Communication between C++ and QML - c++

Question

This page shows how to call C++ functions from within QML.
What I want to do is change the image on a Button via a C++ function (trigger a state-change or however it is done).
How can I achieve this?
UPDATE
I tried the approach by Radon, but immediately when I insert this line:
QObject *test = dynamic_cast<QObject *>(viewer.rootObject());
Compiler complains like this:
error: cannot dynamic_cast '((QMLCppBinder*)this)->QMLCppBinder::viewer.QDeclarativeView::rootObject()' (of type 'struct QGraphicsObject*') to type 'class QObject*' (source is a pointer to incomplete type)
In case it is relevant, QMLCppBinder is a class that I try to build to encapsulate the connections from several QML pages to C++ code. Which seems to be trickier than one might expect.
Here is a skeleton class to give some context for this:
class QMLCppBinder : public QObject
{
Q_OBJECT
public:
QDeclarativeView viewer;
QMLCppBinder() {
viewer.setSource(QUrl("qml/Connect/main.qml"));
viewer.showFullScreen();
// ERROR
QObject *test = dynamic_cast<QObject *>(viewer.rootObject());
}
}

Answer 1

If you set an objectName for the image, you can access it from C++ quite easy:
main.qml
import QtQuick 1.0
Rectangle {
height: 100; width: 100
Image {
objectName: "theImage"
}
}
in C++:
// [...]
QDeclarativeView view(QUrl("main.qml"));
view.show();
// get root object
QObject *rootObject = dynamic_cast<QObject *>(view.rootObject());
// find element by name
QObject *image = rootObject->findChild<QObject *>(QString("theImage"));
if (image) { // element found
image->setProperty("source", QString("path/to/image"));
} else {
qDebug() << "'theImage' not found";
}
// [...]
→ QObject.findChild(), QObject.setProperty()

Answer 2

So, you could set your C++ object as a context property on the QDeclarativeView in C++, like so:
QDeclarativeView canvas;
ImageChanger i; // this is the class containing the function which should change the image
canvas.rootContext()->setContextProperty("imgChanger", &i);
In your ImageChanger class, declare a signal like:
void updateImage(QVariant imgSrc);
Then when you want to change the image, call emit updateImage(imgSrc);.
Now in your QML, listen for this signal as follows:
Image {
id: imgToUpdate;
}
Connections {
target: imgChanger;
onUpdateImage: {
imgToUpdate.source = imgSrc;
}
}
Hope this helps.