Course Title: QML Application Development Section Title: Integrating with C++ Topic: Using QML with C++ backends

Introduction

In this topic, we will explore how to integrate QML with C++ backends, allowing us to leverage the power of C++ in our QML applications. This integration enables us to access C++ functionality, use existing C++ libraries, and optimize performance-critical parts of our application.

Setting up a QML Application with a C++ Backend

Before we dive into the code, let's discuss the general setup of a QML application with a C++ backend. The typical structure consists of the following components:

• QML files (.qml) for the UI
• C++ files (.cpp, .h) for the backend logic
• A main function in C++ (e.g., main.cpp) that initializes the QML engine and sets up the connection between QML and C++
• Possibly, additional resources such as images, fonts, or translations

To set up this structure, you can follow these steps:

1. Create a new Qt project in Qt Creator. Choose the "Applications" template, and then "Qt Quick Application."
2. In the project settings, set up the C++ build environment.
3. Create a new C++ class that will serve as the entry point for your backend logic. For this example, we'll name it Backend.
4. In your main function (e.g., main.cpp), initialize the QML engine and create an instance of your Backend class.

Here is some sample code to get you started:

// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "backend.h"

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

    QQmlApplicationEngine engine;
    Backend backend;
    engine.rootContext()-&gt;setContextProperty("backend", &amp;backend);

    engine.load(QUrl(QStringLiteral("qrc:///main.qml")));

    return app.exec();
}

Creating a C++ Backend

To integrate your C++ backend with the QML frontend, you'll need to create a class that exposes the necessary functionality to QML. For this example, let's say we want to create a backend that provides a calculator-like functionality with basic arithmetic operations.

First, create the backend.h and backend.cpp files and define the Backend class:

// backend.h
#ifndef BACKEND_H
#define BACKEND_H

#include <QObject>

class Backend : public QObject
{
    Q_OBJECT
public:
    Q_INVOKABLE double add(double num1, double num2) { return num1 + num2; }
    Q_INVOKABLE double subtract(double num1, double num2) { return num1 - num2; }
    Q_INVOKABLE double multiply(double num1, double num2) { return num1 * num2; }
    Q_INVOKABLE double divide(double num1, double num2) { return num1 / num2; }

    Q_INVOKABLE QString getGreeting() { return "Hello from C++"; }
};

#endif // BACKEND_H

// backend.cpp
#include "backend.h"

Backend::Backend(QObject *parent) : QObject(parent)
{
}

Note the Q_INVOKABLE macro, which allows QML to call these functions from C++.

Connecting the Backend to QML

To connect the backend to QML, we need to set the backend object as a context property for the QML engine. We've already done this in the main function above.

Here's an example of how you can access the backend functions from QML:

// main.qml
import QtQuick 2.12
import QtQuick.Controls 2.12
import QtQuick.Layouts 1.12

ApplicationWindow {
    title: qsTr("QML with C++ Backend")

    ColumnLayout {
        TextField {
            id: textField1
            placeholderText: "Number 1"
        }
        TextField {
            id: textField2
            placeholderText: "Number 2"
        }

        Button {
            text: "Add"
            onClicked: resultText.text = textField1.text + " + " + textField2.text + " = " + backend.add(parseFloat(textField1.text), parseFloat(textField2.text))
        }
        Button {
            text: "Subtract"
            onClicked: resultText.text = textField1.text + " - " + textField2.text + " = " + backend.subtract(parseFloat(textField1.text), parseFloat(textField2.text))
        }
        Button {
            text: "Multiply"
            onClicked: resultText.text = textField1.text + " * " + textField2.text + " = " + backend.multiply(parseFloat(textField1.text), parseFloat(textField2.text))
        }
        Button {
            text: "Divide"
            onClicked: resultText.text = textField1.text + " / " + textField2.text + " = " + backend.divide(parseFloat(textField1.text), parseFloat(textField2.text))
        }
        Text {
            id: resultText
        }
    }
}

Conclusion and Key Takeaways

In this topic, we've covered how to integrate QML with C++ backends, including:

• Setting up the project structure
• Creating a C++ backend class
• Exposing the backend functionality to QML using Q_INVOKABLE
• Connecting the backend to QML by setting the backend object as a context property for the QML engine
• Calling backend functions from QML

By following these steps, you can leverage the power of C++ in your QML applications, enabling you to optimize performance-critical code and integrate with existing C++ libraries.

Do you have any questions or need further clarification on any of the concepts covered in this topic? Please don't hesitate to ask in the comments.

For more information on QML and Qt, you can refer to the official Qt documentation: https://doc.qt.io/qt-5.15/

Next topic: Exposing C++ Objects to QML