QML

QML, short for Qt Modeling Language, is a declarative language used to design user interfaces and visual components within the Qt framework, especially as part of Qt Quick. It enables developers to describe UI structure and behavior in a concise, readable form, with a focus on fluid and dynamic interfaces.

QML code consists of a hierarchy of elements. The root element is typically an Item, and UI elements such as Rectangle, Text, Image, and various controls are defined as objects with properties. Properties can be bound to expressions, creating automatic updates when underlying values change.

The language integrates JavaScript for logic, data manipulation, and event handling. Signals defined by components can be connected to handlers written in JavaScript or QML, enabling responsive UIs. QML supports property bindings, signals, and the creation of reusable components.

Qt Quick also provides a rich set of built-in types and modules. Import statements load types from modules such as QtQuick (core UI elements) and Qt Quick Controls 2 (widgets and controls). ListView, Repeater, and GridView support item models for dynamic lists and layouts.

Rendering is handled by the Qt Quick scene graph and hardware acceleration. The QML engine executes QML code at runtime, with JavaScript execution for logic. The separation of UI (QML) from application logic (C++) is reinforced by the ability to expose C++ types and objects to QML via a QQmlEngine or QQmlApplicationEngine, context properties, and qmlRegisterType.

QML is used across desktop, embedded, and mobile platforms as part of Qt Quick, providing a fast, maintainable way to build cross-platform user interfaces. It continues to evolve with Qt releases, adding controls, modules, and performance improvements.