MXML

MXML is an XML-based markup language used to declare user interfaces in the early Flex framework, originally developed by Macromedia and later maintained by Adobe and, since the late 2000s, by Apache Flex. It is designed to work with ActionScript and the Flash runtime, allowing developers to describe the structure and behavior of a UI as declarative XML rather than purely imperative code.

MXML files, with the .mxml extension, are compiled by the Flex compiler into ActionScript and then into bytecode that runs on the Flash Player or Adobe AIR. The language relies on XML namespaces to access UI components, such as the MX namespace for classic components and the Spark namespace for Flex 4-style components. UI is built by declaring components like applications, panels, buttons, lists, and containers in a hierarchical markup, and wiring behavior with properties, events, data binding, and optional scripts.

Key features include data binding using expressions in braces, fx:Script blocks for ActionScript code, states and transitions for UI changes, and layout containers such as VGroup, HGroup, and Canvas. Spark introduced more modern, skinable components and a different tag set, often starting with s: for example an Application or a Button.

Today, MXML’s prominence has diminished with the decline of the Flash platform, though the Apache Flex project continues in a limited capacity as a basis for historical knowledge and some legacy applications. It remains a notable example of declarative UI design in a strongly typed, component-based framework.