Alternatives to QMdiArea::paintEvent() for Qt MDI Area Customization

1. Inherits from QWidget
   QMdiArea inherits from QWidget, which means it inherits the paintEvent() function. This base class function provides the framework for handling the painting of any widget in Qt.

2. Triggered by repaint
   The paintEvent() function is called whenever the MDI area needs to be repainted. This can happen due to various reasons like:

   • Resizing the MDI area
   • Minimizing/maximizing child windows
   • Changes in the underlying system (e.g., window decorations)

3. QPaintEvent argument
   The function receives a QPaintEvent object as an argument. This object contains information about the update region, which is the specific area that needs to be repainted.

4. Painting the MDI area (Optional)
   By default, QMdiArea itself might not implement any specific painting logic within paintEvent(). However, you can override this function in a subclass of QMdiArea to:

   • Draw custom backgrounds or borders for the MDI area.
   • Display visual cues like active window indicators.

5. Painting child windows
   The QMdiArea class manages the layout and painting of its child windows (typically QMdiSubWindow instances). These child windows handle their own painting within their respective paintEvent() functions.

Here are some important points to remember

• The viewport() of the QMdiArea provides the drawing surface.
• Use the QPainter object provided by the QPaintEvent to draw on the MDI area.
• If you need custom painting in the MDI area, create a subclass and override paintEvent() in that subclass.
• You typically wouldn't need to modify the base class QMdiArea::paintEvent().

---

#include <QtWidgets>

class MyMdiArea : public QMdiArea {
  Q_OBJECT

public:
  MyMdiArea(QWidget *parent = nullptr) : QMdiArea(parent) {}

protected:
  void paintEvent(QPaintEvent *event) override {
    QMdiArea::paintEvent(event); // Call base class paintEvent first

    QPainter painter(viewport());
    painter.setPen(Qt::blue);
    painter.drawRect(rect() - 2); // Draw a blue border with 2px margin
  }
};

1. We define a class MyMdiArea that inherits from QMdiArea.
2. In the paintEvent() override, we first call the base class paintEvent(event) to ensure default painting behavior.
3. We create a QPainter object using the viewport() of the MyMdiArea.
4. We set the pen color to blue and draw a rectangle with a 2px margin around the entire rect() of the MDI area.

This code will draw a blue border around the MDI area whenever it's repainted.

---

Qt Stylesheets

• This approach is often preferred for simpler visual changes as it promotes separation of concerns and easier maintenance.
• Stylesheets allow you to customize aspects like background colors, borders, text formatting, and more without directly modifying widget code.
• Qt Stylesheets provide a CSS-like mechanism to style Qt widgets. You can define styles for QMdiArea and its child windows (typically QMdiSubWindow).

Custom Widgets

• This approach is suitable for complex visual elements that require more control and interaction beyond basic painting.
• These custom widgets can be placed on top of the QMdiArea using layout managers like QHBoxLayout or QVBoxLayout.
• Instead of overriding paintEvent(), you can create custom widgets to draw specific elements within the MDI area.

QGraphicsScene

• This approach is beneficial for scenarios where you need advanced graphics features or want to decouple the visual representation from the widget hierarchy.
• The QGraphicsView will handle painting and user interaction with the scene elements within the MDI area.
• Then, use a QGraphicsView widget and set its scene to the custom scene.
• You can create a scene containing custom graphics items representing elements you want to display in the MDI area.
• The QGraphicsScene class provides a higher-level framework for managing and painting 2D graphics.

• For advanced graphics or a decoupled visual representation, QGraphicsScene might be a better choice.
• For complex visual elements requiring interaction, custom widgets offer more control.
• For simple visual changes, Qt Stylesheets are often the easiest and most maintainable solution.