Figuring out main.cpp

Understanding the central file of our backend code

The role of main.cpp

While QML is used for the front-end of Kirigami applications, the backend is usually written in C++ thanks to this language's speed and flexibility. While in previous pages we have covered QML in a lot of depth, we'll need to gain an understanding of our backend C++ code in order to make applications that are more useful than what we can achieve with pure QML.

Here, we'll be going over the main.cpp file we created in the Getting Started page so that we can better understand what is going on in the central C++ file of our application. While this is a basic main.cpp, the features we will go over will remain essential no matter what kind of application you decide to create.

What it does

#include <QApplication>
#include <QQmlApplicationEngine>
#include <QtQml>
#include <QUrl>
#include <QQuickStyle>
#include <KLocalizedContext>
#include <KLocalizedString>
#include <KIconTheme>

int main(int argc, char *argv[])
{
    KIconTheme::initTheme();
    QApplication app(argc, argv);
    KLocalizedString::setApplicationDomain("tutorial");
    QApplication::setOrganizationName(QStringLiteral("KDE"));
    QApplication::setOrganizationDomain(QStringLiteral("kde.org"));
    QApplication::setApplicationName(QStringLiteral("Kirigami Tutorial"));
    QApplication::setDesktopFileName(QStringLiteral("org.kde.tutorial"));

    QApplication::setStyle(QStringLiteral("breeze"));
    if (qEnvironmentVariableIsEmpty("QT_QUICK_CONTROLS_STYLE")) {
        QQuickStyle::setStyle(QStringLiteral("org.kde.desktop"));
    }

    QQmlApplicationEngine engine;

    engine.rootContext()->setContextObject(new KLocalizedContext(&engine));
    engine.loadFromModule("org.kde.tutorial", "Main");

    if (engine.rootObjects().isEmpty()) {
        return -1;
    }

    return app.exec();
}

First we must include a number of Qt header files, allowing us to use their functions. In this case, we include a number of Qt headers that handle application logic, and to allow us to read QML files.

We then create a QApplication instance which we call app. Passing argc and argv to the constructor is required for the call.

We also set some metadata relating to the application. These include the organisation that created the application, the organisation's website, and the name of the application. We set these properties by calling QApplication, instantiating an object that comes from QCoreApplication and provides the event loop for applications regardless of whether they have a GUI or not (so if we ran our program without the GUI, this metadata would still be set).

To make our app look good with KDE's Breeze icons and Breeze style on non-Plasma environments such as Windows or GNOME, we need to do three things:

• initialize the theming facilities of KIconThemes on platforms where icon themes aren't part of the system (like Windows or MacOS) with KIconTheme::initTheme()
• set the QStyle with QApplication::setStyle() to force Breeze instead of the native platform style
• set the QtQuick Controls style with QQuickStyle::setStyle() to force Breeze with KDE's qqc2-desktop-style

The call to KIconTheme::initTheme() needs to be done before creating the QApplication and lets the app find Breeze icons to use. Setting the QStyle to Breeze is needed because we used QApplication for our app instead of QGuiApplication. Actual interface controls in the window like buttons and checkboxes will follow Breeze by using qqc2-desktop-style.

The QQmlApplicationEngine lets us load an application from a QML file, which we do in the next line. In engine.loadFromModule("org.kde.tutorial", "Main"); we load our QML from the URI import defined in CMake.

Next, we check if our engine correctly loaded the QML file by checking that the engine's rootObjects() list is not empty. We can then run our application with app.exec().