How to set a pure C++ object instance in a QQuickItem

[Nelson_Piquet]

Hi Guys.....I have QtApp & a pure C++ library. The C++ library exposes a one simple class called MyCppLibApiClass. The QtApp has a class which is embedded on main.qml. Following is the class:

//MyQuickItem.h
class MyQuickItem : public QQuickItem {
  MyQuickItem();
  SetCppObject(MyCppLibApiClass cppObject);
  MyCppLibApiClass m_cppObject;
}

Following the qml code for the quick item:

//MainPage.qml
import MyQuickItem 1.0

MyQuickItem {
  id: myQuickItemID
  visible: true
  objectName: "myQuickItem"
}

Following is my main.cpp showing I load the qml item:

//main.cpp
qmlRegisterType<MyQuickItem>("MyQuickItem", 1, 0, "MyQuickItem");
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
return app.exec();

MyQuickItem needs access to MyCppLibApiClass instance. How do a I get a valid instance to MyQuickItem in main.cpp & call SetCppObject() on it ? I need to set an object of MyCppLibApiClass as member in MyQuickItem. I can do this using a setter method like SetCppObject. But, first of all I need to get a valid instance to MyQuickItem. So How to get access to MyQuickItem in main.cpp ?

I have searched quite a lot before asking this question. I read through this link. But it describes how to set properties with only Qt types like QString & others.Appreciate suggestions to this..

[SGaist]

Hi,

The code sample uses setPropertyas an example of method to call. You have the object at hand so call the methods you need on it.

[p3c0]

@Nelson_Piquet You can also instantiate the MyCppLibApiClass in main.cpp then set it as a context property which will then be accessible from MyQuickItem in QML.
Something like

qmlRegisterType<MyQuickItem>("MyQuickItem", 1, 0, "MyQuickItem");

MyCppLibApiClass myClass;

QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
engine.rootContext()->setContextProperty("myObj", &myClass); // Access myObj from QML

return app.exec();

More info here:
http://doc.qt.io/qt-5/qtqml-cppintegration-contextproperties.html

[Nelson_Piquet]

@p3c0 @SGaist Thanks a lot for your attention. p3c0's answer suggests how to make MyCppLibApiClass recognisable on qml side. but, I cannot take this approach because this requires to make MyCppLibApiClass inherited by QObject if I am correct. MyCppLibApiClass comes from a pure C++ library & I cannot afford to introduce Qt into it. But My primary question is slightly different here. My question is: How to get a valid instance of MyQuickItem & call SetCppObject which accepts a pure Cpp obejct i.e. MyCppLibApiClass ? I have updated my question & sample code in the question to make it clearer

[p3c0]

@Nelson_Piquet So as said earlier in the last post use findChild to get access to QML instance on C++ side. Now once you get this object call the SetCppObject function and pass cppObject to it.

[Nelson_Piquet]

@p3c0 I found that just doing a findChild does not give me access to SetCppObject. Looks like I have to cast it to MyQuickItem to get access to SetCppObject. Following compiles:

QQuickView view;
view.setSource(QUrl::fromLocalFile("main.qml"));
QObject *object = view.rootObject();
QObject *rect = object->findChild<QObject*>("MyQuickItem");
MyQuickItem* quickItemObj = qobject_cast<MyQuickItem*>(rect);
quickItemObj->SetCppObject(theCppObject);

But qobject_cast returns me a null pointer. Q_OBJECT macro is included in MyQuickItem

class MyQuickItem : public QQuickItem {
  Q_OBJECT
public:
  MyQuickItem();
}

Should I do something in MyQuickItem class to get a valid pointer to MyQuickItem ? Or pls correct me if I am going wrong here

[p3c0]

@Nelson_Piquet The object name that findChild accepts is case sensitive. I see a mismatch between this and your first post. ie. myQuickItem and MyQuickItem are totally different.

But qobject_cast returns me a null pointer.

Probably because the object was not found at first.

[Nelson_Piquet]

@p3c0 Thanks for point that out. I corrected the id. Also, the main.qml path I inputted was not the full path. It needs to be like so: qrc:/qml/main.qml. After applying a qobject_cast & I get a pointer to MyQuickItem.

But when I try to access SetCppObject, it crashes reporting an error which says : QQuickView does not support using windows as a root item. If you wish to create your root window from QML, consider using QQmlApplicationEngine instead. But its a separate issue. I will read a little & put a separate question for that. Thanks a lot for helping until here.

[SGaist]

Maybe a silly question but do you really need a QQuickItem based wrapper ? Wouldn't a simple QObject wrapper do the work ?

[p3c0]

@Nelson_Piquet As the error says QQuickView can only load root element of QML types that inherits Item whereas QQmlApplicationEngine loads root elements Window and ApplicationWindow.

[Nelson_Piquet]

@SGaist I really need a QQuickItem based wrapper. it is embedded in qml. I am not sure of how to make it work as a QObject wrapper. but, now I've reached a point with all the help where I am able to retrieve the object of MyQuickItem. But now, MyQuickItem's constructor gets called twice which is not what I need.......aahh !! Following is my main.cpp

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

  QQmlApplicationEngine engine;
  qmlRegisterType<MyQuickItem>("MyQuickItem", 1, 0, "MyQuickItem");
  QQuickView view;
  //Mainpage.qml is where MyQuickItem is located. Its sort of a child page of main.qml 
  view.setSource(QUrl(QStringLiteral("qrc:/qml/pages/MainPage.qml")));
  QObject *object = view.rootObject();
  QObject *rect = object->findChild<QObject*>("myQuickItem"); //pls note that myQuickItem is the object name
  MyQuickItem* quickItem = qobject_cast<MyQuickItem*>(rect);
  quickItem->SetCppObject(theCppObject);
  engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
  return app.exec();
}

My doubt is that view.setSource might need to be avoided but not sure how

[p3c0]

@Nelson_Piquet There is no need to load 2 QML files separetely. This is going to create 2 windows and thus complicate. Just load your main QML using QQuickView or QQmlApplicationEngine and from there load other QML files.

[Nelson_Piquet]

@p3c0 What should I change in my main.cpp to avoid loading the files twice ? If I dont call view.setSource, I will not get hold of the child object. If dont call engine.load, I cannot load my main.qml. How should I change my main.cpp. Since this question is a separate issue, I have posted a new question in this forum here

[Nelson_Piquet]

@p3c0 @SGaist No need to answer the other question I posted. I got it working. Thanks a lot guys. Thanks a many !!!

[SGaist]

About the QObject wrapper, unless I misunderstood, you only want to act on your MyCppLibApiClass and not paint anything related to it thus the QObject wrapper can be used without any dependency on a visual element.

[Nelson_Piquet]

@SGaist understood. thanks