Changing members of Q_GADGET from QML - c++

Question

I am trying to propagate Q_GADGET as a Q_PROPERTY into QML, change it there and pass it back into C++.
I have class that derives from Q_OBJECT, which has the Q_GADGET class as a member.
class Foo : public QObject
{
Q_OBJECT
Q_PROPERTY(QGadgetClass bar READ bar WRITE setBar NOTIFY barChanged)
public:
...
QGadgetClass bar() const { return bar_; }
void setBar(const QGadgetClass &bar) { bar_ = bar; emit barChanged(); }
...
signals:
void barChanged();
private:
QGadgetClass bar_;
}
The Q_GADGET class looks like this:
class QGadgetClass
{
Q_GADGET
Q_PROPERTY(AnotherQGadgetClass test READ test WRITE setTest)
... // there are also properties ID & name
public:
...
// QGadgetClass getMyself() const { return *this; } // function explained later
AnotherQGadgetClass test() const { return test; }
void setTest(const AnotherQGadgetClass &test) { test_ = test; }
...
private:
AnotherQGadgetClass test_;
}
Q_DECLARE_METATYPE(QGadgetClass)
I am trying to access Q_GADGET from QML classic way like accessing a Q_OBJECT, but the setters are not called. If I get AnotherQGadgetClass via getter and change it's properties, the setters are called and everything works, but for some reason I cannot manipulate the QGadgetClass. My code in QML looks like this:
Item {
property var bar: foo.bar
function changeBar()
{
console.log(bar.name) // works
console.log(bar.id) // works
bar.name = "New name" // the WRITE function of Q_PROPERTY(name ...) is not called
console.log(bar.name) // shows old name
console.log(bar.test) // prints out AnotherQGadgetClass correctly
var temp = bar.test // copies AnotherQGadgetClass correctly
console.log(temp.name) // prints AnotherQGadgetClass's name
temp.name = "New temp name" // setter is called
console.log(temp.name) // prints new name
bar.test = temp // constructor is NOT called
console.log(bar.test) // prints out old AnotherQGadgetClass
// following code works and will be explained bellow this code
var aa = bar.getMyself() // calls the "hackish" method
console.log(aa.name) // prints out name of QGadgetClass
aa.name = "New name" // calls the setter
console.log(aa.name) // prints out new name
}
}
I have done some research already, but found nothing but this page. I have also found some very unpretty solution here and it worked, but I find it very hacky.
Note that every Q_GADGET is declared as metatype via Q_DECLARE_METATYPE(...) & is registered before usage via qRegisterMetaType<...>("...").
Is there any prettier solution to access QGadgetClass directly from QML, without need to call getMyself() method? Why are the Q_GADGET class setters not called?

Answer 1

A Q_GADGET is always treated as a value type in QML: it must be passed by copying. So the object that you manipulate in QML is not the same instance that you created in C++, and property changes aren't visible in the original. Many related issues are linked from https://bugreports.qt.io/browse/QTBUG-82443