Changed event for JavaScript Binding in Qml? - qml

Question

I have a property which is defined by a JavaScript expression (currentContainer):
Item {
id: theContainer
property alias currentIndex: theListView.currentIndex
signal onCurrentIndexChanged()
property MyCustomCppContainer currentContainer: {
if(theListView.currentIndex >= 0)
theModel.getCustomContainer(theListView.currentIndex)
else
null
}
signal onCurrentContainerChanged() // nobody calls this signal (yet)
MyCustomCppModel {
id: theModel
}
ListView {
id: theListView
anchors.fill: parent
model: theModel
currentIndex: -1
onCurrentIndexChanged: theContainer.onCurrentIndexChanged()
/* Other properties stripped for example */
}
}
Sadly I always get the last selected container, not the currently selected one:
ContainerItem {
onCurrentIndexChanged: {
//On first change, currentContainer is null
//though the first one was selected
//After selecting the second entry
//I get the result I expected last time
console.log(currentContainer.name);
}
}
I think a solution would be to have another signal for currentContainer: onCurrentContainerChanged().
But who calls this special signal?

Answer 1

I could solve this using a C++ helper class:
class PropertyChangedHelper : public QObject
{
Q_OBJECT
Q_PROPERTY(QVariant theProperty WRITE setTheProperty NOTIFY thePropertyChanged)
public:
PropertyChangedHelper(QObject* parent = nullptr) : QObject(parent) {}
virtual ~PropertyChangedHelper() {}
void setTheProperty(QVariant) {
Q_EMIT thePropertyChanged();
}
Q_SIGNALS:
void thePropertyChanged();
private:
Q_DISABLE_COPY(PropertyChangedHelper)
};
Usage is quite simple:
PropertyChangedHelper {
theProperty: containerItem.currentContainer
onThePropertyChanged: {
console.log(containerItem.currentContainer.name);
}
}
I am not sure if this violates any Qml/QtQuick philosophies, but it works.