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\page qtqml-modules-topic.html
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\brief Description of how to write modules for QML
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A QML module provides versioned types and JavaScript resources in a type
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namespace which may be used by clients who import the module. The types which
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a module provides may be defined in C++ within a plugin, or in QML documents.
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Modules make use of the QML versioning system which allows modules to be
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independently updated.
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Defining of a QML module allows:
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\li The sharing of common QML types within a project - for example, a group of
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UI components that are used by different windows
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\li The distribution of QML-based libraries
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\li The modularization of distinct features, so that applications only load the
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libraries necessary for their individual needs
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\li Versioning of types and resources so that the module can be updated safely
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without breaking client code
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\section1 Defining a QML Module
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A module is defined by a \l{qtqml-modules-qmldir.html}
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{module definition qmldir file}. Each module has an associated type
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namespace, which is the module's identifier. A module can provide QML object
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types (defined either by QML documents or via a C++ plugin) and JavaScript
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resources, and may be imported by clients.
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To define a module, a developer should gather together the various QML
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documents, JavaScript resources and C++ plugins which belong in the module
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into a single directory, and write an appropriate \l{qtqml-modules-qmldir.html}
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{module definition qmldir file} which should also be placed into the directory.
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The directory can then be installed into the
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\l{qtqml-syntax-imports.html#qml-import-path}{QML import path} as a module.
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Note that defining a module is not the only way to share common QML types
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within a project - a simple \l{qtqml-syntax-imports.html#directory-import}
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{QML document directory import} may also be used for this purpose.
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\section1 Supported QML Module Types
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There are two different types of modules supported by QML:
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\li \l{qtqml-modules-identifiedmodules.html}{Identified Modules}
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\li \l{qtqml-modules-legacymodules.html}{Legacy Modules} (deprecated)
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Identified modules explicitly define their identifier and are installed into
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QML import path. Identified modules are more maintainable (due to type
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versioning) and are provided with type registration guarantees by the QML
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engine which are not provided to legacy modules. Legacy modules are only
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supported to allow legacy code to continue to work with the latest version of
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QML, and should be avoided by clients if possible.
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Clients may import a QML module from within QML documents or JavaScript files.
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Please see the documentation about
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\l{qtqml-syntax-imports.html#module-namespace-imports}{importing a QML module}
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for more information on the topic.
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\section1 Providing Types and Functionality in a C++ Plugin
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An application which has a lot of logic implemented in C++, or which defines
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types in C++ and exposes them to QML, may wish to implement a QML plugin. A
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QML extension module developer may wish to implement some types in a C++ plugin
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(as opposed to defining them via QML documents) to achieve better performance
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or for greater flexibility.
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Every C++ plugin for QML has an initialiatization function which is called by
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the QML engine when it loads the plugin. This initialization function must
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register any types that the plugin provides, but must not do anything else
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(for example, instantiating QObjects is not allowed).
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See \l{qtqml-modules-cppplugins.html}{Creating C++ Plugins For QML} for more information.