Qt Quick 101

handler.cpp

#include "handler.h"
#include<QDebug>

Handler::Handler(QObject *parent) : QObject(parent)
{
timesCalled = 0;
}

void Handler::buttonClicked()
{
    qInfo("callsed from C++ handler!");
    timesCalled++;
    valueChanged(timesCalled);
}

handler.h

#ifndef HANDLER_H
#define HANDLER_H

#include <QObject>

class Handler : public QObject
{
    Q_OBJECT

    int timesCalled;
public:
    explicit Handler(QObject *parent = nullptr);

public slots:
    void buttonClicked();

signals:
    void valueChanged(int newValue);
};

#endif // HANDLER_H

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQuickControls2/QtQuickControls2>
#include <QQmlContext>
#include "handler.h"


int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);

    auto ctx = engine.rootContext();
    auto handler = Handler();
    ctx->setContextProperty("handler", &handler);

    engine.load(url);

    auto object = engine.rootObjects()[0];
    auto myButton = object->findChild<QObject*>("myButton3");
    QObject::connect(myButton, SIGNAL(clicked()), &handler, SLOT(buttonClicked()));

    return app.exec();
}

main.qml

import QtQuick 2.15
import QtQuick.Window 2.15
import './' as MyComponents

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("my test app")

    MyComponents.MyApp {
        id: myComponent
        anchors.fill: parent
    }

    Connections {
        target: handler
        onValueChanged: {
            console.log("Data: " + newValue)
            var str_in = "Data: " + newValue
            myComponent.pulicText.text = "Changed to: " + str_in
        }
    }
}

MyApp.qml

import QtQuick 2.0
import QtQuick.Controls 2.15


GroupBox {
    property Text pulicText: textToChange
    id: groupBox
    x: 0
    y: 0
    width: 215
    height: 137
    Button {
        x: 104
        y: 86
        text: 'My button 1!'
        anchors.right: parent.right
        anchors.bottom: parent.bottom
        anchors.rightMargin: 5
        anchors.bottomMargin: 5
        onClicked: groupBox.f()

    }
    Button {
        text: 'My button 2!'
        anchors.left: parent.left
        anchors.top: parent.top
        anchors.leftMargin: 5
        anchors.topMargin: 5
        onClicked: {console.log("hello from 2!")}
    }

    Button {
        text: "My button 3!"
        objectName: "myButton3"
        id: myButton3
        anchors.left: parent.horizontalCenter
        anchors.right: parent.horizontalCenter
        anchors.top: parent.verticalCenter
        anchors.bottom: parent.verticalCenter
        anchors.rightMargin: -53
        anchors.bottomMargin: -17
        anchors.leftMargin: -54
        anchors.topMargin: -18
    }

    Text {
        objectName: "textToChange"
        id: textToChange
        text: "beforeChange"
        anchors.left: parent.horizontalCenter
        anchors.right: parent.horizontalCenter
        anchors.top: parent.verticalCenter
        anchors.bottom: parent.verticalCenter
        anchors.rightMargin: -153
        anchors.leftMargin: -52
        anchors.bottomMargin: -48
        anchors.topMargin: 14
    }

    function f() {console.log("hello! from groupBox")}
}