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

    QGuiApplication app(argc, argv);

    //QQuickView view(QUrl(QStringLiteral("qrc:/main.qml")));
    //QQuickItem *item = view.rootObject();

    Controller control; // qobject based

    QQmlApplicationEngine engine;

    QQmlContext *context = new QQmlContext(engine.rootContext());
    context->setContextProperty("controller", &control);

    //engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    //engine.load(QUrl());
    //if (engine.rootObjects().isEmpty())
    //    return -1;
    QQmlComponent component(&engine);
    component.loadUrl(QUrl(QStringLiteral("qrc:/main.qml")));

    QQuickItem *tobj;

    if (component.isReady()){
        QObject *myObject = component.create();
        tobj = qobject_cast<QQuickItem*>(myObject);
    }else
        qWarning() << component.errorString();

    //QQuickItem *item = engine.rootObjects();
    for(auto item: engine.rootObjects())
    {
        QObject::connect(item, SIGNAL(operateSig()),
                             &control, SIGNAL(operate()));
        break;
    }

    return app.exec();
}

QML file:

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 1.4

/*
  This program is for testing out OPC UA interfaces.
  */

Window {
    id: mainWindow
    visible: true
    width: 640
    height: 480
    title: qsTr("OPC UA Playground")

    signal operateSig()

    Connections {
        target: controller
        onStatus: {
            console.log(message);
        }
    }

    Button {
        id: operateButton
        text: "Operate"
        onClicked: {
            mainWindow.operateSig();
        }
    }

}

However, I get these errors:

qrc:/main.qml:18:5: QML Connections: Cannot assign to non-existent property "onStatus"
qrc:/main.qml:19: ReferenceError: controller is not defined

I don't understand why it cannot see the "controller" object. I have set contexts before in other apps and it has worked. I have to be missing something really really simple.