6.1. 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 handlera 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 page's 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 won't 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.