Building a reusable QML module

The example below demonstrates how to create a library which exposes C++ to QML. The directory structure for the example looks like this:

├── CMakeLists.txt
└── example
    └── mylib
        ├── CMakeLists.txt
        ├── mytype.cpp
        ├── mytype.h

The toplevel CMakeLists.txt file does some basic setup, and then uses add_subdirectory to include the one in mylib. The subdirectory structure corresponds to the QML module’s URI, but with the dots replaced by slashes. That’s the same logic the engine uses when it searches for a module in the import paths. mytype.h declares a class and uses the declarative registration macros to expose it to the engine.

In the subdirectory’s CMakeLists.txt we again call qt6_add_qml_module. However, the invocation is slightly different:

qt6_add_qml_module(mylib
    URI example.mylib
    VERSION 1.0
    SOURCES
        mytype.h mytype.cpp
)

To add C++ types, the SOURCES parameter needs to be specified. The target for mylib is not created. Therefore, if the target passed to qt6_add_qml_module does not exist, a library target is automatically created, which is needed in this case.

When the project is built, in addition to the library, a QML plugin is also built. The plugin's auto-generated class extends from QQmlEngineExtensionPlugin. The mylib library itself already contains the code to register the types with the engine. However, that is only useful in cases where we can link against the library. To make the module usable in a QML file loaded by qml, the QML Runtime Tool, a plugin is needed that can be loaded. The plugin is then responsible for actually linking against the library, and ensuring that the types get registered.

Note that the automatic plugin generation is only possible if the module does not do anything besides registering the types. If it needs to do something more advanced like registering an image provider in initializeEngine, you still need to manually write the plugin. qt6_add_qml_module has support for this with NO_GENERATE_PLUGIN_SOURCE.

Also, following the directory layout convention helps tooling. That layout is mirrored in the build directory. Which means that you can pass the path to your build directory to the QML tool (via the -I flag), and it will find the plugin.

Before concluding add a QML file to the module. In the lib subfolder, add a Mistake.qml file

import example.mylib

MyType{
    answer: 43
}

and adjust the qt6_add_qml_module call:

qt6_add_qml_module(mylib
    URI example.mylib
    VERSION 1.0
    SOURCES
        mytype.h mytype.cpp
    QML_FILES
        Mistake.qml
)

As mentioned, we made a mistake because answer is actually a read-only property. This illustrates qmllint integration: CMake creates a qmllint target, and once we run it, qmllint warns about the issue:

$> cmake --build . --target mylib_qmllint
...
Warning: Mistake.qml:4:13: Cannot assign to read-only property answer
    answer: 43
            ^^