How does setContextProperty() fail in this situation? - c++

Question

I have got three classes created in C++ as Window, PropertyList and MyParams and I want to pass PropertyList and MyParams these two classes to qml.
class Window
{
public:
PropertyList* getPropertyList();
private:
PropertyList* propertyList;
};
class PropertyList : public QObject
{
Q_OBJECT
public:
MyParams* getMyParams();
Q_INVOKABLE void test();
private:
MyParams* myParams;
};
class MyParams : public QObject
{
Q_OBJECT
public:
Q_INVOKABLE void test();
};
int main(int argc, char *argv[])
{
Window* window = new Window();
PropertyList* propertyList = window->getPropertyList();
MyParams* myParam = propertyList->getMyParams();
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty(QStringLiteral("myParams"), myParam);
engine.rootContext()->setContextProperty(QStringLiteral("propertyList"), propertyList);
}
Why does this work:
engine.rootContext()->setContextProperty(QStringLiteral("propertyList"), propertyList);
And this fail:
engine.rootContext()->setContextProperty(QStringLiteral("myParams"), myParam);
Is it because I declare PropertyList as Q_OBJECT? And how can I fix this? Thanks a lot.
propertyList.test() can be called succesfully while myParams.test() cannot be called and crashed the qml.

Answer 1

I cannot see any problems in your code example, because the sample is not complete (Consider: https://stackoverflow.com/help/reprex).
Nevertheless I implemented a solution for you. I hope it will help. (Warning: there will be memory leaks around class Window, but it should helpt to understand C++ and Qml binding)
Here is your PropertyList.h:
#ifndef PROPERTYLIST_H
#define PROPERTYLIST_H
#include "myparams.h"
#include <QObject>
class PropertyList : public QObject
{
Q_OBJECT
public:
explicit PropertyList(QObject *parent = nullptr) : QObject (parent) { }
MyParams* getMyParams()
{
return myParams;
}
Q_INVOKABLE void test()
{
qDebug() << "PropertyList test";
}
private:
MyParams* myParams = new MyParams();
};
#endif // PROPERTYLIST_H
Here is your MyParams.h:
#ifndef MYPARAMS_H
#define MYPARAMS_H
#include <QObject>
#include <QDebug>
class MyParams : public QObject
{
Q_OBJECT
public:
explicit MyParams(QObject *parent = nullptr) : QObject (parent) { }
Q_INVOKABLE void test()
{
qDebug() << "MyParams test";
}
};
#endif // MYPARAMS_H
Here is your main.cpp:
#include "propertylist.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlEngine>
#include <QQmlContext>
#include <QDebug>
class Window
{
public:
PropertyList* getPropertyList()
{
return propertyList;
}
private:
PropertyList* propertyList = new PropertyList(nullptr);
};
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
Window* window = new Window();
PropertyList* propertyList = window->getPropertyList();
MyParams* myParam = propertyList->getMyParams();
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty(QStringLiteral("myParams"), myParam);
engine.rootContext()->setContextProperty(QStringLiteral("propertyList"), propertyList);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
{
return -1;
}
return app.exec();
}
Here is your main.qml:
import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
ColumnLayout {
anchors.centerIn: parent
Button {
text: "myParams"
onClicked: {
myParams.test()
}
}
Button {
text: "propertyList"
onClicked: {
propertyList.test()
}
}
}
}