Qt Window State Management with QWindow::windowState() and Related Functions

Purpose

• The window state refers to how the window is currently occupying the screen real estate, such as maximized, minimized, or normal.
• It retrieves the current window state of a QWindow object, which represents a window on the screen.
• QWindow::windowState() is a member function of the QWindow class in Qt's GUI framework.

Return Value

• Common Qt::WindowState flags include:
  • Qt::WindowNoState (normal window)
  • Qt::WindowMinimized
  • Qt::WindowMaximized
  • Qt::WindowFullScreen
• The function returns a Qt::WindowState value, which is a bitwise OR of flags indicating the window's state.

Example Usage

#include <QWindow>
#include <Qt>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QWindow window;
    window.show();  // Create and display the window

    Qt::WindowState state = window.windowState();

    if (state & Qt::WindowMaximized) {
        qDebug() << "Window is maximized";
    } else if (state & Qt::WindowMinimized) {
        qDebug() << "Window is minimized";
    } else {
        qDebug() << "Window is in normal state";
    }

    return app.exec();
}

• Qt provides platform-specific behavior for window states, so the exact interpretation of each flag might vary slightly depending on the operating system.
• The windowStateChanged() signal is emitted whenever the window state changes, allowing you to react to these events accordingly.
• To modify the window state, use the setWindowState() function on the QWindow object.

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Reacting to Window State Changes

This example shows how to connect to the windowStateChanged() signal and perform actions based on the new state:

#include <QWindow>
#include <Qt>

class MyWindow : public QWindow {
    Q_OBJECT

public:
    MyWindow(QWidget *parent = nullptr);

signals:
    void stateChanged(Qt::WindowState state);

private slots:
    void onWindowStateChanged(Qt::WindowState state);

private:
    QLabel *label;
};

MyWindow::MyWindow(QWidget *parent) : QWindow(parent) {
    label = new QLabel("Window state:");
    label->setAlignment(Qt::AlignCenter);
    setCentralWidget(label);

    connect(this, &MyWindow::windowStateChanged, this, &MyWindow::onWindowStateChanged);
}

void MyWindow::onWindowStateChanged(Qt::WindowState state) {
    QString newState;
    if (state & Qt::WindowMaximized) {
        newState = "Maximized";
    } else if (state & Qt::WindowMinimized) {
        newState = "Minimized";
    } else {
        newState = "Normal";
    }
    label->setText("Window state: " + newState);
}

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    MyWindow window;
    window.show();

    return app.exec();
}

In this code:

• The windowStateChanged signal is connected to the onWindowStateChanged slot, which updates a label to reflect the new window state.
• We create a custom MyWindow class that inherits from QWindow.

Maximizing/Minimizing the Window

This example shows how to programmatically maximize or minimize the window:

#include <QWindow>
#include <Qt>
#include <QPushButton>

class MyWindow : public QWindow {
    Q_OBJECT

public:
    MyWindow(QWidget *parent = nullptr);

private slots:
    void onMaximizeClicked();
    void onMinimizeClicked();

private:
    QPushButton *maximizeButton;
    QPushButton *minimizeButton;
};

MyWindow::MyWindow(QWidget *parent) : QWindow(parent) {
    maximizeButton = new QPushButton("Maximize");
    minimizeButton = new QPushButton("Minimize");

    QHBoxLayout *layout = new QHBoxLayout;
    layout->addWidget(maximizeButton);
    layout->addWidget(minimizeButton);

    QWidget *widget = new QWidget;
    widget->setLayout(layout);
    setCentralWidget(widget);

    connect(maximizeButton, &QPushButton::clicked, this, &MyWindow::onMaximizeClicked);
    connect(minimizeButton, &QPushButton::clicked, this, &MyWindow::onMinimizeClicked);
}

void MyWindow::onMaximizeClicked() {
    setWindowState(windowState() | Qt::WindowMaximized);
}

void MyWindow::onMinimizeClicked() {
    setWindowState(windowState() | Qt::WindowMinimized);
}

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    MyWindow window;
    window.show();

    return app.exec();
}

• Clicking the buttons calls the corresponding slots, which use setWindowState() to modify the window state.
• We create buttons to trigger maximization and minimization.

Toggling Fullscreen Mode

This example demonstrates toggling the window between normal and fullscreen states:

#include <QWindow>
#include <Qt>
#include <QPushButton>

class MyWindow : public QWindow {
    Q_OBJECT

public:
    MyWindow(QWidget *parent = nullptr);

private slots:
    void onToggleFullscreenClicked();

private:
    QPushButton *fullscreenButton;
    bool isFullscreen;
};

MyWindow::MyWindow(QWidget *parent) : QWindow(parent) {
    fullscreenButton = new QPushButton("Toggle Fullscreen");
    setCentralWidget(fullscreenButton);

    connect(fullscreenButton, &QPushButton::clicked, this, &MyWindow::onToggleFullscreenClicked);
    isFullscreen = false;
}

void MyWindow::onToggleFullscreenClicked() {
    if (isFullscreen) {
        setWindowState(windowState() & ~Qt::WindowFullScreen);
        fullscreenButton->setText("Toggle Fullscreen

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Using isMinimized(), isMaximized(), and isFullScreen()

These are separate functions offered by QWindow that individually check if the window is minimized, maximized, or fullscreen, respectively. They provide a more granular approach compared to the bitwise nature of windowState().

Overriding changeEvent()

The changeEvent() function is a virtual function inherited from QWidget (which QWindow inherits from) that is called whenever the widget's state changes. By overriding this function in your custom window class, you can check for specific events like QEvent::WindowStateChange and determine the new window state using event()->attributes() & Qt::WindowStateFlags.

However, this approach requires more code and might be less efficient compared to using signals and slots.

Using windowStateChanged() Signal

If you're more interested in reacting to window state changes rather than retrieving the current state, consider using the windowStateChanged() signal emitted by QWindow. This signal is triggered whenever the window state changes, allowing you to execute code based on the new state passed as an argument.

Choosing the Right Approach

The best approach depends on your specific needs:

• Wanting to react to state changes and perform actions
  Use windowStateChanged() signal.
• Checking for specific states individually
  Use isMinimized(), isMaximized(), and isFullScreen().
• Retrieving the current state occasionally
  Use windowState().