[QT][QML] How import a simple variable from CPP class to QML View

Shadow.01

Hi all,

in my QT6 project (created with CMake / OS Ubuntu 20.04) I have a UIConfig file with some global configuration const for the entire app (ground basic color, window size, welcome message and so on). I tried to use it in my QML view, but I see an error. Below all detail. How can I solve the problem? Thank you very much!

UIConfig header:

#ifndef UICONFIG_H
#define UICONFIG_H

#include <QQuickPaintedItem>
#include <QObject>

class UIConfig
{
    QML_ELEMENT

public:
    static double const FOOBAR;
    static int const WINDOW_DESKTOP_WIDTH;
    static int const WINDOW_DESKTOP_HEIGHT;


};

#endif // UICONFIG_H

UIConfig cpp:

#include "uiconfig.h"

double const UIConfig::FOOBAR = 42;
int const UIConfig::WINDOW_DESKTOP_WIDTH = 1200;
int const UIConfig::WINDOW_DESKTOP_HEIGHT = 800;

A fragment of my QML view:

import QtQuick 2.0
import QtQuick.Layouts 1.3
import QtQuick.Controls 2.5
import com.my.app

Window {
    width: WINDOW_DESKTOP_WIDTH
    height: 800
    visible: true
    title: qsTr("Container page")
    color: "#ffffff"

    Component.onCompleted: {

        x = Screen.width / 2 - width / 2
        y = Screen.height / 2 - height / 2
    }
(...)

CMakeLists file:

cmake_minimum_required(VERSION 3.14)

project(myapp VERSION 0.1 LANGUAGES CXX)

set(CMAKE_INCLUDE_CURRENT_DIR ON)

set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(QT NAMES Qt6 COMPONENTS Core Quick REQUIRED)
find_package(Qt6 COMPONENTS Core Quick REQUIRED)

set(CMAKE_EXPORT_COMPILE_COMMANDS 1)

set(PROJECT_SOURCES
        main.cpp
        qml.qrc
        images.qrc
        global/uiconfig.h
        global/uiconfig.cpp
)

qt_add_executable(myapp
    MANUAL_FINALIZATION
    ${PROJECT_SOURCES}
)

target_compile_definitions(myapp
  PRIVATE $<$<OR:$<CONFIG:Debug>,$<CONFIG:RelWithDebInfo>>:QT_QML_DEBUG>)
target_link_libraries(myapp
  PRIVATE Qt6::Core Qt6::Quick)

set_target_properties(myapp PROPERTIES
    QT_QML_MODULE_VERSION 1.0
    QT_QML_MODULE_URI com.my.myapp
)

list(APPEND QML_IMPORT_PATH .)

qt6_qml_type_registration(myapp)
qt_import_qml_plugins(myapp)
qt_finalize_executable(myapp)

Runtime error if I try to import a var, i.e. WINDOW_DESKTOP_WIDTH assign to window width:

ReferenceError: WINDOW_DESKTOP_WIDTH is not defined

VRonin

@Shadow-01 said in [QT][QML] How import a simple variable from CPP class to QML View:

in which part of my app should I add the row you wrote? In the main.cpp?

yes, you should have something similar to:

#include "uiconfig.h"
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("WINDOW_DESKTOP_WIDTH", &UIConfig::WINDOW_DESKTOP_WIDTH);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    return app.exec();
}

VRonin

See https://qmlbook.github.io/ch18-extensions/extensions.html#

Shadow.01

Doesn't work and it is totally different from my case. I am not opening a QML from a CPP class, but from other QML. I simply want a const/var, I can define in a certain point of my app, and recall anywhere. Other ideas? Thank you very much.

VRonin

From your code above is hard to understand your goal as you don't show the assignment of WINDOW_DESKTOP_WIDTH nor your main().
What I understood is that you have a variable defined in C++ and you want to use it in QML and that's what that section of the QML book covers

//QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("WINDOW_DESKTOP_WIDTH", &UIConfig::WINDOW_DESKTOP_WIDTH);

Shadow.01

@VRonin you right, sorry. I edited the question. As you can see now, the way I want use this var is by setting as a width of my mentioned qml window the WINDOW_DESKTOP_WIDTH value defined in UIConfig (see please section "A fragment of my QML view:".

Now, in which part of my app should I add the row you wrote? In the main.cpp? Thanks again for your helping.

Shadow.01

I DID IT!

Thanks for helping.

The way was, put the variable in the cpp and h files mentioned in my original question (it was correct); put your row in the main.cpp file; call the global path of the app in the qml file and call the desired var in the qml code.

I didn't see before, because in your link was mentioned a specific cpp file for a qml view; in my case, some view are not linked to a cpp file. Thank you so much, you given me a big help!

VRonin

@Shadow-01 said in [QT][QML] How import a simple variable from CPP class to QML View:

in which part of my app should I add the row you wrote? In the main.cpp?

yes, you should have something similar to:

#include "uiconfig.h"
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("WINDOW_DESKTOP_WIDTH", &UIConfig::WINDOW_DESKTOP_WIDTH);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    return app.exec();
}