本文实例讲述了JavaScript编程设计模式之观察者模式。分享给大家供大家参考,具体如下:
简介
简单的解释观察者模式,就是一个对象(subject)维护一个依赖他的对象(observers)列表,当自身状态发生变化时,自动通知所有观察者对象。当某个对象不需要获得通知时,可以从对象列表中删除掉。
从上面的解释中我们可以提炼出三个componet: Subject, ObserverList和Observer,用JS实现很简单:
function ObserverList(){ this.observerList = []; } ObserverList.prototype.Add = function( obj ){ return this.observerList.push( obj ); }; ObserverList.prototype.Empty = function(){ this.observerList = []; }; ObserverList.prototype.Count = function(){ return this.observerList.length; }; ObserverList.prototype.Get = function( index ){ if( index > -1 && index < this.observerList.length ){ return this.observerList[ index ]; } }; ObserverList.prototype.Insert = function( obj, index ){ var pointer = -1; if( index === 0 ){ this.observerList.unshift( obj ); pointer = index; }else if( index === this.observerList.length ){ this.observerList.push( obj ); pointer = index; } return pointer; }; ObserverList.prototype.IndexOf = function( obj, startIndex ){ var i = startIndex, pointer = -1; while( i < this.observerList.length ){ if( this.observerList[i] === obj ){ pointer = i; } i++; } return pointer; }; ObserverList.prototype.RemoveAt = function( index ){ if( index === 0 ){ this.observerList.shift(); }else if( index === this.observerList.length -1 ){ this.observerList.pop(); } }; // Extend an object with an extension function extend( extension, obj ){ for ( var key in extension ){ obj[key] = extension[key]; } }
Subject拥有增加和删除Observer的能力
function Subject(){ this.observers = new ObserverList(); } Subject.prototype.AddObserver = function( observer ){ this.observers.Add( observer ); }; Subject.prototype.RemoveObserver = function( observer ){ this.observers.RemoveAt( this.observers.IndexOf( observer, 0 ) ); }; Subject.prototype.Notify = function( context ){ var observerCount = this.observers.Count(); for(var i=0; i < observerCount; i++){ this.observers.Get(i).Update( context ); } };
最后定义一个观察者对象,实现update方法
// The Observer function Observer(){ this.Update = function(){ // ... }; }
当有多个观察者,只需扩展上面的基本对象,并重写Update方法。
尽管观察则模式被广泛使用,但在JS中经常使用它的变体: 发布订阅模式
发布订阅模式通过一个topic/event通道,解耦了观察者模式中Subject(发布者)和Observer(订阅者)之间耦合的问题,在JS中被广泛使用。
下面简单的例子说明了使用发布订阅模式的基本结构
// A very simple new mail handler // A count of the number of messages received var mailCounter = 0; // Initialize subscribers that will listen out for a topic // with the name "inbox/newMessage". // Render a preview of new messages var subscriber1 = subscribe( "inbox/newMessage", function( topic, data ) { // Log the topic for debugging purposes console.log( "A new message was received: ", topic ); // Use the data that was passed from our subject // to display a message preview to the user $( ".messageSender" ).html( data.sender ); $( ".messagePreview" ).html( data.body ); }); // Here's another subscriber using the same data to perform // a different task. // Update the counter displaying the number of new // messages received via the publisher var subscriber2 = subscribe( "inbox/newMessage", function( topic, data ) { $('.newMessageCounter').html( mailCounter++ ); }); publish( "inbox/newMessage", [{ sender:"hello@google.com", body: "Hey there! How are you doing today" }]); // We could then at a later point unsubscribe our subscribers // from receiving any new topic notifications as follows: // unsubscribe( subscriber1, ); // unsubscribe( subscriber2 );
发布订阅模式的实现
许多Js库都很好的实现了发布订阅模式,例如Jquery的自定义事件功能。
// Publish // jQuery: $(obj).trigger("channel", [arg1, arg2, arg3]); $( el ).trigger( "/login", [{username:"test", userData:"test"}] ); // Dojo: dojo.publish("channel", [arg1, arg2, arg3] ); dojo.publish( "/login", [{username:"test", userData:"test"}] ); // YUI: el.publish("channel", [arg1, arg2, arg3]); el.publish( "/login", {username:"test", userData:"test"} ); // Subscribe // jQuery: $(obj).on( "channel", [data], fn ); $( el ).on( "/login", function( event ){...} ); // Dojo: dojo.subscribe( "channel", fn); var handle = dojo.subscribe( "/login", function(data){..} ); // YUI: el.on("channel", handler); el.on( "/login", function( data ){...} ); // Unsubscribe // jQuery: $(obj).off( "channel" ); $( el ).off( "/login" ); // Dojo: dojo.unsubscribe( handle ); dojo.unsubscribe( handle ); // YUI: el.detach("channel"); el.detach( "/login" );
简单实现
var pubsub = {}; (function(q) { var topics = {}, subUid = -1; // Publish or broadcast events of interest // with a specific topic name and arguments // such as the data to pass along q.publish = function( topic, args ) { if ( !topics[topic] ) { return false; } var subscribers = topics[topic], len = subscribers "htmlcode">// Another simple message handler // A simple message logger that logs any topics and data received through our // subscriber var messageLogger = function ( topics, data ) { console.log( "Logging: " + topics + ": " + data ); }; // Subscribers listen for topics they have subscribed to and // invoke a callback function (e.g messageLogger) once a new // notification is broadcast on that topic var subscription = pubsub.subscribe( "inbox/newMessage", messageLogger ); // Publishers are in charge of publishing topics or notifications of // interest to the application. e.g: pubsub.publish( "inbox/newMessage", "hello world!" ); // or pubsub.publish( "inbox/newMessage", ["test", "a", "b", "c"] ); // or pubsub.publish( "inbox/newMessage", { sender: "hello@google.com", body: "Hey again!" }); // We cab also unsubscribe if we no longer wish for our subscribers // to be notified // pubsub.unsubscribe( subscription ); // Once unsubscribed, this for example won't result in our // messageLogger being executed as the subscriber is // no longer listening pubsub.publish( "inbox/newMessage", "Hello! are you still there" );更多关于JavaScript相关内容可查看本站专题:《javascript面向对象入门教程》、《JavaScript切换特效与技巧总结》、《JavaScript查找算法技巧总结》、《JavaScript错误与调试技巧总结》、《JavaScript数据结构与算法技巧总结》、《JavaScript遍历算法与技巧总结》及《JavaScript数学运算用法总结》
希望本文所述对大家JavaScript程序设计有所帮助。
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