Qt: when can I access dynamic properties from qtcreator?

Question

I have a couple of widgets in QtCreator that are promoted from the same class. However, I'd like them to have subtle differences between the two, so I'd like to pass some differences in the UI file that the promoted class can use to distinguish itself. Dynamic properties seem like the way to go so in the UI editor I've assigned a dynamic property to each promoted widget. In the code I tried accessing the property, but noticed it seems to only be available post construction (probably because Qt is calling setProperty() after the object is created.

MyWidget::MyWidget(QWidget* parent) : QGLWidget(parent)
{
    this->property("someProperty").toString(); // returns blank
}

void MyWidget::initializeGL()
{
    this->property("someProperty").toString(); // returns string set in UI file
}

So my question how do people use these properties for constructor-type stuff? I could just do that in initializeGL, but that seems odd since these properties might not be related to initializing OpenGL. I imagine I could also connect to the property changed signal and do it there. Is that the common way to handle this?

Answer 1

If the generated code for setupUi() from your .ui file does something like this:

MyWidget *w = new MyWidget;
w->setProperty(...);

then your constructor is accessing a meta property that does not yet exist.

You can reimplement QObject::event() to capture QDynamicPropertyChangeEvents, letting you act once the property is initialized.

bool MyWidget::event(QEvent *ev)
{
    if (ev->type() == QEvent::DynamicPropertyChange) {
        if (QDynamicPropertyChangeEvent *propEv = static_cast<QDynamicPropertyChangeEvent *>(ev)) {
            if (propEv->propertyName() == "someProperty")
                ...
        }
    }
}

Bear in mind that this code will be called every time there is a dynamic property change.

A better approach may be to create a function to perform the necessary initilization on the widget after setupUi() etc. are called and the dynamic property is created.

void setupMyWidget(MyWidget *w)
{
    QString s = w->property("someProperty").toString();
    ...
}

Typically, dynamic properties are assigned a default value in the constructor so that they are always available and non-null later.

setProperty("someProperty", defaultValue);