Introduction to Ruby on Rails with Ajax
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In this first article in a two-part series, you will learn how Ruby on Rails implements Ajax. We will use the specific example of a slideshow to demonstrate our points. This article is excerpted from chapter six of the book
Ruby on Rails: Up and Running, written by Bruce A. Tate and Curt Hibbs (O'Reilly, 2006; ISBN: 0596101325). Copyright © 2006 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.
Ajax is one of the most important emerging trends in web applications. Web sites like Google Maps and Gmail dramatically demonstrate that web applications do not have to be slow, clunky, page-at-a-time web forms. Ajax techniques can reclaim some of the fluidity and responsiveness that was lost when we moved from desktop applications to web applications.
Ajax (which stands for the cryptic "Asynchronous JavaScript and XML") is a technique for building web pages that are more interactive, exciting, and dynamic. Ajax is asynchronous: JavaScript libraries can communicate with the server at any time, and the web page need not be frozen while waiting for a response. Ajax uses JavaScript on the browser, any language on the server, and XML to specify messages.
When you use this emerging technique, a web page can communicate with the server at any time, updating only those portions of the display that need it. Users experience more responsive web pages, with immediate feedback. Even though using Ajax techniques usually requires significantly more sophisticated design and implementation skills, the benefits to the end user are so great that Ajax-enabled web applications will soon become the rule, not the exception. Fortunately, Rails makes Ajax so simple that, for typical cases, using Ajax is almost as easy as not using it.
How Rails Implements Ajax Rails has a simple, consistent model for how it implements Ajax operations. Once the browser has rendered and displayed the initial web page, different user actions cause it to display a new web page (like any traditional web application) or trigger an Ajax operation:
Some trigger fires
This trigger could be the user clicking on a button or link, the user making changes to the data on a form or in a field, or just a periodic trigger (based on a timer).
The web client calls the server
A JavaScript method, XMLHttpRequest, sends data associated with the trigger to an action handler on the server. The data might be the ID of a checkbox, the text in an entry field, or a whole form.
The server does something
The server-side action handler--a Rails controller action (for our purposes)--does something with the data and returns an HTML fragment to the web client.
The client receives the response
The client-side JavaScript, which Rails creates automatically, receives the HTML fragment and uses it to update a specified part of the current pages HTML, often the content of a <div> tag.
These steps are the simplest way to use Ajax in a Rails application, but with a little extra work, you can have the server return any kind of data in response to an Ajax request, and you can create custom JavaScript in the browser to perform more involved interactions. We'll stick to HTML fragments in this chapter.
Rails uses the Prototype and script.aculo.us JavaScript libraries to implement browser support for Ajax. You can use these libraries independently of Rails, but with their seamless integration with Rails, you probably wont want to. Throughout this chapter, we'll exploit the Ajax and special-effects capabilities that come with Rails to implement missing features in our Photo Share application.
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 This article is excerpted from chapter six of the book Ruby on Rails: Up and Running, written by Bruce A. Tate and Curt Hibbs (O'Reilly, 2006; ISBN: 0596101325). Check it out today at your favorite bookstore. Buy this book now.
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