How to bind a property to a singleton object property from QML - qt

Question

There is a question about how to bind from a singleton object property to a QML property. But what about if we like to bind a QML property to a singleton object.
Here is the singleton class definition,
class Singleton : public QObject {
Q_OBJECT
Q_PROPERTY(QString name READ name WRITE setName)
public:
explicit Singleton(QObject *parent = nullptr);
QString name() const;
void setName(const QString &name);
private:
QString m_name;
};
And on QML
property string qmlName: textField.text
TextField {
id: textField
}
I would like to bind textField.text to Singleton object name property. It is possible to bind it with a workaround like,
onQmlNameChanged: {
Singleton.name = qmlName;
}
But that won't be an Property Binding actually, because it is an assignment.
So is there a more nature binding way to a singleton object property?

Answer 1

You could try to assign the binding like this:
Component.onCompleted: Singleton.name = Qt.binding(function() { return qmlName })
It works for normal QML-Objects, not sure it works with a singleton class, though. Anyhow, you can read more about this approach in the section "Creating Property Bindings from JavaScript".

Answer 2

That is essentially what a property binding does, at least I assume it is what it does - it connects the changed() signals of the related variables to reevaluating the bound expression which references them.
So this in practice is a binding:
onQmlNameChanged: {
Singleton.name = qmlName;
}
You will only have a problem if you only execute the assignment once, but if it is attached to a signal it will keep updating as expected form a binding.
That would be identical to a Singleton.name : qmlName, unfortunately, the QML syntax does not allow to do it in this form. So for all intents and purposes, you do have a binding, even if it uses a different syntax to achieve it.
In fact this shouldn't be much different from what QML does under the carpet for you. For example the binding:
someProp : anotherProp + yetAnotherProp
is implement as something like this:
function unnamed(this) { return this.anotherProp + this.yetAnotherProp }
anotherPropChanged.connect(function(this) { this.someProp = unnamed(this) })
yetAnotherPropChanged.connect(function(this) { this.someProp = unnamed(this) })
Obviously, that is quite cumbersome to do manually especially as the expression becomes more complex and references more objects, so QML is doing it for you.