Skip to main content

Pushing Ajax responses using Observer Pattern in JavaScript

Last week I'd played around with couple of design patterns in JavaScript (Constructor and Module pattern). I really liked the Module pattern i.e. the approach taken in JavaScript to enable encapsulation of data in a Class (functions in JavaScript). I was building an application using these Design patterns but found that making Ajax request inside a function in a module was not the right approach. JavaScript would send a request and start executing the next statement. I wanted to use an approach that would push the data from the Model whenever there was some new data available. This lead me to try the 'Observer pattern' in JavaScript.

After 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.

Here the Subject class defines an array 'observerList' and two methods 'addObserver' and 'notifyObserver'. The addObserver method would add the observer object to the array and notifyObserver would iterate through the list of observers and send notifications whenever there is a new message. Now the Twitter class can extend the Subject class:

The constructor of Twitter class would call the parent constructor and then add the observer object (by invoking addObserver) to the list. Now it's ready to fetch the tweets for the provided handlerName. The init function then sends an Ajax request to fetch the tweets and once the data is received it would call the notifyObserver function. Note that the notifyObserver function is defined in the Subject class. The implementation of the Observer is pretty simple:

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.


Popular posts from this blog

Custom validation messages for HTML5 Input elements using the constraint validation API

HTML5 has introduced several input types such as EMAIL, URL, RANGE, SEARCH, DATE, TIME, etc,. Most of the modern browsers have implemented them and are ready to be used in a HTML document. Another exciting feature introduced in HTML5 is the form validation. Instead of writing JavaScript to validate users input, browsers can now validate it and show an appropriate message if the validation fails. The validation message is shown in line with the field for which the validation has failed. The default error message is shown when the validation fails. In this post I'll explain how these error messages can be changed.

Adding beforeRender and afterRender functions to a Backbone View

I was working on a Backbone application that updated the DOM when a response was received from the server. In a Backbone View, the initialize method would perform some operations and then call the render method to update the view. This worked fine, however there was scenario where in I wanted to perform some tasks before and after rendering the view. This can be considered as firing an event before and after the function had completed its execution. I found a very simple way to do this with Underscore's wrap method.

De-obfuscating javascript code in Chrome Developer Tools

I had blogged about JavaScript debugging with Chrome Developer Tools some time back, wherein I have explained how these developer tools can help in debugging javascript code. Today Google Chrome 12 was released and my Chrome browser was updated to this version. As with every release, there have been some improvements made on performance, usability etc,. One feature that stood out for me is the ability to De-obfuscate the javascript code.

What is Minification?

Minification is the process of removing unnecessary characters such as white spaces, comments, new lines from the source code. These otherwise would be added to make the code more readable. Minifying the source code helps in reducing the file size and thereby reducing the time taken to download the file. This is the reason why most of the popular javascript libraries such as jQuery are minified. A minified jQuery file is of 31 KB in size where as an uncompressed one is about 229 KB. Unfortunately, debugging minified javascript file…