Calling C++ method from QML (Qt Quick application) - c++

Question

So, I've done some searching and none of the similar questions I've read have had advice that worked.
I'm using Qt Creator (and I'm not too familiar with Qt) so I'm not sure what voodoo it's done in the background. However, I'm using a standard Qt Quick Application project.
Essentially, I want to call a C++ function from QML that returns a string that replaces some text in the layout, periodically.
Here is main.cpp:
#include <QtGui/QApplication>
#include "qmlapplicationviewer.h"
#include <QDeclarativeContext>
class testClass : public QObject
{
Q_OBJECT
public:
Q_INVOKABLE QString gimmeText() {
return QString("new text");
}
};
Q_DECL_EXPORT int main(int argc, char *argv[])
{
QScopedPointer<QApplication> app(createApplication(argc, argv));
QmlApplicationViewer viewer;
viewer.setOrientation(QmlApplicationViewer::ScreenOrientationAuto);
viewer.setMainQmlFile(QLatin1String("qml/Picenter2/main.qml"));
testClass t;
viewer.rootContext()->setContextProperty("testOb", &t);
viewer.showFullScreen();
return app->exec();
}
And here is a snippet of the layout (as most of it is obviously irrelevant):
Text {
id: text1
x: 105
y: 156
color: "#ffffff"
text: qsTr("text")
font.pixelSize: 12
Timer {
interval: 1000; running: true; repeat: false
onTriggered: text1.text = testOb.gimmeText();
}
The errors given are:
invalid use of incomplete type 'struct QDeclarativeContext' main.cpp (28)
forward declaration of 'struct QDeclarativeContext' qdeclarativeview.h (60)
EDIT: with QDeclarativeContext included, the above disappear, giving these errors:
(.text.startup+0x3e):-1: error: undefined reference to `vtable for testClass'
(.text.startup+0xcf):-1: error: undefined reference to `vtable for testClass'
(.text.startup+0x12b):-1: error: undefined reference to `vtable for testClass'
:-1: error: collect2: ld returned 1 exit status
I haven't done much C++ programming, so I'm not entirely familiar with what that means. Following advice for essentially identical problems has only given me vtable errors or more incomprehensible things.
What really confuses me is that, looking at the header file, the QmlApplicationViewer is derived from QDeclarativeView, which is exactly what the Qt documentation uses here to do almost exactly what I want. Thanks for any suggestions anyone has.

Answer 1

You have to register your class for using it with QML. You can do this in the main function. You have to import it in the QML code too. Your code should look like this :
main.cpp :
#include <QtGui/QApplication>
#include "qmlapplicationviewer.h"
#include <QDeclarativeContext>
#include <QtDeclarative> // Required for registration
class testClass : public QObject
{
Q_OBJECT
public:
Q_INVOKABLE QString gimmeText() {
return QString("new text");
}
};
Q_DECL_EXPORT int main(int argc, char *argv[])
{
QScopedPointer<QApplication> app(createApplication(argc, argv));
qmlRegisterType<testClass>("MyCustomQMLObjects", 2, 35, "testClassNameInQML");
QmlApplicationViewer viewer;
viewer.setOrientation(QmlApplicationViewer::ScreenOrientationAuto);
viewer.setMainQmlFile(QLatin1String("qml/Picenter2/main.qml"));
testClass t;
viewer.rootContext()->setContextProperty("testOb", &t);
viewer.showFullScreen();
return app->exec();
}
QML code :
// ...
import MyCustomQMLObjects 2.35
// ...
property testClassNameInQML testOb
// ...
Text {
id: text1
x: 105
y: 156
color: "#ffffff"
text: qsTr("text")
font.pixelSize: 12
Timer {
interval: 1000; running: true; repeat: false
onTriggered: text1.text = testOb.gimmeText();
}
// ...

Answer 2

I have no experience with qt and I cant see what in the code triggers the error. However when such errors occur, it is because a class (struct QDeclarativeContext) has been forward declared, but is used as if the entire definition is known (access member, declare variable of this type, etc). To fix this you need to include the header which has this type's definition.