Cannot connect MyClass::data(QVariant) to (null)::updateData(QVariant)

[p3c0]

Hi,

Are you using Qt 4.x or Qt 5.x ?
QDeclarativeView wont work with QtQuick 2.2 which was introduced in Qt 5

[houmingc]

did try 2 attempt and failed also
@
---------1st attempt ---------------------------
QQmlEngine engine;
QQmlComponent component(&engine, QUrl("qrc:/file/main.qml"));

QObject *root=component.create();

QObject::connect(&myClass, SIGNAL(data(QVariant)), root, SLOT(updateData(QVariant)));

error:
aggregate "QQmlEngine Engine" has incomplete type and cannot be defined
variable 'QQmlComponent component' has initializer but incomplete type

---------2rd attempt ---------------------------

QQuickView view;
view.setSource(QUrl("qrc:/file/main.qml"));

QObject rootObject= dynamic_cast<QObject>(view.rootObject());
QObject::connect(&myClass, SIGNAL(data(QVariant)), root, SLOT(updateData(QVariant)));

error:
cannot dynamic_cast view.QQuickView::rootObject to type 'Class QObject'
main.o Error 1

@

[houmingc]

using Qt Creator 3.1.2 (opensource)
will be upgrading the Qt end of the week.

Not able to create an QML instance meant i can't continue anything, isn't it??

[p3c0]

2nd Attempt will not work since Window requires QQmlApplicationEngine and not QQuickView to load it.

Have a look at "this":http://qt-project.org/doc/qt-5/qtqml-cppintegration-interactqmlfromcpp.html#invoking-qml-methods for an example.

Now instead of using QQmlEngine directly you can use more convenient QQmlApplicationEngine:
@
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));

QObject *rootObject= engine.rootObjects().first();
qDebug() << rootObject->objectName();

QVariant returnedValue;
QVariant msg = "Hello from C++";

QMetaObject::invokeMethod(rootObject, "updateData",
Q_RETURN_ARG(QVariant, returnedValue),
Q_ARG(QVariant, msg));

qDebug() << "QML function returned:" << returnedValue.toString();
@

[houmingc]

i am running Qt on Ubuntu not windows

[p3c0]

It's Window not Windows. Window here refers to the root element which you have used in the QML file.
I think you must start with the basics of QML first.

[houmingc]

Thanks, QMetaObject::InvokeMethod works better than signals/slot for C++ invoking QML. From same document, QML invoking C++, i could not compile, below is my code. Can help?
@
-----------------header file myclass----------------
#include <QDebug>

class MyClass: public QObject
{
Q_OBJECT
public slots:
void cppSlot(const QString &msg){
qDebug()<<"Called the c++ slot with message"<<msg;
}
};

-----------------cpp file----------------
#include <QApplication>
#include <QQmlApplicationEngine>
#include <QQuickView>
#include <QDebug>
#include "myclass.h"

int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);

QQuickView view("qrc:///main.qml");
QObject *item= view.rootObject();

MyClass myClass;
QObject::connect(item,SIGNAL(qmlSignal(QString)),&myClass,SLOT(cppSlot(QString)));

return app.exec();

}

-----------------qml file----------------
import QtQuick 2.2
import QtQuick.Controls 1.1

ApplicationWindow {
id: item
visible: true
width: 640
height: 480
title: qsTr("Hello World")

signal qmlSignal(string msg)

MouseArea{
anchors.fill:parent
onClicked: item.qmlSignal("Hello from QML")
}
}
@

[p3c0]

What error do you get during compiling?

[p3c0]

To use ApplicationWindow/Window you will need to use QQmlApplicationEngine and not QQuickView. The error should be at runtime not compiletime.

[houmingc]

error at QQuickView and QObject statement.

@

no matching function for call to 'QQuickView::QQuickView(const char[16])'
cannot convert 'QQuickItem' to 'QObject *' in initialization.
@

[p3c0]

Use QQmlApplicationEngine for loading ApplicationWindow/Window.
@
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
QObject *rootObject= engine.rootObjects().first();
QObject::connect(rootObject,SIGNAL(qmlSignal(QString)),
&myClass,SLOT(cppSlot(QString)));
@

Above should work.

[houmingc]

insert statement 'MyClass myclass;' before QObject::connect.
2 error encounter:
undefined reference to 'vtable for MyClass'
ld returned 1 exit status

[p3c0]

Run qmake and then Rebuild.

[houmingc]

I created a new header file for class 'MyClass'
Now my error is
QQmlApplicationEngine failed to load component
Assert:"!isEmpty()" in file /include/QtCore/qlist.h
The program has unexpectedly finished.

[p3c0]

I don't know. You must have misplaced something somewhere. Better would be to create a new project with that class and qml file. Use the code that i posted above to launch the qml file. It is not a very complicated code.

[houmingc]

Thanks very much.