MXMLs

MXMLs refer to files written in MXML, an XML-based markup language used to declare the user interface of Flex applications. MXML describes the visual component tree, layouts, data bindings, and basic behavior of an application, allowing developers to define UI structure in a declarative way while coupling with ActionScript for logic.

MXML originated with Macromedia’s Flex framework and was later developed under Adobe Systems. After Adobe’s focus shifted away from Flash Player, the open-source Apache Flex project took over maintenance. MXML files use the .mxml extension and are processed by the Flex SDK’s MXML compiler to produce executable code, typically within a SWF or AIR context.

The language favors a hierarchical, tag-based syntax where components are represented as XML elements within namespace-qualified tags (for example, mx:Application or spark:Button). The root element defines the application container, and nested tags instantiate UI components, layout containers, data bindings, and event handlers. Attributes set component properties such as size, position, and styles, while child elements or attributes express data binding and state changes. MXML can include ActionScript code blocks and references to separate ActionScript classes, enabling logic, data models, and custom components.

Key features commonly used with MXML include data binding, component styling, and state management, along with support for both MX and Spark component sets, particularly during Flex 4 transitions. MXML files thus serve as the primary, declarative entry point for constructing rich user interfaces in Flex-based applications, with the build process integrating them with ActionScript to produce interactive software for web and desktop environments.