JavaScript编程设计模式之观察者模式(Observer Pattern)实例详解

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 extensionfunction 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 Observerfunction 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 receivedvar mailCounter = 0;// Initialize subscribers that will listen out for a topic// with the name "inbox/newMessage".// Render a preview of new messagesvar 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 publishervar 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 );