JavaScript觀察者模式實例詳解

本文介紹了JavaScript程式設計模式之觀察者模式(Observer Pattern),簡單說明了觀察者模式的概念、原理並結合實例形式詳細給出了觀察者模式的相關實現與使用技巧,需要的朋友可以參考下，希望能幫助大家。

簡介

簡單的解釋觀察者模式，就是一個物件（subject）維護一個依賴他的物件（observers）列表，當自身狀態改變時，自動通知所有觀察者物件。當某個物件不需要獲得通知時，可以從物件清單中刪除掉。

從上面的解釋我們可以提煉出三個componet： Subject， ObserverList和Observer，用JS實作很簡單：

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擁有增加和刪除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 );
 }
};

最後定義一個觀察者對象，實作update方法

// The Observer
function Observer(){
 this.Update = function(){
  // ...
 };
}

當有多個觀察者，只需擴展上面的基本對象，並重寫Update方法。

儘管觀察值模式被廣泛使用，但在JS中經常使用它的變體： 發布訂閱模式

發布訂閱模式通過一個topic/event通道，解耦了觀察者模式中Subject（發布者）和Observer（訂閱者）之間耦合的問題，在JS中被廣泛使用。

下面簡單的範例說明了使用發布訂閱模式的基本結構

// 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 );

發布訂閱模式的實作

許多Js函式庫都很好的實作了發布訂閱模式，例如Jquery的自訂事件功能。

// 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" );

簡單實作

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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