Connecting C++ signal to QML slot

Mary_M

Hi,
I am new to qt and want to pass a simple signal from a c++ widget to qml. I followed the structure in this document with a few changes. . The code complies but nothing shows up due to this error:
QObject::connect: No such slot QQuickItem::qmlSlot(QVariant) in ../qmltest/mainwindow.cpp:15
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

qmltest.pro:

QT += core gui qml quickwidgets
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
CONFIG += c++11
...

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

mainwindow.cpp:
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
{
auto cppObj = new Widget();
auto quickWidget = new QQuickWidget(QUrl("qrc:/main.qml"));
auto qmlObj = quickWidget->rootObject();

// Connect C++ signal to QML slot
connect(cppObj, SIGNAL(cppSignal(QVariant)),
        qmlObj, SLOT(qmlSlot(QVariant)));

}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
mainwindow.h:
class MainWindow : public QMainWindow
{
Q_OBJECT

public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
};
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
widget.cpp:

Widget::Widget(QWidget *parent)
: QWidget(parent)
{

button = new QPushButton("Click Me!", this);
connect(button, &QPushButton::clicked, [=] {
    emit cppSignal("hello");
});

}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
widget.h:

class Widget : public QWidget
{
Q_OBJECT
QPushButton *button;
signals:
void cppSignal(const QVariant& sentMsg) const;

public:
Widget(QWidget *parent = 0);
~Widget();
};
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
main.qml:
import QtQuick 2.15
import QtQuick.Controls 2.15

Item {
Rectangle {
width: 100; height: 100
function qmlSlot(receivedMsg) {
console.log("QML received: " + receivedMsg)
}

}

Can anyone help me with this? I also used MetaObject::invokeMethod(qmlObj, "qmlSlot"); same error.
Note that in this project I designed a keyboard by QWidgets to type and want to pass the pushed keys to the qml which is a chat GUI in this work. So I have to use qquickwidgets to show both keyboard and chat GUI in the same window.

raven-worx

@Mary_M
you try to connect to a method of the rootItem(). But in your qml file the object with the desired method is not the root item. It has a parent Item.

Its probably anyway easier to do the connection inside QML and not in C++.

Mary_M

@raven-worx Thank you! I tried with the root item, no error but the text doesn't show up! Also, I need the signal to be emitted from C++.

eyllanesc

@Mary_M It is recommended to make the connection in qml:

*.cpp

auto cppObj = new Widget();
auto quickWidget = new QQuickWidget();
quickWidget->rootContext()->setContextProperty("cppObj", cppObj);
quickWidget->setSource(QUrl("qrc:/main.qml"));

main.qml

import QtQuick 2.15
import QtQuick.Controls 2.15

Item {
    Rectangle {
        width: 100; height: 100
    }
    Connections{
        target: cppObj
        function onCppSignal(receivedMsg){
            console.log("QML received: " + receivedMsg)
        }
    }
}

GabrielRR

Try something like this:

Q_INVOKABLE void triggerEvent(QString text) {
 emit signalEvent(text);
}
Person() {
 startTimer(1000);
}
void timerEvent(QTimerEvent *e) {
 QString text = "Hello";
 triggerEvent(text);
}

signals: 
void signalEvent(QString text);

//In qml:

id: myQmlObject
onSignalEvent: {
 console.log(" Message " + text);
}

Mary_M

@eyllanesc Thank you so much! but when I use connections this is the error:
QObject::connect: Cannot connect Widget::cppSignal(QVariant) to (nullptr)::qmlSlot(QVariant)

I also tried this one but still the same:
Item {
Rectangle {
id: rect
width: 100; height: 100
Text{id:myText} }
Connections{
target: cppObj
function onCppSignal(receivedMsg){
MyText.text = receivedMsg;
}
}
}

Mary_M

@GabrielRR Thanks! but how can I use this in widget.cpp not in mainwindow?

eyllanesc

@Mary_M Have you deleted your C++ connection attempt? That error message is not from QML but from C++, also remove build folder and compile.

Mary_M

@eyllanesc You are right! I deleted connect in mainwindow now and built it again. No error now but no output either! Only empty window showed up. At the end I used Qframe and gridline to add button and rect alongside. Many thanks!

Pablo J. Rogina

@Mary_M If your issue is solved please don't forget to mark the post as such!