interface to publish notification */ jps.publish = function (notification) { if(notification.getName && typeof notification.getName === 'function') { var subs = jps._getSubscribersFromMap(notification.getName()).concat(); subs.forEach(function (ele) { ele.executeNotification(notification); }); } else { throw new Error('notification must implement INotification'); } }; /** * interface to create new notification object */ jps.createNotification = function (name, data, type) { if(typeof name === 'string') { var Notification = function (name, data, type) { jps.INotification.call(this, name, data, type); }; jps._utils.extendClass(Notification, jps.INotification); return new Notification(name, data, type); } else { throw new Error('notification name must be String type'); } }; /** * interface to unsubscribe notification interested by subscriber */ jps.unsubscribe = function (notificationname, subscriber) { if(subscriber.listNotificationInterested && typeof subscriber.listNotificationInterested === 'function') { if(typeof notificationname === 'string') { jps._removeSubscriberFromMap(notificationname, subscriber); } else { throw new Error('interested notification name must be String type'); } } else { throw new Error('subscriber must implement ISubscriber'); } }; /** * interface to unsubscribe all the notification interested by subscriber */ jps.unsubscribeAll = function (subscriber) { if(subscriber.listNotificationInterested && typeof subscriber.listNotificationInterested === 'function') { if(typeof notificationname === 'string') { jps._removeSubscribersFromMap(subscriber); } else { throw new Error('interested notification name must be String type'); } } else { throw new Error('subscriber must implement ISubscriber'); } }; /** * the map from notification name to subscriber */ jps._notificationMap = {}; /** * get the subscribers from map by the notification name */ jps._getSubscribersFromMap = function (notificationname) { if(jps._notificationMap[notificationname] === undefined) { return []; } else { return jps._notificationMap[notificationname]; } }; /** * get the interested names from map by the subscriber */ jps._getInterestedNamesFromMap = function (subscriber) { var retArr = []; var arr; for(var name in jps._notificationMap) { arr = jps._notificationMap[name].filter(function (ele) { return ele === subscriber; }); if(arr.length > 0) { retArr.push(name); } } return retArr; }; /** * check the name is subscribed by the subscriber from map */ jps._hasSubscriberFromMap = function (name, subscriber) { var names = jps._getInterestedNamesFromMap(subscriber); var retArr = names.filter(function (ele) { return ele === name; }); return retArr.length > 0; }; /** * add the subscriber to the map */ jps._addSubscriberToMap = function (name, subscriber) { if(jps._hasSubscriberFromMap(name, subscriber)) { //do nothing } else { var subs = jps._getSubscribersFromMap(name); if(subs.length === 0) { subs = jps._notificationMap[name] = []; } subs.push(subscriber); } return true; }; /** * remove the subscriber from the map */ jps._removeSubscriberFromMap = function (name, subscriber) { var subs = jps._getSubscribersFromMap(name); var idx = subs.indexOf(subscriber); if(idx > -1) { subs.splice(idx, 1); } else { //do nothing } return subs; }; /** * remove all observed notification for the subscriber */ jps._removeSubscribersFromMap = function (subscriber) { var subs; var names = jps._getInterestedNamesFromMap(subscriber); names.forEach(function (name) { var subs = jps._removeSubscriberFromMap(name, subscriber); if(subs && subs.length === 0) { delete jps._notificationMap[name]; } }); return true; }; jps._utils = { 'extendClass': function (child, parent) { if(typeof child !== 'function') throw new TypeError('extendClass child must be function type'); if(typeof parent !== 'function') throw new TypeError('extendClass parent must be function type'); if(child === parent) return ; var Transitive = new Function(); Transitive.prototype = parent.prototype; child.prototype = new Transitive(); return child.prototype.constructor = child; } };

这里的代码有点长，我们仔细来看一遍：

• 首先是一个工具对象jps._utils，里面是有一些辅助方法，目前我们需要的类继承。
• 其次是一个key/value的map：jps._notificationMap，用来建立订阅主题名与订阅者的映射。

• 最后是管理者的实现。

  在这些实现中，我们看到，无非是增删改查。但是我们需要对于异常进行检查并处理。
  还有便是一个习惯的约定，以_开头的方法，一般表示private，私有成员。

封装为类库jpslib

这里，我们可以借助类似于webpack之类的工具进行代码的处理。
最后发布为npm。
这里的详细过程就省略了，代码可以到github上获取。

当然此处可以使用ES6来写，会更易读一些，建议读者亲自去实现一遍，会有深刻体会。

封装一个组件基类

还是先上代码，我们拿React组件来举例：

'use strict';import React from 'react';import jpslib from 'jpslib';class ComSubscriber extends jpslib.ISubscriber {    constructor(name, callback, scope) {        super();        this._name = name;        this._callback = callback;        this._scope = scope;    }    get name() {        return this._name;    }    get callback() {        return this._callback;    }    listNotificationInterested() {        return [this._name];    }    executeNotification(notice) {        this._callback.call(this._scope || {}, notice.getBody());    }}class PSComponent extends React.Component {    constructor(props) {        super(props);        this._subscribers = [];    }    hasSubscriber(name, callback) {        for(let i = 0; i < this._subscribers.length; i++) {            let sub = this._subscribers[i];            if(sub.name === name && sub.callback === callback) {                return true;            }        }        return false;    }    addSubscriber(name, callback) {        if(typeof name === 'string') {            if(typeof callback === 'function') {                if(this.hasSubscriber(name, callback)) {                    return false;                }                let subscriber = new ComSubscriber(name, callback, this);                jpslib.subscribe(subscriber);                this._subscribers.push(subscriber);                return true;             } else {                throw new Error('addSubscriber parameter callback should be type of function');            }        } else {            throw new Error('addSubscriber parameter name should be type of string');        }    }    removeSubscriber(name, callback) {        if(typeof name === 'string' && typeof callback === 'function') {            for(let i = 0; i < this._subscribers.length; i++) {                let sub = this._subscribers[i];                if(sub.name === name && sub.callback === callback) {                    const subscriber = this._subscribers.splice(i, 1)[0];                    jpslib.unsubscribe(name, subscriber);                    return true;                }            }        } else if(typeof name === 'string' && callback === undefined) {            for(let i = 0; i < this._subscribers.length; i++) {                let sub = this._subscribers[i];                if(sub.name === name) {                    const subscirber = this._subscribers.splice(i, 1);                    jpslib.unsubscribe(name, subscriber);                    i--;                }            }            return true;        }        return false;    }    sendNotification(name, body) {        if(typeof name === 'string') {            const notice = jpslib.createNotification(name, body);            jpslib.publish(notice);        }    }}export default PSComponent;

所有继承自PSComponent的组件，便可以使用jpslib的通讯了，例如：

先创造一个消息名的枚举：

'use strict';class NoticeTypes {    static ICONBUTTON_CLICK = 'iconbutton_click';}export default NoticeTypes;

然后进行消息的订阅及回调处理：

'use strict';import React from 'react';import PSComponent from './pscomponent';import ComNotice from './notice';class App extends PSComponent {    constructor(props) {        super(props);    }    componentDidMount() {        this.addSubscriber(ComNotice.ICONBUTTON_CLICK, this._iconbuttonClickHandler);    }        componentWillUnmount() {        this.removeSubscriber(COMNotice.ICONBUTTON_CLICK, this._iconbuttonClickHandler);    }        _iconbuttonClickHandler() {        console.log('救命啊，我被点了');    }        render() {        ...    }};export default App;

在需要的地方广播消息：

this.sendNotification(COMNotice.ICONBUTTON_CLICK, {});

这样封装之后，是不是我们在写代码的时候就会发现好用很多了，简单清晰。

相关项目源码，请浏览github。
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