You can use ActionScript code in an tag to change the state of a component at run time.

Flex developers can use ActionScript to extend the functionality of their Adobe® Flex® applications. ActionScript provides flow control and object manipulation features that are not available in MXML.

ActionScript a flash language is an implementation of ECMAScript and is similar to OOP based JavaScript. In Adobe Flash Player this is the main programming tool. It consists of built-in objects and functions, and allows its users to build their own objects and functions. With this strong tool one can extend its competency in creating RIAs. Its coding is done inside the tag . We create ActionScript files and call them inside the main MXML file with either ‘source’ attribute of tag or with ‘include’ statement inside tag by passing the relative path names of ActionScript files in these attributes. Each and every ActionScript file should be saved with ‘.as’ extension.

MXML is an XML markup language that you use to lay out user interface components. You also use MXML to declaratively define nonvisual aspects of an application, such as access to data sources on the server and data bindings between user interface components and data sources on the server.

Like HTML, MXML provides tags that define user interfaces. MXML will seem very familiar if you have worked with HTML. However, MXML is more structured than HTML, and it provides a much richer tag set. For example, MXML includes tags for visual components such as data grids, trees, tab navigators, accordions, and menus, as well as nonvisual components that provide web service connections, data binding, and animation effects. You can also extend MXML with custom components that you reference as MXML tags.

One of the biggest differences between MXML and HTML is that MXML-defined applications are compiled into SWF files and rendered by Adobe® Flash® Player or Adobe® AIR™, which provides a richer and more dynamic user interface than page-based HTML applications.

You can write an MXML application in a single file or in multiple files. MXML also supports custom components written in MXML and ActionScript files.

tag defines the Application container that is always the root tag of a Flex application.
tag represents an Application container. A container is a user-interface component that contains other components and has built-in layout rules for positioning its child components. By default, an Application container lays out its children vertically from top to bottom. You can nest other types of containers inside an Application container, such as the Panel container.

A common coding practice is to divide your Flex application into functional units, or modules, where each module performs a discrete task. In Flex, you can divide your application into separate MXML files and ActionScript files, where each file corresponds to a different module. By dividing your application into modules, you provide many benefits, including the following:

*Ease of development

*Different developers or development groups can develop and debug modules independently of each other.

*Reusability

*You can reuse modules in different applications so that you do not have to duplicate your work.

*Maintainability

*You can isolate and debug errors faster than if your application is developed in a single file.

MXML development is based on the same iterative process used for other types of web application files such as HTML, JavaServer Pages (JSP), Active Server Pages (ASP), and ColdFusion Markup Language (CFML). Developing a useful Flex application is as easy as opening your favorite text editor, typing some XML tags, saving the file, requesting the file’s URL in a web browser, and then repeating the same process.

In the Flex model-view design pattern, user interface components represent the view. The MXML language supports two types of user interface components: controls and containers. Controls are form elements, such as buttons, text fields, and list boxes. Containers are rectangular regions of the screen that contain controls and other containers.

You use container components for laying out a user interface, and for controlling user navigation through the application. Examples of layout containers include the HBox container for laying out child components horizontally, the VBox container for laying out child components vertically, and the Grid container for laying out child components in rows and columns. Examples of navigator containers include the TabNavigator container for creating tabbed panels, the Accordion navigator container for creating collapsible panels, and the ViewStack navigator container for laying out panels on top of each other.

The Container class is the base class of all Flex container classes. Containers that extend the Container class add their own functionality for laying out child components. Typical properties of a container tag include id, width, and height.
A container defines a rectangular region of the drawing surface of Adobe® Flash® Player. Within a container, you define the components, both controls and containers, that you want to appear within the container. Components defined within a container are called children of the container. Adobe® Flex®provides a wide variety of containers, ranging from simple boxes through panels and forms, to elements such as accordions or tabbed navigators that provide built-in navigation among child containers.

At the root of a Flex application is a single container, called the Application container, that represents the entire Flash Player drawing surface. This Application container holds all other containers and components.

A container has predefined rules to control the layout of its children, including sizing and positioning. Flex defines layout rules to simplify the design and implementation of rich Internet applications, while also providing enough flexibility to let you create a diverse set of applications.

Flex applications can be written using Adobe Flex Builder or by using the freely available Flex compiler from Adobe.

The initial release in March 2004 by Macromedia included a software development kit, an IDE, and a J2EE integration application known as Flex Data Services. Since Adobe acquired Macromedia in 2005, subsequent releases of Flex no longer require a license for Flex Data Services, which has become a separate product rebranded as LiveCycle Data Services.

In February 2008, Adobe released the Flex 3 SDK under the open source Mozilla Public License. Adobe Flash Player, the runtime on which Flex applications are viewed, and Adobe Flex Builder, the IDE built on the open source Eclipse platform and used to build Flex applications, remain proprietary.

Eclipse is a multi-language software development environment comprising an IDE and a plug-in system to extend it. It is written primarily in Java and can be used to develop applications in Java and, by means of the various plug-ins, in other languages as well, including C, C++, COBOL, Python, Perl, PHP, and others.

Adobe Flash Builder 4 is an integrated development environment (IDE) built on the Eclipse platform meant for developing rich Internet applications (RIAs) and cross-platform desktop applications, particularly for the Adobe Flash platform.

The Adobe Flash Player is software for viewing animations and movies using computer programs such as a web browser; in common usage, Flash lets you put animation and movies on a web site

A widget is anything that can be embedded within a web page. A widget adds some content to that page that is not static.

widget engine is a software service available to users for running and displaying applets on a graphical user interface.

GUI widget is an element of a graphical user interface (GUI) that displays an information arrangement changeable by the user, such as a window or a text box.

A widget toolkit, widget library, or GUI toolkit is a set of widgets used in designing applications with graphical user interfaces (GUIs).

A web widget is a portable chunk of code that can be installed and executed within any separate HTML-based web page by an end user without requiring additional compilation. They are derived from the idea of code reuse.

Desktop widgets are interactive virtual tools that provide single-purpose services such as showing the user the latest news, the current weather, the time, a calendar, a dictionary, a map program, a calculator, desktop notes, photo viewers, or even a language translator, among other things.