CMakeLists verstehen

Getting to grips with how CMakeLists.txt files work

CMake

In our introductory tutorial, we used CMake as the build system for our application, but we only really paid close attention to one of our CMakeLists.txt files. Here, we're going to go over how it works in a bit more detail.

CMake is useful because it allows us to automate much of the stuff that needs to be done before compilation.

The root CMakeLists.txt

You might remember this CMakeLists.txt file from the first tutorial:

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cmake_minimum_required(VERSION 3.20)
project(kirigami-tutorial)

find_package(ECM 6.0.0 REQUIRED NO_MODULE)
set(CMAKE_MODULE_PATH ${ECM_MODULE_PATH})

include(KDEInstallDirs)
include(KDECMakeSettings)
include(KDECompilerSettings NO_POLICY_SCOPE)
include(ECMFindQmlModule)
include(ECMQmlModule)

find_package(Qt6 REQUIRED COMPONENTS
    Core
    Quick
    Test
    Gui
    QuickControls2
    Widgets
)

find_package(KF6 REQUIRED COMPONENTS
    Kirigami
    I18n
    CoreAddons
    QQC2DesktopStyle
    IconThemes
)

ecm_find_qmlmodule(org.kde.kirigami REQUIRED)

add_subdirectory(src)

install(PROGRAMS org.kde.tutorial.desktop DESTINATION ${KDE_INSTALL_APPDIR})

feature_summary(WHAT ALL INCLUDE_QUIET_PACKAGES FATAL_ON_MISSING_REQUIRED_PACKAGES)

The first line, cmake_minimum_required() sets the version of CMake we will be calling.

After that, project(kirigami-tutorial) defines the name of the project.

Then we get to a section where we include a number of necessary CMake and KDE settings by using extra-cmake-modules. They provide a set of useful utilities:

KDEInstallDirs provides convenience variables such as ${KDE_INSTALL_TARGETS_DEFAULT_ARGS}, ${KDE_INSTALL_QMLDIR}, ${KDE_INSTALL_BINDIR} and ${KDE_INSTALL_LIBDIR}.
KDECMakeSettings provides things like CMAKE_AUTORCC ON, an uninstall target that can be used with cmake --build build/ --target uninstall, and ENABLE_CLAZY.
KDECompilerSettings provides a minimum C++ standard, compiler flags such as -pedantic, and best practices macros like -DQT_NO_CAST_FROM_ASCII to require explicit conversions such as QStringLiteral().
ECMFindQmlModule provides a way to ensure a runtime QML dependency is found at compile time.
ECMQmlModule provides CMake commands like ecm_add_qml_module() and ecm_target_qml_sources().

The following section is important, because it specifies which dependencies we'll be bringing in at compile time. Let's look at the first:

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find_package(Qt6 REQUIRED COMPONENTS
    Core
    Quick
    Test
    Gui
    QuickControls2
    Widgets
)

find_package(KF6 REQUIRED COMPONENTS
    Kirigami
    I18n
    CoreAddons
    QQC2DesktopStyle

find_package() finds and loads the external library and its components.
REQUIRED tells CMake to exit with an error if the package cannot be found.
COMPONENTS is a parameter that precedes the specific components of the framework we will include.
Each word after COMPONENTS refers to a specific component of the library.

Note

If you are looking to add any components listed in the KDE API documentation to your application, you may check the right sidebar for how to add the component with CMake. For instance, for Kirigami, you will find something like find_package(KF6Kirigami), which with the addition of ECM becomes:

find_package(KF6 COMPONENTS Kirigami)

Pay close attention to your included components, as omitting ones used in our code will stop our application from compiling.

The install line instructs CMake to install the desktop file in ${KDE_INSTALL_APPDIR}, which on Linux translates to $XDG_DATA_DIRS/applications, usually /usr/share/applications, and on Windows translates to C:/Program Files/${PROJECT_NAME}/bin/data/applications:

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add_subdirectory(src)

The final line lets CMake print out which packages it has found, and it makes compilation fail immediately if it encounters an error:

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install(PROGRAMS org.kde.tutorial.desktop DESTINATION ${KDE_INSTALL_APPDIR})

And above that, add_subdirectory(src) points CMake into the src/ directory, where it finds another CMakeLists.txt file.

src/CMakeLists.txt

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add_executable(kirigami-hello)

ecm_add_qml_module(kirigami-hello
    URI
    org.kde.tutorial
)

target_sources(kirigami-hello
    PRIVATE
    main.cpp
)

ecm_target_qml_sources(kirigami-hello
    SOURCES
    Main.qml
)

target_link_libraries(kirigami-hello
    PRIVATE
    Qt6::Quick
    Qt6::Qml
    Qt6::Gui
    Qt6::QuickControls2
    Qt6::Widgets
    KF6::I18n
    KF6::CoreAddons
    KF6::IconThemes
)

install(TARGETS kirigami-hello ${KDE_INSTALL_TARGETS_DEFAULT_ARGS})

While the first file handled metadata and finding libraries, this one will consist of handling dependencies and installing the application. It has the following CMake calls:

add_executable() creates the executable target we will use to run our project.
ecm_add_qml_module() creates a QML module target that will be accessible via the "org.kde.tutorial" import.
target_sources() adds C++ source files to the executable target.
ecm_target_qml_sources() adds QML files to the module.
target_link_libraries() links the C++ libraries used in our code to our executable. Kirigami is not included here because we are using only its QML module.
install() installs the executable to the system.

The documentation for the two ECM commands can be found in the extra-cmake-modules API for ECMQmlModule.

The call to ecm_add_qml_module() was used here to modify the traditional C++ source code executable target and turn it into something that can accept QML files and C++ source code that is accessible from QML in what is called using the executable as backing target for a QML module. This means the QML files are run directly as part of the application, which is often the case for applications.

You may also create a separate QML module that does not use the executable as backing target using ecm_add_qml_module(). In this case, you'd create a library target using add_library(), link it to an existing executable target using target_link_libraries(), and in addition to installing the library with install() you will need to finalize the QML module with ecm_finalize_qml_module() so it can generate two files: qmldir and qmltypes. These files are used by QtQuick applications to find separate QML modules.

The method for creating a separate QML module is better exemplified in Using separate files.

These are additions provided by extra-cmake-modules to make the use of Qt declarative registration (the replacement to Qt resource files) easier.

Note

These libraries should match the components that we included in our previous CMakeLists.txt file, otherwise these components will not be included and our application won't compile.

The documentation for all three commands can be found in the extra-cmake-modules API for ECMQmlModule.

src/components/CMakeLists.txt

In the tutorial about how to split your code into separate files, a new CMake file was introduced to allow for separate QML modules:

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add_library(kirigami-hello-components)

ecm_add_qml_module(kirigami-hello-components
    URI "org.kde.tutorial.components"
    GENERATE_PLUGIN_SOURCE
)

ecm_target_qml_sources(kirigami-hello-components
    SOURCES
    AddDialog.qml
    KountdownDelegate.qml
)

ecm_finalize_qml_module(kirigami-hello-components)

install(TARGETS kirigami-hello-components ${KDE_INSTALL_TARGETS_DEFAULT_ARGS})

The requirement for this file to be read by CMake is adding a call to add_subdirectory() in the src/CMakeLists.txt pointing to it.

We create a new target called kirigami-hello-components and then turn it into a QML module using ecm_add_qml_module() under the import name org.kde.tutorial.components and add the relevant QML files.

The call to add_library() generates a new target called kirigami-hello-components. This target will have its own set of source code files, QML files, link its own libraries and so on, but it needs to be linked to the executable, but once it is compiled it needs to be linked to the executable created in the src/CMakeLists.txt. This is done by adding the target name to the list of libraries that will be linked to the executable in target_link_libraries().

The call to ecm_add_qml_module() changes the library to allow it to accept QML files as before, but this time we need to use GENERATE_PLUGIN_SOURCE. When the executable is used as a backing target (like with kirigami-hello) it doesn't need to generate plugin code since it's built into the executable; with separate QML modules like kirigami-hello-components the plugin code is necessary.

Upstream Qt's qt_add_qml_module() by default generates a plugin together with the QML module, but KDE's ecm_add_qml_module() by default does not for backwards compatibility.

Another thing that is necessary for separate QML modules is to finalize the target. This mainly means CMake generates two files, qmldir and qmltypes, which describe the QML modules we have and exports their symbols for use in the library. They are important when installing your application so that the executable being run is able to find where the QML files for each module are, so they are automatically added to the target.

You can then just install the target as before.

Next time you need to add more QML files, remember to include them in this file. C++ files that use the QML_ELEMENT keyword which we will see much later in the tutorial can also be added here using target_sources(). You can logically separate your code by creating more QML modules with different imports as needed.

This setup will be useful when developing most Kirigami apps.

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