Creating and changing qml objects from c++

[SeDi]

Hi,
I create a component in c++ and use it as a sourceComponent for a Loader in QML, but - though setProperty returns true - I can't see the properties set by c++.

The loader approach here is kind of a necessity, because in the end, the object to be created will be a complex, nested structure of qml files (I want to display math formulas). Most of these qml elements have loaders themselves (again: later to be set by c++), here I use the simplest qml (like an end branch of the nested structure).

I try it this way (shortened code):

main.cpp:

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

    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    QObject *root = engine.rootObjects()[0]; // yet unused

   QQmlComponent* component = new QQmlComponent(&engine, QUrl(QStringLiteral("qrc:/ElementVariable.qml")));
    QQuickItem *item = qobject_cast<QQuickItem*>(component->create());

    qDebug()<< item->setProperty("text", "myNewText");  //returns true
    qDebug()<<item->setProperty("visible", true); //returns true

    engine.rootContext()->setContextProperty("EquationComponent", QVariant::fromValue<QQmlComponent*>(component));
 
    return app.exec();
}

main.qml:

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    Component {
        id: emptyComponent
        Item {
        }
    }
    Loader {
        objectName: "equationLoader"
        sourceComponent: typeof EquationComponent !== "undefined" ? EquationComponent : emptyComponent;
        width: childrenRect.width
        height: childrenRect.height
    }
}

ElementVariable.qml:

import QtQuick 2.0

Rectangle {
    property alias text: textItem.text
    width: childrenRect.width
    height: childrenRect.height
    Text {
        id: textItem
        width: implicitWidth + 10
        height: implicitHeight
        text: "textUnchanged"  // this I want to change
        verticalAlignment: Text.AlignVCenter
        horizontalAlignment: Text.AlignHCenter
    }
}

Result: I do see the element, but I don't see the text that is set in the main.cpp, I only see the Component's default text.

Any ideas?

[dheerendra]

@SeDi said in Creating and changing qml objects from c++:

I can't see the properties set by c++.

Where are you setting the properties in C++ which you would like see on QML ? Atleast I don't see that code here.

[SeDi]

@dheerendra - thanks for your reply!

I set it right before the end of the main.cpp. It is a bit "hidden" because I have prepended it with qDebug()<<

    qDebug()<< item->setProperty("text", "myNewText");  //returns true
    qDebug()<<item->setProperty("visible", true); //returns true

[dheerendra]

Here you are changing the properties of object called item. This object is not going to change your component.

[SeDi]

@dheerendra - oh, I see. Thank you for your time and effort!

I have changed

engine.rootContext()->setContextProperty("EquationComponent", QVariant::fromValue<QQmlComponent*>(component));

into

engine.rootContext()->setContextProperty("EquationComponent", QVariant::fromValue<QQuickItem*>(item));

but I get the error:
"qrc:/main.qml:17:26: Unable to assign ElementVariable_QMLTYPE_2 to QQmlComponent"

This seems understandable - it's called sourceComponent: after all.

Having reverted the above idea (back to setting the component, not the item) I have also tried to set the properties on the component - doesn't work either:

    qDebug()<< component ->setProperty("text", "myNewText");  //returns false
    qDebug()<<component ->setProperty("visible", true); //returns false

It appears that I might have a misconception about Components/Items and that I lack sufficient knowledge about the Loader QML element.

• is a Component more like a template / a class?
• is an Item its realisation, like an actual object?
• How can I "force-feed" the Loader from c++ with a specific Object created from there?

[dheerendra]

Yes component is like class. Once done you can’t change. you can change properties of object. You can specify the properties while loading. You can check documentation for this.

[SeDi]

Thank you very much! That solved it. The creation process is being done automatically by the Loader in QML. It didn't make sense to make a second and unrelated "component->create()" in c++. I have to set the Component first and then get the Item from the Loader via property("item") afterwards.

Here's the code involved:

loader->setProperty("sourceComponent", QVariant::fromValue<QQmlComponent*>(component));
QQuickItem* loaderItem = qvariant_cast<QQuickItem*>(loader->property("item"));
loaderItem->setProperty("text", "myNewText");

[dheerendra]

NO need to set the property from C++ by getting item. You can do something like
Loader { sourceComponent: EquationComponent; text: "PthinkS" }