Programming Flex 3

Using Flex Elements

The Flex framework includes a core set of languages and libraries that are the basis of any Flex application. Using MXML, ActionScript, and the Flex class library you can construct and compile .swf content that you can then deploy to Flash Player.

MXML

MXML is an XML-based markup language primarily for describing screen layout. In that respect, it is much like HTML. Using MXML tags you can add components such as form controls and media playback components to layout containers such as grids.

In addition to screen layout, you can use MXML to describe effects, transitions, data models, and data binding. MXML is robust enough that it is possible to build many applications entirely with MXML. Flex Builder enables you to construct MXML with a WYSIWYG approach, enabling you to build basic Flex applications without writing any code.

Although the WYSIWYG approach is helpful for basic prototypes and simple applications, writing MXML code is still necessary for more complex tasks. Additionally, sophisticated Flex applications generally require both MXML and ActionScript.

MXML is a declarative way to create Flex content, but the simplicity should not fool you into thinking that MXML is not powerful. MXML provides a fast and powerful way to create layout and UI content. However, MXML documents get compiled in several steps, the first of which converts the MXML to an ActionScript class. This means MXML documents provide you with all the power of object-oriented design, but with the convenience of a markup language. Furthermore, MXML documents are treated as ActionScript classes at runtime.

ActionScript

ActionScript is the programming language understood by Flash Player and is the fundamental engine of all Flex applications. MXML simplifies screen layout and many basic tasks, but all of what MXML does is made possible by ActionScript, and ActionScript can do many things that MXML cannot do. For example, ActionScript is necessary to respond to events such as mouse clicks.

Although it is possible to build an application entirely with MXML or entirely with ActionScript, it is more common and more sensible to build applications with the appropriate balance of both MXML and ActionScript. Each offers benefits, and they work well together. MXML is best suited for screen layout and basic data features. ActionScript is best suited for user interaction, complex data functionality, and any custom functionality not included in the Flex class library.

ActionScript is supported natively by Flash Player and does not require any additional libraries to run. All the native ActionScript classes (classes that are built into Flash Player) are packaged in the flash package or in the top-level package. In contrast, the Flex framework is written in ActionScript, but those classes are included in an .swf file at compile time. All the Flex framework classes are in the mx package.

Note

You can learn much more about ActionScript in Chapter 4. In fact, if any of the terms we’ve used in this section are unfamiliar to you, you will have an opportunity to learn more about them in that chapter.