How to detect dataChanged() and resourcesChanged() from QML - qt

Question

The codes below:
Item{
onDataChanged: console.log("Data changed")
}
Item{
onResourcesChanged: console.log("Resources changed")
}
throw Cannot assign to non-existent property "onDataChanged" and Cannot assign to non-existent property "onResourcesChanged" respectively.
This is not the case with the childrenChanged() signal. The reason for this is that in qtdeclarative/src/quick/items/qquickitem.h, children property is declared with:
Q_PRIVATE_PROPERTY(QQuickItem::d_func(), QQmlListProperty<QQuickItem> children READ children NOTIFY childrenChanged DESIGNABLE false)
but this is not the case for data or resources. They are declared with:
Q_PRIVATE_PROPERTY(QQuickItem::d_func(), QQmlListProperty<QObject> data READ data DESIGNABLE false)
Q_PRIVATE_PROPERTY(QQuickItem::d_func(), QQmlListProperty<QObject> resources READ resources DESIGNABLE false)
with no changed() signal. Why is this design choice to particularly hide the change on non-visible children made? Moreover, how can the change on data be detected from QML?

Answer 1

Why do you need this ?
One possible workaround is to listen for child events. I wrote a quick attached type PoC :
ChildListener.h :
#ifndef CHILDLISTENER_H
#define CHILDLISTENER_H
#include <QObject>
#include <QtQml>
class ChildListener : public QObject {
Q_OBJECT
public:
ChildListener(QObject *object) : QObject(object) {
if (object)
object->installEventFilter(this);
}
static ChildListener *qmlAttachedProperties(QObject *object) {
return new ChildListener(object);
}
signals:
void childAdded(QObject* child);
void childRemoved(QObject* child);
protected:
bool eventFilter(QObject *obj, QEvent *event) override {
Q_UNUSED(obj)
if (QChildEvent *childEvent = dynamic_cast<QChildEvent*>(event)) {
if (childEvent->added())
emit childAdded(childEvent->child());
if (childEvent->removed())
emit childRemoved(childEvent->child());
}
return false;
}
};
QML_DECLARE_TYPEINFO(ChildListener, QML_HAS_ATTACHED_PROPERTIES)
#endif // CHILDLISTENER_H
Register it with qmlRegisterUncreatableType<ChildListener>("fr.grecko.ChildListener", 1, 0, "ChildListener", "ChildListener can only be accessed as an attached type."); and you can now use it like so :
import fr.grecko.ChildListener 1.0
/* ... */
Timer {
id: foo
objectName: "My name is foo"
}
Item {
ChildListener.onChildAdded: print("child added : ", child)
data: [foo];
}
This outputs : qml: child added : QQmlTimer(0x7ffe22f538e0, "My name is foo") in the console