Pushing Ajax Responses using Observer Pattern in JavaScript
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Join For FreeAfter reading a bit on the Observer pattern, I realized that it's used in almost all client side JavaScript programs in the form of event handlers. All browser events are examples of this pattern i.e. whenever an event takes place, the registered event handler is fired. Here, you want a particular function to be executed whenever there is a change in the state of another object. The function can be viewed as an Observer which performs the necessary action when the state of another object - let's call it the 'Subject' changes. The 'Subject' is responsible for notifying the Observers of the change in state and pass the necessary data to it's Observers. To do this the Subject has to maintain a list of Observers who are interested in getting notifications.
In my application, I was trying to fetch tweets by sending an Ajax request to Twitter server. Once the tweets are available, I wanted all my observers to be notified with the recent tweets. Here, my Subject is 'Twitter' which is responsible for fetching the tweets and notifying all its observers whenever the data is available. Also, it is responsible for maintaining a list of observers who are interested in receiving notifications. The way I modeled this application is to define a generic class 'Subject' and a class 'Twitter' which would extend the Subject class.
/*
* Define a generic class Subject with two methods
* 'addObserver' and 'notifyObserver'
*/
function Subject() {
this.observerList = [];
}
Subject.prototype.addObserver = function(observerObj) {
this.observerList.push(observerObj)
}
Subject.prototype.notifyObserver = function(msg) {
for (var i = 0, length = this.observerList.length; i < length; i++) {
this.observerList[i]['updateFn'](msg);
}
}
/*
* Define a Twitter class which extends the Subject class
*/
function Twitter(handlerName,observerObj) {
//Call the parent class - Subject
Subject.call(this);
//Handler name is the username for which tweets has to be retrieved
this.handlerName = handlerName;
//add the observer object
this.addObserver(observerObj);
//fetch the status messages
this.init();
}
Twitter.prototype = Object.create(Subject.prototype);
//Define the init method that will fetch the status messages
Twitter.prototype.init = function() {
parent = this;
$.ajax({
url: 'http://twitter.com/statuses/user_timeline/' + parent.handlerName + '.json?callback=?',
success: function(data) {
//iterate over the list of tweets
$.each(data,function(index,value)
{
var message = value.text;
//ignore the reply messages i.e. messages that start with '@'
if(message[0] != '@')
{
//Check whether the status message contains a link
if((index=message.indexOf('http://')) > 0)
{
substr1 = message.substr(index);
nextIndex = substr1.indexOf(' ');
substr2 = (nextIndex > 0)? substr1.substr(0,substr1.indexOf(' ')):substr1.substr(0);
pos1 = message.indexOf(substr2);
pos2 = pos1 + substr2.length;
newMessage = [message.slice(0,pos1),'<a href='+ substr2+ ' target="_blank" >' + substr2 + '</a>',message.slice(pos2)].join('');
//notify the Observer of the new message
parent.notifyObserver(newMessage);
}
else {
parent.notifyObserver(message);
}
}
});
},
//if there is an error, throw an Error message
error: function(data) {
parent.notifyObserver("Error");
},
dataType: 'json'
});
}
//Define an Observer class
function Observer(name, updateFn) {
this.name = name;
this.updateFn = updateFn;
}
//Create an instance of Observer class
observerObj = new Observer("observer1", function(msg) { console.log(msg); });
//Create an instance of the Subject and provide the observer object
tObject = new Twitter("sagarganatra",observerObj);
The Observer needs to provide the function (or the handler) that should be called when the Subject has a new message. In this case the updateFn would log the messages that it receives from the Subject.
Published at DZone with permission of Sagar Ganatra, DZone MVB. See the original article here.
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