In my project I'm trying to expose a singleton type to QML from C++. Initially I registered it with qmlRegisterSingletonInstance and it worked fine. But when I tried to replace this way with QML_ELEMENT and QML_SINGLETON macros it compiles fine but when QML tries to use the class it complains that the type is not defined.
My project has two modules, one consists solely of qml files and the second - of C++ files, where I try to expose the class from and use it in the first module. The structure is a bit complicated so I reproduced the issue with a simpler test project. I noticed that QML cannot recognize neither the singleton type nor a non-singleton type.
The project has a following structure:
│ CMakeLists.txt
│ main.cpp
│
└───logicmodule
CMakeLists.txt
Main.qml
Message.h
MySingleton.h
The code:
// logicmodule/MySingleton.h
#pragma once
#include <QObject>
#include <QQmlEngine>
#include <QString>
class MySingleton : public QObject
{
Q_OBJECT
QML_ELEMENT
QML_SINGLETON
public:
MySingleton(QObject *parent = nullptr)
: QObject{parent}
{}
public slots:
QString exec(const QString &str) { return "somestring"; }
};
// logicmodule/Message.h
#pragma once
#include <QObject>
#include <QQmlEngine>
#include <QString>
class Message : public QObject
{
Q_OBJECT
QML_ELEMENT
Q_PROPERTY(QString author MEMBER m_author NOTIFY authorChanged)
public:
Message() {}
signals:
void authorChanged();
private:
QString m_author;
};
// logicmodule/Main.qml
import QtQuick
import QtQuick.Controls
Window {
width: 640
height: 480
visible: true
title: qsTr("Hello World")
// property Message msg: Message {}
Button {
id: button
anchors.centerIn: parent
text: "Click me"
onClicked: {
label.text = MySingleton.exec(label.text)
}
}
Label {
id: label
anchors.top: button.bottom
}
}
// logicmodule/CMakeLists.txt
project(logicmodule VERSION 0.1 LANGUAGES CXX)
qt_add_library(${PROJECT_NAME} STATIC)
qt6_add_qml_module(${PROJECT_NAME}
URI Main
VERSION 1.0
NO_PLUGIN
RESOURCE_PREFIX "/qt/qml"
QML_FILES
Main.qml
SOURCES
MySingleton.h
Message.h
)
target_link_libraries(${PROJECT_NAME} PUBLIC
Qt6::Core
Qt6::Gui
Qt6::Qml
Qt6::Quick
)
target_include_directories(${PROJECT_NAME}
PUBLIC
.
)
// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "Message.h"
#include "MySingleton.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
const QUrl url(u"qrc:/qt/qml/Main/Main.qml"_qs);
QObject::connect(
&engine,
&QQmlApplicationEngine::objectCreated,
&app,
[url](QObject *obj, const QUrl &objUrl) {
if (!obj && url == objUrl)
QCoreApplication::exit(-1);
},
Qt::QueuedConnection);
engine.addImportPath(QCoreApplication::applicationDirPath() + "/qml");
engine.addImportPath(":/");
engine.load(url);
if (engine.rootObjects().isEmpty()) {
return -1;
}
return app.exec();
}
// CMakeLists.txt
cmake_minimum_required(VERSION 3.16)
project(typetest VERSION 0.1 LANGUAGES CXX)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)
set(QT_QML_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/qml)
qt_standard_project_setup(REQUIRES 6.5)
add_subdirectory(logicmodule)
qt_add_executable(apptypetest
main.cpp
)
target_link_libraries(apptypetest PUBLIC
logicmodule
)
# No effect
# target_include_directories(apptypetest
# PUBLIC
# "${PROJECT_SOURCE_DIR}/logicmodule"
# logicmodule
# )
# include_directories(logicmodule)
It all compiles fine but at runtime I get an error when trying to call MySingleton:
qrc:/qt/qml/Main/Main.qml:16: ReferenceError: MySingleton is not defined
I tried to research this issue but cannot find exactly my case, the closest I got is this one: qmltyperegistration include path does not acknowledge subdirectories
But the files with custom types seems to be included just fine without any prefixes.
I also noticed that in the build folder I have a file build\Debug\logicmodule\logicmodule_qmltyperegistrations.cpp and it contains calls to qmlRegisterTypesAndRevisions but it seems that it doesn't have an effect?
Also tried a project structure without any additional libraries (so without logicmodule lib, everything is in the project root and I have only one CMakeLists) and it seem to be working fine and all the custom types are recognized by QML.
Could somebody point me to what am I doing wrong here with the library? Why can't QML recognize the type when it's defined inside a lib?
The problem was in using NO_PLUGIN in qt_add_qml_module() call. After removing it and linking the executable to *plugin target instead of backing lib everything worked.
Final CMakeLists:
# logicmodule/CMakeLists.txt
project(logicmodulelib VERSION 0.1 LANGUAGES CXX)
qt_add_library(${PROJECT_NAME} STATIC)
qt_add_qml_module(${PROJECT_NAME}
URI logicmodule
VERSION 1.0
RESOURCE_PREFIX "/qt/qml"
QML_FILES
Main.qml
SOURCES
MySingleton.h
Message.h
)
target_link_libraries(${PROJECT_NAME} PUBLIC
Qt6::Core
Qt6::Gui
Qt6::Qml
Qt6::Quick
)
target_include_directories(${PROJECT_NAME}
PUBLIC
.
)
# CmakeLists.txt
cmake_minimum_required(VERSION 3.16)
project(typetest VERSION 0.1 LANGUAGES CXX)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)
qt_policy(SET QTP0001 NEW)
set(QT_QML_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/qt/qml)
set(QML_IMPORT_PATH "${CMAKE_BINARY_DIR}/qt/qml" CACHE STRING "Qt Creator extra qml import paths"
FORCE)
qt_standard_project_setup(REQUIRES 6.5)
add_subdirectory(logicmodule)
qt_add_executable(apptypetest
main.cpp
)
target_link_libraries(apptypetest PUBLIC
Qt6::Gui
Qt6::Qml
logicmodulelibplugin
)
Thanks to people on qt forum that helped me solve this: