How to register Qt C++ object to use it in QML

Question

I try to register ColorsSource *pSource object to use it in QML but I get erreor in string: "QQmlContext *context = myObject->rootContext();" undefined reference to __imp__ZNK12QQuickWidget11rootContextEv

main.cpp

#include "ColorsSource.h"

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQuickWidgets/QQuickWidget>
#include <QQmlContext>

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);

    engine.load(url);

    ColorsSource *pSource = new ColorsSource;

    QQuickWidget *myObject = static_cast<QQuickWidget*>(engine.rootObjects().first());

    QQmlContext *context = myObject->rootContext();
    context->setContextProperty("ColorSource", pSource);

    return app.exec();
}

T3 H40 · Accepted Answer · 2020-06-17 09:23:01Z

4

You are missing the quickwidgets Qt module. Thats why it cannot find these Symbols. If you use qmake, add QT += quickwidgets to your pro file.

But that won't be your Problem. QQuickWidgets are widgets to display some QQuick code, to be used in a Widgets application. But as you use a QQmlApplicationEngine, you are working in Quick. Your rootObject() won't be a QQuickWidget. So instead of looking for a QQuickWidget, access the engines rootContext - replace:

QQuickWidget *myObject = static_cast<QQuickWidget*>(engine.rootObjects().first());
QQmlContext *context = myObject->rootContext();
context->setContextProperty("ColorSource", pSource);

with

QQmlContext *context = engine.rootContext();
context->setContextProperty("ColorSource", pSource);

This will make your code a lot less prone to errors.

Also, in an unrelated note, you should always check your pointers, to make sure your static_cast went allright. Otherwise you will get crashes if the engines rootObjects are empty or the first one is not a QQuickWidget.

edited Jun 17, 2020 at 9:23

answered Jun 17, 2020 at 9:13

T3 H40

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4 Comments

Евгений Дружинин Over a year ago

I use CMake instead of qmake

T3 H40 Over a year ago

@ЕвгенийДружинин - Similar procedure but again, you will not need the quickwidgets module if you are using the QQmlApplicationContext. Strip all the QQuickWidget includes and replace the stuff I wrote in my answer, this should fix things for you

Евгений Дружинин Over a year ago

Thanks it helped me, engine.rootContext() is really works

T3 H40 Over a year ago

Good to hear. As a tip: Have a look into the differences between a QWidget based application and a QQuick based application. These are two entirely different concepts and understanding what you are working with makes a huge difference in your development.

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