QQmlEngine

QQmlEngine is the runtime engine for QML in the Qt framework. It provides the execution environment for QML code and the embedded JavaScript context, handling the loading, parsing, and evaluation of QML sources. The engine coordinates the interaction between QML and native C++ objects, enabling data exchange and property bindings between the two layers.

Each QQmlEngine exposes a QQmlContext that serves as the global context for QML code. Developers can populate the context with properties and objects using rootContext() so that QML files can access them as global variables. The engine also manages module imports and the resolution of QML types, including import paths that guide where QML looks for modules and components.

QQmlEngine works together with other QML APIs such as QQmlComponent, which is used to load and instantiate QML types in the scope of a given engine. A component created with a specific engine can be instantiated to produce a QObject hierarchy that lives in the same thread as the engine. The engine supports controlling object ownership to determine whether objects created in QML are owned by QQmlEngine or by C++ code.

Common usage involves creating a QQmlEngine in a C++ application, optionally configuring import paths and context properties, and then loading QML via QQmlComponent or by evaluating QML strings. The engine is not thread-safe and is typically used within the thread that created it, with separate engines allocated per thread as needed.

Historically, QQmlEngine replaced QDeclarativeEngine as part of Qt’s QML 2 architecture, aligning with the QtQml module and newer QML runtime features.