Customizing Keyboard Shortcuts in Qt MDI SubWindows

Understanding QMdiSubWindow and keyPressEvent()

• keyPressEvent(QKeyEvent* event)
  This is a protected member function inherited from QWidget. It's called whenever a key press event occurs within the QMdiSubWindow. This function allows you to handle keyboard input specifically for the sub-window.

• QMdiSubWindow
  This class in Qt Widgets represents a sub-window within an MDI (Multiple Document Interface) container. It's used to create child windows that can be arranged, resized, and docked within the main MDI window.

What happens in keyPressEvent()

When a user presses a key while the focus is on a widget within the QMdiSubWindow, Qt triggers the keyPressEvent() function. This function receives a QKeyEvent object as an argument, which provides information about the pressed key, modifiers (like Ctrl, Shift, Alt), and other details about the event.

Default behavior (optional)

By default, keyPressEvent() in QMdiSubWindow doesn't perform any specific actions related to keyboard input. It might simply pass the event on to its parent widget or the main application for further handling.

Customizing keyPressEvent() for specific behavior

1. Include necessary headers
   

   #include <QKeyEvent>
   #include <QMdiSubWindow>
   

2. Create a subclass of QMdiSubWindow
   

   class MySubWindow : public QMdiSubWindow {
       Q_OBJECT
   
   public:
       explicit MySubWindow(QWidget *parent = nullptr, Qt::WindowFlags flags = Qt::WindowFlags());
   
   protected:
       void keyPressEvent(QKeyEvent *event) override;
   };
   

3. Override keyPressEvent()
   

   void MySubWindow::keyPressEvent(QKeyEvent *event) {
       // Handle specific key presses here
       if (event->key() == Qt::Key_Escape) {
           close(); // Example: Close the sub-window on Escape key press
       } else if (event->key() == Qt::Key_F1 && event->modifiers() & Qt::ControlModifier) {
           // Example: Handle Ctrl+F1 for help
       } else {
           // Pass the event on for default processing (optional)
           QWidget::keyPressEvent(event);
       }
   }
   

Key points

• Optionally, call QWidget::keyPressEvent(event) to pass the event on for further default processing in Qt's event handling mechanism.
• You can implement any desired actions based on the pressed key and modifiers.
• Check for modifier keys using bitwise operations with Qt:: modifiers (e.g., Qt::ControlModifier, Qt::ShiftModifier).
• Use event->key() to identify the pressed key (e.g., Qt::Key_A for 'A').

---

Example 1: Close sub-window on Escape

This code demonstrates closing the sub-window when the user presses the Escape key:

#include <QKeyEvent>
#include <QMdiSubWindow>

class MySubWindow : public QMdiSubWindow {
    Q_OBJECT

public:
    explicit MySubWindow(QWidget *parent = nullptr, Qt::WindowFlags flags = Qt::WindowFlags());

protected:
    void keyPressEvent(QKeyEvent *event) override;
};

MySubWindow::MySubWindow(QWidget *parent, Qt::WindowFlags flags)
    : QMdiSubWindow(parent, flags)
{
    // Set this sub-window to be focusable for key press events
    setFocusPolicy(Qt::ClickFocus);
}

void MySubWindow::keyPressEvent(QKeyEvent *event) {
    if (event->key() == Qt::Key_Escape) {
        close(); // Close the sub-window
    } else {
        QWidget::keyPressEvent(event); // Pass on for default processing (optional)
    }
}

Example 2: Toggle Fullscreen on F11

This code shows toggling the sub-window between maximized and normal states on pressing F11:

#include <QKeyEvent>
#include <QMdiSubWindow>

class MySubWindow : public QMdiSubWindow {
    Q_OBJECT

public:
    explicit MySubWindow(QWidget *parent = nullptr, Qt::WindowFlags flags = Qt::WindowFlags());

protected:
    void keyPressEvent(QKeyEvent *event) override;

private:
    bool isFullScreen = false;
};

MySubWindow::MySubWindow(QWidget *parent, Qt::WindowFlags flags)
    : QMdiSubWindow(parent, flags)
{
    setFocusPolicy(Qt::ClickFocus);
}

void MySubWindow::keyPressEvent(QKeyEvent *event) {
    if (event->key() == Qt::Key_F11) {
        isFullScreen = !isFullScreen;
        if (isFullScreen) {
            showMaximized();
        } else {
            showNormal();
        }
    } else {
        QWidget::keyPressEvent(event); // Pass on for default processing (optional)
    }
}

• Consider using Qt's QShortcut class for creating global keyboard shortcuts that are not specific to a sub-window.
• Ensure your sub-window is focusable using setFocusPolicy(Qt::ClickFocus).
• Include necessary headers (<QKeyEvent>, <QMdiSubWindow>) in your code.
• These are basic examples. You can extend them to handle more complex key combinations and actions.

---

QShortcut Class

• Example
  

  #include <QShortcut>
  #include <QMdiSubWindow>
  
  class MySubWindow : public QMdiSubWindow {
      Q_OBJECT
  
  public:
      explicit MySubWindow(QWidget *parent = nullptr, Qt::WindowFlags flags = Qt::WindowFlags());
  
  private:
      QShortcut *closeShortcut;
  };
  
  MySubWindow::MySubWindow(QWidget *parent, Qt::WindowFlags flags)
      : QMdiSubWindow(parent, flags)
  {
      // Create a shortcut for closing the sub-window on Ctrl+W
      closeShortcut = new QShortcut(QKeySequence(tr("Ctrl+W", "Close")), this);
      connect(closeShortcut, &QShortcut::activated, this, &MySubWindow::close);
  }
  

• Description
  The QShortcut class allows you to define global keyboard shortcuts that can be triggered anywhere in your application, regardless of which widget has focus. This is a good option for actions that should be accessible from any sub-window or the main window.

Event Filters

• Example
  (This is a simplified example to demonstrate the concept)

  #include <QEvent>
  #include <QMdiSubWindow>
  
  class MySubWindowEventFilter : public QObject {
      Q_OBJECT
  
  public:
      bool eventFilter(QObject *obj, QEvent *event) override {
          if (event->type() == QEvent::KeyPress) {
              // Handle key press event here
              return true; // Stop event propagation (optional)
          }
          return QObject::eventFilter(obj, event); // Pass on other events
      }
  };
  
  class MySubWindow : public QMdiSubWindow {
      Q_OBJECT
  
  public:
      explicit MySubWindow(QWidget *parent = nullptr, Qt::WindowFlags flags = Qt::WindowFlags());
  
  private:
      MySubWindowEventFilter filter;
  };
  
  MySubWindow::MySubWindow(QWidget *parent, Qt::WindowFlags flags)
      : QMdiSubWindow(parent, flags)
  {
      // Install the event filter on the sub-window
      installEventFilter(&filter);
  }
  

• Description
  You can install an event filter on a widget (including QMdiSubWindow) to intercept all events (including key press events) before they reach the widget itself. This approach allows for more granular control over event handling, but it can be more complex to manage.

• Consider event filters only if you need very fine-grained control over event handling within the sub-window, but be aware of the increased complexity.
• Use QMdiSubWindow::keyPressEvent() for handling keyboard input that's specifically relevant to the sub-window and its contents.
• Use QShortcut for global keyboard shortcuts accessible from anywhere in the application.