How to create QML singleton

[KejPi]

I do not know how to attach file, so this is cope and paste of minimal example:

CMakeList.txt

cmake_minimum_required(VERSION 3.16)

project(SingletonTest VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(Qt6 6.4 REQUIRED COMPONENTS Quick)

qt_standard_project_setup()

qt_add_executable(appSingletonTest
    main.cpp
)

set_source_files_properties(qml/Colors.qml PROPERTIES QT_QML_SINGLETON_TYPE TRUE)
qt_add_qml_module(appSingletonTest
    URI SingletonTest
    VERSION 1.0
    QML_FILES qml/main.qml
    QML_FILES qml/Colors.qml
)

# Qt for iOS sets MACOSX_BUNDLE_GUI_IDENTIFIER automatically since Qt 6.1.
# If you are developing for iOS or macOS you should consider setting an
# explicit, fixed bundle identifier manually though.
set_target_properties(appSingletonTest PROPERTIES
#    MACOSX_BUNDLE_GUI_IDENTIFIER com.example.appSingletonTest
    MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION}
    MACOSX_BUNDLE_SHORT_VERSION_STRING ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
    MACOSX_BUNDLE TRUE
    WIN32_EXECUTABLE TRUE
)

target_link_libraries(appSingletonTest
    PRIVATE Qt6::Quick
)

include(GNUInstallDirs)
install(TARGETS appSingletonTest
    BUNDLE DESTINATION .
    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
)

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(u"qrc:/SingletonTest/qml/main.qml"_qs);
    QObject::connect(
        &engine,
        &QQmlApplicationEngine::objectCreationFailed,
        &app,
        []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

qml/main.qml:

import QtQuick
import SingletonTest

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Rectangle {
        width: parent.width/2
        height: parent.height/2
        anchors.centerIn: parent
        color: Colors.myColor
    }
}

qml/Colors.qml:

pragma Singleton
import QtQuick

QtObject {
    id: colors
    readonly property color myColor: "red"
}

When I run it I get this message:

qrc:/SingletonTest/qml/main.qml:14: TypeError: Colors was a singleton at compile time, but is not a singleton anymore.

[GrecKo]

What if you put the QML files in the root folder of the project? Colors.qml instead of qml/Colors.qml

Also I'd remove the second QML_FILES in qt_add_qml_module

[KejPi]

If I move Colors.qml to root folder and change CMakeList.txt accordingly:

set_source_files_properties(Colors.qml PROPERTIES QT_QML_SINGLETON_TYPE TRUE)
qt_add_qml_module(appSingletonTest
    URI SingletonTest
    VERSION 1.0
    QML_FILES qml/main.qml
              Colors.qml
)

I get this error when I run the application:

qrc:/SingletonTest/qml/main.qml:14: ReferenceError: Colors is not defined

[KejPi]

It seems all QML files must be in root folder including some other custom components files.

If I do this, it works:

set_source_files_properties(Colors.qml PROPERTIES QT_QML_SINGLETON_TYPE TRUE)
qt_add_qml_module(appSingletonTest
    URI SingletonTest
    VERSION 1.0
    QML_FILES main.qml
              Colors.qml
              SomeComponent.qml
)

This strange, so far I put all my QML files to dedicated folder (qml/) to separate it from C++ and everything was working (see my first example code) - only the singleton causes some problem.

I could consider this as workaround but I do not like it since I will mix QML with C++ and it will be a mess :-(

EDIT: It seems that this construction works too:

set_source_files_properties(qmlfiles/Colors.qml PROPERTIES QT_QML_SINGLETON_TYPE TRUE)
set_source_files_properties(qmlfiles/main.qml PROPERTIES QT_RESOURCE_ALIAS main.qml)
set_source_files_properties(qmlfiles/Colors.qml PROPERTIES QT_RESOURCE_ALIAS Colors.qml)
set_source_files_properties(qmlfiles/SomeComponent.qml PROPERTIES QT_RESOURCE_ALIAS SomeComponent.qml)
qt_add_qml_module(appSingletonTest
    URI SingletonTest
    VERSION 1.0
    QML_FILES qmlfiles/main.qml
              qmlfiles/Colors.qml
              qmlfiles/SomeComponent.qml
)

[ekkescorner]

@KejPi open a bugreport because QML SINGLETON doesn't work if not in root folder

[Mesrine]

@ekkescorner

I do not know why the one from the post does not work, but singletons from subdirectories work.

I used here

[ekkescorner]

@Mesrine aaah - good to know. just porting my apps from QMake to CMake. next step will be to modernize my apps, where I'll need QML SINGLETONS, too

[Janson]

This post is deleted!

[KejPi]

I have a small example project that clearly shows it does not work as expected. I can share it but I do not know how to do it here. Maybe I did something wrong - I have juste created project in QtCreator and moved all QML files in subdirectory.

[KejPi]

@Mesrine Could it be that you only have singleton in the module while I have all my QML files there?

[Mesrine]

@KejPi

I have used your minimal example and works ok :) .

The only thing I changed was that instead of CMakeList.txt i named CMakeLists.txt.

I have tested it with qt(shared libraries) 6.5,6.6,6.7 in Linux.

Make sure you are using the correct CMakeLists.txt.
And let us know your platform and Qt version.

[KejPi]

Yes, it was only typo, I use the CMakeLists.txt created by Qt Creator.
I am running it on MacOS 14.2, Qt version 6.5.3

[Mesrine]

@KejPi
Do you run the application that is on the build directory or the installed one?

Try putting
set_source_files_properties(qml/Colors.qml PROPERTIES QT_QML_SINGLETON_TYPE TRUE)

before

qt_add_executable(appSingletonTest
    main.cpp
)

in your minimal example.
If that does not work try removing some macos properties in the CMakeLists.txt.
If nothing works I would say you can open a bug as suggested by @ekkescorner

[KejPi]

@Mesrine I run the application directly from Qt Creator after building it.
Moving set_source_files_properties did not help, the problem is the same.

Bug report created https://bugreports.qt.io/browse/QTBUG-121758