Flex 4 Cookbook

Chapter 1. Flex and ActionScript Basics

A Flex application consists primarily of code written in two different languages: ActionScript and MXML. In its 3.0 incarnation, ActionScript went from a prototype-based scripting language to a fully object-oriented, strictly typed language. MXML is a markup language that will feel comfortable to anyone who has spent time working with Hypertext Markup Language (HTML), Extensible Markup Language (XML), or a host of newer markup-based languages.

Many newcomers to Flex wonder how MXML and ActionScript relate to one another. The MXML compiler (mxmlc), after parsing through the different idioms, translates them into the same objects, so that this:

<s:Button id="btn" label="My Button" height="100"/>

and this:

var btn:Button = new Button();
btn.label = "My Button";
btn.height = 100;

produce the same object. The major difference is that while creating that object in ActionScript (the second example) creates the button and nothing else, creating the object in MXML adds the button to whatever component contains the MXML code. The Flex Framework handles calling the constructor of the object described in MXML and either adding it to the parent or setting it as a property of the parent.

MXML files can include ActionScript within a <fx:Script> tag, but ActionScript files cannot include MXML. Although it’s tempting to think of MXML as describing the appearance and components that make up your application and of ActionScript as describing the event handlers and ...