Qt - When the Keys Go Up: Exploring QWindow::keyReleaseEvent()

Purpose

• It's called whenever a key is released while the window has focus, allowing you to handle key release events within your custom window class.
• The QWindow::keyReleaseEvent() function is a protected member of the QWindow class in Qt.

Parameters

• QKeyEvent* event: A pointer to a QKeyEvent object that contains information about the key that was released, including the key code, modifiers used (like Ctrl, Shift, Alt), and other details.

Functionality

• Based on this information, you can implement custom behavior specific to your application.
• Inside this function, you can inspect the QKeyEvent object's properties to determine which key was released and what modifiers were used.
• This triggers the keyReleaseEvent() function in your custom window class that inherits from QWindow.
• When a key is released within a window, Qt's event loop dispatches a QKeyEvent object with the QEvent::KeyRelease type to the window.

Common Use Cases

• Implementing custom shortcuts
  You can create custom keyboard shortcuts for your application's functionality. When the user releases the final key in the shortcut, keyReleaseEvent() can be used to trigger the desired action.
• Toggling behavior based on modifier keys
  If a key combination is used for specific actions, you can leverage keyReleaseEvent() to check for modifier keys like Ctrl, Shift, or Alt and execute different actions depending on the combination.
• Disabling functionality upon key release
  You might use keyReleaseEvent() to disable certain features when a specific key is released (e.g., stopping movement in a game when the arrow key is released).

Example

#include <QApplication>
#include <QWindow>

class MyWindow : public QWindow {
    Q_OBJECT

public:
    explicit MyWindow(QWidget *parent = nullptr) : QWindow(parent) {}

protected:
    void keyReleaseEvent(QKeyEvent *event) override {
        if (event->isAutoRepeat()) {
            return; // Ignore auto-repeat events
        }

        switch (event->key()) {
            case Qt::Key_A:
                if (event->modifiers() & Qt::ControlModifier) {
                    // Handle Ctrl+A combination (e.g., select all)
                    qDebug() << "Ctrl+A released";
                } else {
                    // Handle key 'A' release alone
                    qDebug() << "Key 'A' released";
                }
                break;
            case Qt::Key_Escape:
                // Handle Escape key release for closing the window
                close();
                break;
            default:
                // Handle other key releases as needed
                break;
        }
    }
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    MyWindow window;
    window.show();
    return app.exec();
}

In this example:

• Based on the combination, it performs specific actions, such as printing a message to the console or closing the window.
• It then examines the key that was released and any modifier keys that were held down concurrently.
• The keyReleaseEvent() checks for auto-repeat events and ignores them to avoid unnecessary processing.

---

Disabling movement upon key release (game scenario)

#include <QApplication>
#include <QWindow>

class Player {
public:
    void moveUp() { /* ... */ }
    void moveDown() { /* ... */ }
    void moveLeft() { /* ... */ }
    void moveRight() { /* ... */ }
};

class GameWindow : public QWindow {
    Q_OBJECT

public:
    explicit GameWindow(QWidget *parent = nullptr) : QWindow(parent), player(new Player) {}

protected:
    void keyReleaseEvent(QKeyEvent *event) override {
        if (event->isAutoRepeat()) {
            return;
        }

        switch (event->key()) {
            case Qt::Key_W:
                player->moveUp(); // Stop moving up when W is released
                break;
            case Qt::Key_S:
                player->moveDown(); // Stop moving down when S is released
                break;
            case Qt::Key_A:
                player->moveLeft(); // Stop moving left when A is released
                break;
            case Qt::Key_D:
                player->moveRight(); // Stop moving right when D is released
                break;
            default:
                break;
        }
    }

private:
    Player* player;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    GameWindow window;
    window.show();
    return app.exec();
}

In this example, the keyReleaseEvent() stops the player's movement in the corresponding direction when the arrow key is released.

Toggling fullscreen mode with Ctrl+F

#include <QApplication>
#include <QWindow>

class MainWindow : public QWindow {
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr) : QWindow(parent) {}

protected:
    void keyReleaseEvent(QKeyEvent *event) override {
        if (event->isAutoRepeat()) {
            return;
        }

        if (event->key() == Qt::Key_F && event->modifiers() & Qt::ControlModifier) {
            if (isFullScreen()) {
                showNormal(); // Exit fullscreen mode
            } else {
                setWindowState(windowState() | Qt::WindowFullScreen); // Enter fullscreen mode
            }
        }
    }
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    MainWindow window;
    window.show();
    return app.exec();
}

Here, keyReleaseEvent() checks for the Ctrl+F combination and toggles the window's fullscreen mode accordingly.

Implementing a custom shortcut (Ctrl+Z for undo)

#include <QApplication>
#include <QWindow>
#include <QDebug>

class EditorWindow : public QWindow {
    Q_OBJECT

public:
    explicit EditorWindow(QWidget *parent = nullptr) : QWindow(parent) {}

protected:
    void keyReleaseEvent(QKeyEvent *event) override {
        if (event->isAutoRepeat()) {
            return;
        }

        if (event->key() == Qt::Key_Z && event->modifiers() & Qt::ControlModifier) {
            // Handle Ctrl+Z for undo functionality (replace with your actual undo logic)
            qDebug() << "Undo performed";
        }
    }
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    EditorWindow window;
    window.show();
    return app.exec();
}

This example demonstrates a Ctrl+Z shortcut for undo functionality. You'll need to replace the qDebug statement with your application's specific undo logic.

---

QKeyEvent in a QWidget

• This approach allows you to handle key releases specifically for that widget.
• If you're working with a specific widget instead of the entire window, you can leverage the keyReleaseEvent() function within the widget class itself (inherited from QWidget).

QShortcut

• When the user releases the final key in the shortcut, the connected slot will be triggered.
• You can create a QShortcut object, specify the key combination and optionally a context (like a specific widget), and connect it to a custom slot using connect.
• The QShortcut class in Qt provides a more convenient way to define keyboard shortcuts.

Global Event Filters

• In the eventFilter() function, you can check for QEvent::KeyRelease events and handle them as needed.
• Install this event filter on the application instance using QApplication::instance()->installEventFilter(yourFilterObject).
• You can create a class that inherits from QObject and implements the eventFilter() function.
• For handling key releases globally across your application, you can use Qt's event filter mechanism.

• Use global event filters when you need to capture key releases across the entire application, but this might be less common in typical Qt GUI development.
• For defining and handling keyboard shortcuts, QShortcut offers a cleaner and more structured way.
• If you need to handle key releases specifically for a widget, keyReleaseEvent() in the widget class is the most suitable choice.

Approach	Use Case
keyReleaseEvent() (in QWidget)	Handling key releases specific to a particular widget
QShortcut	Defining and handling keyboard shortcuts for specific actions
Global Event Filters	Capturing key releases globally across the entire application (less common in typical Qt GUI development)