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c++qtqmlqt5qt-signals
laurapons
laurapons

Reputation: 1103

Connecting signals from QML to Qt: var to QList<QString>

I am trying to connect a signal from QML to a SLOT from Qt. The signal passes a QList variable.

QML:

property var cases: ["A", "B", "C"]

signal casesClicked(list cases)

Qt:

d->m_view->setSource(QUrl("qrc:/qml/main.qml"));
d->m_item = d->m_view->rootObject();
QObject::connect(d->m_item, SIGNAL(casesClicked(QList<QString>)), this, SLOT(onCasesClicked(QList<QString>)));

The issue I am having is that I don't know how to declare QList from the QML side, so it is taken directly. If I declare it with:

signal casesClicked(var cases)

then, the signal is not connected, and I if I declare it as a list or an Array it says "Invalid signal parameter type: list/Array"

Any tip? I don't have any problems with single int, bool or string. Thanks,

Upvotes: 2

Views: 1925

Answers (2)

eyllanesc
eyllanesc

Reputation: 244282

I do not think it is appropriate to make the connection on the C ++ side since when compiling it does not know the signal created in QML, a possible solution is to make the connection on the QML side. In the following I show an example

main.cpp

#include <QGuiApplication>
#include <QQuickView>
#include <QQmlContext>
#include <QDebug>
class Test: public QObject{
QQuickView *view;
Q_OBJECT
public:
    Test(QObject *parent=Q_NULLPTR):QObject(parent)
    {
        view = new QQuickView;
        view->rootContext()->setContextProperty("Test", this);
        view->setSource(QUrl("qrc:/main.qml"));
        view->show();
    }
public slots:
    void onCasesClicked(QVariantList cases){
        qDebug()<<cases;
    }
};

int main(int argc, char *argv[])
{
#if defined(Q_OS_WIN)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif

    QGuiApplication app(argc, argv);
    Test test;
    return app.exec();
}

#include "main.moc"

main.qml

import QtQuick 2.9

Item {
    id: it
    visible: true
    width: 640
    height: 480

    signal casesClicked(var cases)

    MouseArea {
        id: mouseArea
        anchors.fill: parent
        onClicked: it.casesClicked(["B", 1, "D"])
    }

    Connections{
        target: it
        onCasesClicked: Test.onCasesClicked(cases)
    }
    // or
    // onCasesClicked: Test.onCasesClicked(cases)
    // if the connection is made in the same item
}

Output:

(QVariant(QString, "B"), QVariant(int, 1), QVariant(QString, "D"))

Upvotes: 1

dtech
dtech

Reputation: 49329

IIRC JS arrays are converted to QVariantList in C++, so try using that instead.

EDIT:

Ok, it appears that although the above is the default automatic conversion, it doesn't work for some reason when the connection is made from C++.

In this scenario the signal value is converted to a QVariant instead, which can directly be converted to a QStringList which is convenient. However that approach will not work if you have a JS array with different data types in it, which is perfectly legal and quite frequently used in JS.

QVariantList will still work if you pass the JS array as a parameter to a C++ function call thou. So you can iterate each value and handle a "polymorphic" array.

Whether or not it is recommended practice to create connections from to QML objects from C++ depends on the usage scenario, overall, I'd say that it isn't. The recommended practice is to expose the C++ core interface to QML and do the connections from there and to avoid touching any QML from C++ whatsoever. But there are a few corner cases where exceptions can be made.

Upvotes: 1

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