Binding does not update property but Connections object does

patrickkidd

There must be something I am missing about how the Qml property binding system works. The following code does not update the Text.text property when the notify signal for either of the properties personAName and personBName are changed:

Text {
    id: titleLabel
    text: "%1, %2".arg(marriageModel.personAName).arg(marriageModel.personBName)
}

However, using a connection object does work:

Text {
    id: titleLabel
    function updateText() {
        this.text = "%1, %2".arg(marriageModel.personAName).arg(marriageModel.personBName)
        print(text)
    }
    Connections {
        target: marriageModel
        onPersonANameChanged: titleLabel.updateText()
        onPersonBNameChanged: titleLabel.updateText()
    }
}

Surely these two cases are functionally equivalent according to the Qml spec? What are the conditions under which the Text.text in the first example wouldn't update when personANameChanged and personBNameChanged signals are emitted, but it is updated with explicit Connections to the signals in the second example?

Thanks!

patrickkidd

I figured it out. I was setting the Qt property to constant in the declaration. Apparently Qml caches the property somewhere in the Qml internals and never queries for it again.

I had misunderstood the constant property attribute to mean that it was read-only, i.e. there was no setter. In fact, constant means it never changes over the course of the application's life cycle.

So that is one additional condition where which the Text.text would not update when the property notify signals are emitted.

fcarney

It looks to me that the signal from marriageModel is not the same as marriageModel.personAName. Is there a signal associated with personAName and personBName?

Edit:
I just saw that there are signals. It should be updating then. What does your property code look like?

patrickkidd

@fcarney said in Binding does not update property but Connections object does:

It looks to me that the signal from marriageModel is not the same as marriageModel.personAName. Is there a signal associated with personAName and personBName?

Edit:
I just saw that there are signals. It should be updating then. What does your property code look like?

Could you please clarify? The second example shows the notify signal names associated with the properties.

GrecKo

What is the type of marriageModel and is personAName a property of it?

Can you share the .h where you declared your properties?

patrickkidd

Marriage model is a QObject and the two signals we’re added as the notify signals for the two properties. Qml automatically derived the onPersonANameChanged property name from the standard naming scheme.

These properties are automatically generated in python and I unfortunately can’t share the code as it’s too complicated. So I am asking more from a carefully theoretical perspective about the Qml binding mechanism. So far as I can tell, the example should be self explanatory.

GrecKo

text: "%1, %2".arg(marriageModel.personAName).arg(marriageModel.personBName)

This binding should update itself if the personAName or personBName notify signal is emitted.
Unless you share a minimal reproducible example we can't really help you more.

patrickkidd

I figured it out. I was setting the Qt property to constant in the declaration. Apparently Qml caches the property somewhere in the Qml internals and never queries for it again.

I had misunderstood the constant property attribute to mean that it was read-only, i.e. there was no setter. In fact, constant means it never changes over the course of the application's life cycle.

So that is one additional condition where which the Text.text would not update when the property notify signals are emitted.