Exposing C++ backend to QML

Marek

Hi

I can see two methods: qmlRegisterType and setContextProperty
Is it possible to register backend.h in main.cpp which then will be exposed in main.qml

import io.qt.examples.backend 1.0

BackEnd {
        id: backend
    }

And then if I create "QList<QObject*> dataList" of some sort in backend.cpp or its child classes I can use this as a model for example for Repeater somewhere in qml files eg. like this:

Repeater {
    model: backend.someClass.dataList

The idea is to have single "entry point" from QML to C++ classes

Will this work?

Best Regards
Marek

Marek

Well this will not work.

Digging deeper, I would like some clarify this:
I have created subclassed QAbstractListModel based on this Sailfish nice example
If I register this model:

qmlRegisterType<DemoModel>("com.example", 1, 0, "DemoModel");

I can't pass any arguments during registration - according to this: QML needs to be able to instantiate the type, calling it with only parent QObject.
So if my model needs to get the data from C++ part of the application, eg. from xmlMgr that reads files from directory, it can't be exposed to QML with qmlRegisterType - am I get this right?

In case model needs to get data from C++ part I need to use

demoModel=new DemoModel(xmlMgr);
demoModel->init();
view.rootContext()->setContextProperty("demoModel", demoModel);

Is this how it supposed to be, or am I missing something?

Best Regards
Marek

Eeli K

@Marek If you have one object which you have created in C++ you can expose that object to be used in QML with setContextProperty. If a model is based on QAbstractItemModel and can be used automatically in C++ views it should work automatically with QML views, too. So I think your last comment is correct - you need only setContextProperty for it to work.

Marek

@Eeli-K Thanks for answer.
I hoped someone will add something about qmlRegisterType used with models (first approach), because as I see it now... it pretty much useless for registering models

Best Regards
Marek

osmial

Hi @Marek,

you don't need to create any instances of DemoModel in your C++. qmlRegisterType is enough, but you also need somehow to trigger update of your model. In my case I've created Q_INVOKABLE void update(); method in my model's class implementation which is being called each time data in model should be refreshed.
Of course don't forget to override:
int rowCount(const QModelIndex &parent);
QVariant data(const QModelIndex &index, int role);
methods in your model implementation.
Here you have great description of model-view-delegate concept: http://doc.qt.io/qt-5/model-view-programming.html

Marek

Hi @osmial

I don't create DemoModel instance in C++. I know it will be created by QML and I know about Q_INVOKABLE.
Lets say you have some cpp class dataMgr - it reads data from external source like tcp socket.
This Data is needed for DemoModel, and you have DemoModel defined in cpp.
How do you call DemoModel Q_INVOKABLE void update() from QML if DemoMode does not know about dataMgr ?
DemoModel is instantiated in QML without pointer to dataMgr and you cant access DataModel from cpp.

I assume that your update() from DemoModel reads data from some source but this process is completely isolated from the rest of cpp part of application, right?

Best Regards
Marek

Marek

I have ended up with such a solution:

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    BackEnd backend(&engine);

    engine.load(QUrl(QLatin1String("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    backend.init();

    return app.exec();
}

Backend contructor is before engine load so the models are defined in QML
Backend init is after engine load to get rootObject, signals etc.

Backend - C++ part of the application

#include "backend.h"

BackEnd::BackEnd(QQmlApplicationEngine *engine) {
    this->engine=engine;

    xmlMgr=new XmlMgr(this);
    itemModel=new ItemModel(this);

    xmlMgr->init();
    itemModel->loadData(xmlMgr);

    context = engine->rootContext();
    context->setContextProperty("ItemModel",itemModel);
}
void BackEnd::init() {

    rootObject=engine->rootObjects().first();

    connect(rootObject,SIGNAL(itemClicked(int)),this,SLOT(itemClicked(int)));
}

void BackEnd::itemClicked(int item_id) {
    qDebug()<<"BackEnd::itemClicked item_id:"<<item_id;
    int catalog_id=itemModel->getCatalogId(item_id);
    if(catalog_id)
        itemModel->setSelectedCatalog(catalog_id);
}

QML part

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

    signal itemClicked(int value)

    header: Header {
        height: 60;
        width: parent.width
    }
    SwipeView {
        id: swipeView
        anchors.fill: parent

        Page1 {

        }
        Page {
            MainItem2 {
                id: mainItem
                visible: true
            }
         }
     }
}

This works well for me.

Best Regards
Marek