QML instantiates C++ objects. How do I access their methods? - c++

Question

Here's what my main.cpp looks like:
int main(int argc, char **argv)
{
QGuiApplication app(argc, argv);
QCoreApplication::addLibraryPath("./");
QQuickView view;
view.setResizeMode(QQuickView::SizeRootObjectToView);
view.setSource(QUrl("qrc:/myqml.qml"));
view.show();
return app.exec();
}
As you can see, it creates things from myqml. Well, myqml instantiates a C++ class MyClass.
How do I access this C++ methods from the object QQuickView view? For example, I'd like to do something of the type view.MyClassInstance.myMethod1()

Answer 1

You want to obtain an object created in QML by C++, here it does not matter if the target has a prototype created in C ++ or not. If you want this you must obtain the object through findChild since all the objects created in QML have a kinship relationship with the window.
main.cpp
#include <QtQuick>
#include <QtGui>
class MyClass: public QObject
{
Q_OBJECT
public:
using QObject::QObject;
Q_INVOKABLE void invokable(){
qDebug()<< "invokable";
}
Q_SLOT void slot(){
qDebug()<< "slot";
}
void function(){
qDebug()<< "function";
}
};
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
qmlRegisterType<MyClass>("foo", 1, 0, "MyClass");
QGuiApplication app(argc, argv);
QQuickView view;
view.setResizeMode(QQuickView::SizeRootObjectToView);
view.setSource(QUrl(QStringLiteral("qrc:/main.qml")));
view.show();
if(MyClass* myclass_instance = view.findChild<MyClass *>("myclass_instance")){
myclass_instance->invokable();
myclass_instance->slot();
myclass_instance->function();
}
return app.exec();
}
#include "main.moc"
main.qml
import QtQuick 2.9
import foo 1.0
Rectangle {
color: "salmon"
width: 640
height: 480
MyClass{
objectName: "myclass_instance"
}
}
But this method has several drawbacks such as who manages the life cycle of the object is QML, not C ++ so the pointer could at some point point to an unreserved address. Another disadvantage is that there is a dependency of C++ to QML since if the objectName is changed in QML the code in C ++ would have to be changed.
Another approach is to create a helper class that is exported to QML with setContextProperty() and that interacts with the MyClass object.
main.cpp
#include <QtQuick>
#include <QtGui>
class MyClass: public QObject
{
Q_OBJECT
public:
using QObject::QObject;
Q_INVOKABLE void invokable(){
qDebug()<< "invokable";
}
Q_SLOT void slot(){
qDebug()<< "slot";
}
void function(){
qDebug()<< "function";
}
};
class Helper: public QObject
{
Q_OBJECT
public:
using QObject::QObject;
void call_function(){
emit called();
}
Q_SIGNAL void called();
};
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
qmlRegisterType<MyClass>("foo", 1, 0, "MyClass");
QGuiApplication app(argc, argv);
Helper helper;
QQuickView view;
view.rootContext()->setContextProperty("helper", &helper);
view.setResizeMode(QQuickView::SizeRootObjectToView);
view.setSource(QUrl(QStringLiteral("qrc:/main.qml")));
view.show();
helper.call_function();
return app.exec();
}
#include "main.moc"
main.qml
import QtQuick 2.9
import foo 1.0
Rectangle {
color: "salmon"
width: 640
height: 480
MyClass{
id: myclass
}
Connections{
target: helper
onCalled:{
myclass.invokable()
myclass.slot()
}
}
}
The advantage of this method is that there is no dependence between the logic of C++ and QML, besides the life cycle does not generate problems since the myclass objects are not handled directly in QML. The disadvantage is that you write a little more code and you can only call the Q_INVOKABLE or Q_SLOT.