Creating the main window

This tutorial shows you the magic of an application's most important thing: the main window.

Summary

This tutorial carries on from our Hello World project and will introduce the KXmlGuiWindow class.

In the previous tutorial, the program caused a dialog box to pop up. Now we are going to take steps towards creating a functioning application with a more advanced window structure.

KXmlGuiWindow

KXmlGuiWindow provides a full main window view with menubars, toolbars, a statusbar and a main area in the centre for a large widget. For example, the help menu is predefined. Most KDE applications will derive from this class as it provides an easy way to define menu and toolbar layouts through XML files (this technology is called KXmlGui ). While we will not be using it in this tutorial, we will use it in the next.

In order to have a useful KXmlGuiWindow , we must subclass it. So we create two files, mainwindow.cpp and mainwindow.h, which will contain our code.

mainwindow.h

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#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <KXmlGuiWindow>

class KTextEdit;
 
class MainWindow : public KXmlGuiWindow
{
public:
    explicit MainWindow(QWidget *parent = nullptr);
 
private:
    KTextEdit *textArea;
};
 
#endif // MAINWINDOW_H

First we subclass KXmlGuiWindow with class MainWindow : public KXmlGuiWindow, then we declare the constructor with MainWindow(QWidget *parent = nullptr);.

Finally, we declare a pointer to the object that will make up the bulk of our program, turning it into a text editor. KTextEdit is a generic rich text editing widget with some niceties like cursor auto-hiding and spell checking.

mainwindow.cpp

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#include "mainwindow.h"

#include <KTextEdit>

MainWindow::MainWindow(QWidget *parent) : KXmlGuiWindow(parent)
{
    textArea = new KTextEdit();
    setCentralWidget(textArea);
    setupGUI();
}

First, of course, we have to include the header file containing the class declaration.

Inside the constructor of our subclassed KXmlGuiWindow , we initialize our KTextEdit textArea. Because textArea derives from QWidget and our KXmlGuiWindow MainWindow derives from QMainWindow , we can call QMainWindow::setCentralWidget() to make our textArea occupy the empty area in the central section of our window.

Finally, KXmlGuiWindow::setupGUI() is called which does a lot of behind-the-scenes stuff and creates the default menus (Settings, Help).

Back to main.cpp

In order to actually run this window, we need to add a few lines in main.cpp:

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#include <QApplication>
#include <QCommandLineParser>
#include <KAboutData>
#include <KLocalizedString>
#include "mainwindow.h"
 
int main (int argc, char *argv[])
{
    using namespace Qt::Literals::StringLiterals;

    QApplication app(argc, argv);
    KLocalizedString::setApplicationDomain("mainwindow");
    
    KAboutData aboutData(
        u"mainwindow"_s,
        i18n("Main Window"),
        u"1.0"_s,
        i18n("A simple text area"),
        KAboutLicense::GPL,
        i18n("(c) 2015"),
        i18n("Some text..."),
        u"https://example.kde.org/"_s,
        u"submit@bugs.kde.org"_s);

    aboutData.addAuthor(i18n("Name"), i18n("Task"),
        u"your@email.com"_s,
        u"https://your.website.com"_s,
        u"OSC Username"_s);

    KAboutData::setApplicationData(aboutData);
 
    QCommandLineParser parser;
    aboutData.setupCommandLine(&parser);
    parser.process(app);
    aboutData.processCommandLine(&parser);
    
    MainWindow *window = new MainWindow();
    window->show();
    
    return app.exec();
}

Again, we include our new header file mainwindow.h. This lets us create our new MainWindow object which we then display near the end of the main function (by default, new window objects are hidden).

CMake

The best way to build the program is to use CMake. We add mainwindow.cpp to the sources list, include the XmlGui and TextWidgets frameworks, and replace all helloworld text with mainwindow.

CMakeLists.txt

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cmake_minimum_required(VERSION 3.20)

project(mainwindow)

set(QT_MIN_VERSION "6.6.0")
set(KF_MIN_VERSION "6.0.0")

find_package(ECM ${KF_MIN_VERSION} REQUIRED NO_MODULE)
set(CMAKE_MODULE_PATH ${ECM_MODULE_PATH} ${CMAKE_CURRENT_SOURCE_DIR}/cmake)

include(KDEInstallDirs)
include(KDECMakeSettings)
include(KDECompilerSettings NO_POLICY_SCOPE)
include(FeatureSummary)

find_package(Qt6 ${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS
    Core    # QCommandLineParser, QStringLiteral
    Widgets # QApplication 
)

find_package(KF6 ${KF_MIN_VERSION} REQUIRED COMPONENTS
    CoreAddons      # KAboutData
    I18n            # KLocalizedString
    XmlGui          # KXmlGuiWindow
    TextWidgets     # KTextEdit
)
    
add_executable(mainwindow)

target_sources(mainwindow
    PRIVATE
    main.cpp
    mainwindow.cpp)

target_link_libraries(mainwindow
    Qt6::Widgets
    KF6::CoreAddons
    KF6::I18n
    KF6::XmlGui
    KF6::TextWidgets
)

install(TARGETS mainwindow ${KDE_INSTALL_TARGETS_DEFAULT_ARGS})

feature_summary(WHAT ALL INCLUDE_QUIET_PACKAGES FATAL_ON_MISSING_REQUIRED_PACKAGES)

Running our application

For mature projects, the best way to compile, link and run KDE software is to set up a correct build environment. But for a simple tutorial like this, it's enough to just create a build directory and build from there. Like before:

cmake -B build/
cmake --build build/
./build/bin/mainwindow

Hello World!

Using actions