JavaScript模式之观察者模式(Observer Pattern)

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:"[email protected]", 
  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: "[email protected]", 
  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?" );