I have a C++ object with a signal that I connected to a QML handler. However, even though I am passing arguments to the signal, the QML arguments show up as undefined
.
With Balloon
and Air
as custom QObject
s, I'm connecting Balloon
's signal void popped(Air const& air, int volume, QString message);
to a QML handler:
Balloon.qml:
import QtQuick 2.12
import QtQuick.Controls 2.12
import ReproImpl 0.1 as Impl
Item {
id: root
width: 500
height: 500
property int sharpness: 50
readonly property Impl.Balloon impl: Impl.Balloon {
onPopped: {
// Output: {}
console.log(JSON.stringify(arguments));
// TypeError: cannot read property 'type' of undefined
console.log("Bang!", air.type, volume, message);
}
}
Button {
anchors.centerIn: parent
text: "Click me"
onClicked: {
console.log("Clicked!");
impl.prick(sharpness)
}
}
}
main.cpp:
#include <QGuiApplication>
#include <QObject>
#include <QQmlEngine>
#include <QQuickView>
namespace my::ns {
// Used as a parameter of the signal
class Air : public QObject {
Q_OBJECT
Q_PROPERTY(QString type READ type)
public:
Air(QString type, QObject* parent = nullptr)
: QObject{parent}
, type_{type}
{}
Air(QObject* parent = nullptr)
: Air{"", parent}
{}
QString type() const { return type_; }
private:
QString type_;
};
class Balloon : public QObject {
Q_OBJECT
public:
Balloon(int toughness, QObject* parent = nullptr)
: QObject{parent}
, toughness{toughness}
{}
Balloon(QObject* parent = nullptr)
: Balloon{10, parent}
{}
Q_SIGNALS:
void popped(Air const& air, int volume, QString message);
public Q_SLOTS:
void prick(int sharpness)
{
if (sharpness > toughness) {
Air air{"Hello"};
Q_EMIT popped(air, 10, "POP!");
}
}
private:
int toughness;
};
void registerModule()
{
qmlRegisterModule("ReproImpl", 0, 1);
qmlRegisterType<Air>("ReproImpl", 0, 1, "Air");
qmlRegisterType<Balloon>("ReproImpl", 0, 1, "Balloon");
qmlProtectModule("ReproImpl", 0);
}
}
int main(int argc, char** argv)
{
QGuiApplication app(argc, argv);
my::ns::registerModule();
QQuickView window(QUrl("Balloon.qml"));
window.show();
return app.exec();
}
#include "main.moc"
What can I do to fix this?
This occurs in Qt 5.12.0 and 5.13.0.
For whatever reason, QML doesn't support reference signal parameters. You are passing the
air
byconst&
:To get this working, you need to pass a non-const pointer:
Note that it is safe to pass a pointer to the stack allocated
air
; QML does not take ownership of it: