Beyond paintEvent(): Alternative Approaches for QTabWidget Styling in Qt

Default behavior

• By default, paintEvent() handles the painting of the following elements in a QTabWidget:
  • The tab bar at the top (or wherever positioned) containing the labels and possibly icons for each tab.
  • The background area behind the tabs.

Customization

• Within the reimplemented function, you can use the Qt drawing API (provided by the QPainter class) to draw any custom elements or modify the appearance of the default elements.
• You can subclass QTabWidget and reimplement paintEvent() to achieve custom visual styles for your tab widget.
• While paintEvent() provides the default rendering, it's intended to be overridden for customization.

Things to consider when overriding paintEvent()

• Drawing with QPainter
  The QPainter class within the Qt framework offers various functions for drawing shapes, lines, text, and images. You can use these functions to create your custom visual elements for the tab widget.
• QStyle option
  The paintEvent() function receives a QPaintEvent argument. You can access the QStyleOption object from this argument, which provides information about the widget's style and state. This information can be helpful for drawing elements according to the current style settings.
• Base class behavior
  It's essential to call the base class QWidget::paintEvent() function at the beginning of your custom paintEvent() implementation. This ensures that the standard widget functionalities like background and borders are drawn correctly.

• It's generally recommended to avoid modifying the default behavior unless you have a specific visual customization goal.
• Overriding paintEvent() requires a good understanding of the Qt drawing API and Qt styles.

---

#include <QApplication>
#include <QTabWidget>

class CustomTabWidget : public QTabWidget {
  Q_OBJECT

public:
  CustomTabWidget(QWidget* parent = nullptr) : QTabWidget(parent) {}

protected:
  void paintEvent(QPaintEvent* event) override {
    // Call base class paintEvent to draw default elements
    QWidget::paintEvent(event);

    // Get the widget area
    QRect rect = this->rect();

    // Create a painter object
    QPainter painter(this);

    // Set a custom background color
    painter.fillRect(rect, QColor(Qt::lightgray)); // Adjust color as needed

    // Draw any additional custom elements here (optional)
  }
};

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

  CustomTabWidget tabWidget;
  tabWidget.addTab(new QWidget(), "Tab 1");
  tabWidget.addTab(new QWidget(), "Tab 2");
  tabWidget.show();

  return app.exec();
}

This example:

1. Creates a custom widget class CustomTabWidget inheriting from QTabWidget.
2. Overrides the paintEvent() function.
3. Calls the base class QWidget::paintEvent() first.
4. Gets the widget area using rect().
5. Creates a QPainter object to draw on the widget.
6. Sets a custom background color using painter.fillRect().
7. (Optional) You can add code here to draw additional custom elements on top of the background.

This approach allows for simple background color changes without directly modifying the default rendering behavior.

---

Qt Style Sheets

• This approach is generally easier to maintain and offers a more platform-independent way to customize styles.
• You can use them to modify various aspects of the QTabWidget's appearance, such as background colors, fonts, borders, and margins, without directly touching the widget's code.
• Qt Style Sheets provide a declarative way to define the visual styles of widgets.

Subclassing QStyle

• However, this is a more complex approach and requires a deep understanding of Qt's styling system. It's recommended for advanced customization needs where style sheets fall short.
• This approach involves creating a custom subclass of the QStyle class. This allows you to completely override the default way Qt draws various widgets, including the QTabWidget.

Custom Widgets

• This approach requires building the tab behavior and visual elements from scratch, offering complete control but also involving more development effort.
• In some cases, you might find it more efficient to create a custom widget that mimics the functionality of a tab widget. This could be suitable for scenarios where the default QTabWidget doesn't meet your specific needs.

Approach	Ease of Use	Customization Level	Maintainability
Qt Style Sheets	Easy	Moderate	High
Subclassing QStyle	Complex	High	Moderate
Custom Widgets	Medium	Very High	Low