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 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 ? subscribers.length : 0;
while (len--) {
subscribers[len].func( topic, args );
}
return this;
};
// Subscribe to events of interest
// with a specific topic name and a
// callback function, to be executed
// when the topic/event is observed
q.subscribe = function( topic, func ) {
if (!topics[topic]) {
topics[topic] = [];
}
var token = ( ++subUid ).toString();
topics[topic].push({
token: token,
func: func
});
return token;
};
// Unsubscribe from a specific
// topic, based on a tokenized reference
// to the subscription
q.unsubscribe = function( token ) {
for ( var m in topics ) {
if ( topics[m] ) {
for ( var i = 0, j = topics[m].length; i < j; i++ ) {
if ( topics[m][i].token === token) {
topics[m].splice( i, 1 );
return token;
}
}
}
}
return this;
};
}( pubsub ));
使用方法
// 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?" );
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