findChild() always returns 0 for QML objects

Erlend E. Aasland

Hi,

I've got a QML component embedded in a Qt C++ app via QQuickView. Everything loads nice and neat, but now I need to control the QML module by modifying its properties. I've tried a bunch of findChild<QObject *>(''tag''), but I always get null. I've got objectName: ''tag'' set in the QML component, so I assume the tag is set properly. Current code:

form.cpp:

QObject *obj = this->findChild<QObject *>("tag");
if (obj) {
    obj->setProperty("ticks", tick_count);
} else {
    qDebug() << "Could not find QML component!";
}

component.qml:

Item {
    id: tag
    objectName: "tag"
    property int ticks: 0
    /* … */
}

sierdzio

@Erlend-E.-Aasland said in findChild() always returns 0 for QML objects:

this->findChild<QObject *>("tag");

What is the this object? Is it the QML engine or something that actually holds your QML components?

objectName: "tag"

That'c correct. findChild<>() looks for named objects.

component.qml:

Are you sure that component is instantiated? Is there only one copy of it?

In general, I'd rather recommend a different approach. Instead of forcing properties like this, use the Connections element. Add some QObject "controller" which will manage C++ - QML communiacation, add it as engine context property, then refer to it in your QML code. Either like this or like this.

Erlend E. Aasland

@sierdzio Thanks for your input! The this-object was the parent widget holding the QQuickView. I solved it by saving the root object of the QQuickView:

QObject *obj = quickview->rootObject();
if (obj)
    obj->setProperty("ticks", tick_count);
else
    qDebug() << "Could not set tick count!";

Anyway, I'll have a look at the different approach you suggest. It sound like a more robust way of interacting with QML.

Best regards, E