Writing QML Modules▲

You should declare a QML module using the CMake QML Module API to:

  • Generate qmldir and *.qmltypes files.

  • Register C++ types annotated with QML_ELEMENT.

  • Combine QML files and C++-based types in the same module.

  • Invoke qmlcachegen on all QML files.

  • Use the pre-compiled versions of QML files inside the module.

  • Provide the module both in the physical and in the resource file system.

  • Create a backing library and an optional plugin. Link the backing library into the application to avoid loading the plugin at run time.

All the above actions can also be configured separately. For more information, see CMake QML Module API.

Multiple QML Modules in One Binary▲

You can add multiple QML modules into the same binary. Define a CMake target for each module and then link the targets to the executable. If the extra targets are all static libraries, the result will be one binary, which contains multiple QML modules. In short you can create an application like this:

 
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myProject
    | - CMakeLists.txt
    | - main.cpp
    | - main.qml
    | - onething.h
    | - onething.cpp
    | - ExtraModule
        | - CMakeLists.txt
        | - Extra.qml
        | - extrathing.h
        | - extrathing.cpp

To begin, let's assume main.qml contains an instantiation of Extra.qml:

 
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import ExtraModule
Extra { ... }

The extra module has to be a static library so that you can link it into the main program. Therefore, state as much in ExtraModule/CMakeLists.txt:

 
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# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: BSD-3-Clause

qt_add_library(extra_module STATIC)
qt_add_qml_module(extra_module
    URI "ExtraModule"
    VERSION 1.0
    QML_FILES
        Extra.qml
    SOURCES
        extrathing.cpp extrathing.h
    RESOURCE_PREFIX /
)

This generates two targets: extra_module for the backing library, and extra_moduleplugin for the plugin. Being a static library too, the plugin cannot be loaded at runtime.

In myProject/CMakeLists.txt you need to specify the QML module that main.qml and any types declared in onething.h are part of:

 
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qt_add_executable(main_program main.cpp)

qt_add_qml_module(main_program
    VERSION 1.0
    URI myProject
    QML_FILES
        main.qml
    SOURCES
        onething.cpp onething.h

)

From there, you add the subdirectory for the extra module:

 
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add_subdirectory(ExtraModule)

To ensure that linking the extra module works correctly, you need to:

  • Define a symbol in the extra module.

  • Create a reference to the symbol from the main program.

QML plugins contain a symbol you can use for this purpose. You can use the Q_IMPORT_QML_PLUGIN macro to create a reference to this symbol. Add the following code to the main.cpp:

 
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#include <QtQml/QQmlExtensionPlugin>
Q_IMPORT_QML_PLUGIN(ExtraModulePlugin)

ExtraModulePlugin is the name of the generated plugin class. It's composed of the module URI with Plugin appended to it. Then, in the main program's CMakeLists.txt, link the plugin, not the backing library, into the main program:

 
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