Understanding QStyleOptionTabWidgetFrame::midLineWidth in Qt Widgets

Context

• Qt Widgets is a module within Qt that provides a set of pre-built widget classes for common UI elements like buttons, text boxes, and tab widgets.
• Qt is a cross-platform application framework for creating user interfaces with rich graphics.

QStyleOptionTabWidgetFrame Class

• It holds various properties that control the appearance of the frame, including its size, position, and visual style.
• This class is used to provide information to Qt's styling system (QStyle) about how to draw the frame surrounding a QTabWidget (a widget that displays tabs for organizing content).

midLineWidth Property

• A positive value (e.g., 1 or 2) will create a line with the specified width in the middle of the frame.
• By default, midLineWidth is set to 0, meaning no middle line is drawn.
• This property specifically controls the width (in pixels) of a middle line that's often used to create a visual effect like a sunken or raised frame.

How It's Used

• If midLineWidth is greater than 0, the style system draws a line with the specified width in the middle of the frame, typically using the frame's border color or a slightly darker shade.
• It then examines the midLineWidth property to determine if a middle line needs to be drawn.
• When a QTabWidget is drawn, the styling system (QStyle) retrieves the QStyleOptionTabWidgetFrame object associated with the widget.

Customization

• To change the appearance of the middle line in QTabWidget frames, you might need to create a custom style that overrides the default behavior of QStyle. However, this is an advanced technique and is usually not necessary for basic customization.
• You can't directly modify the appearance of Qt widgets using raw code. Qt's styling system provides a way to customize the look and feel of widgets through stylesheets or custom styles.

• While midLineWidth controls the width of the line, other styling properties might affect its color or other visual aspects.
• The exact visual representation of the middle line depends on the chosen style (e.g., it might be a solid line, a dotted line, or have a specific color).

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Using Stylesheets

#include <QApplication>
#include <QWidget>
#include <QTabWidget>

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

  QWidget window;

  QTabWidget tabWidget;
  tabWidget.setStyleSheet("QTabWidget { border: none; }");

  window.setCentralWidget(&tabWidget);
  window.show();

  return app.exec();
}

In this code:

• The stylesheet removes the border by setting border: none; for the QTabWidget. While this doesn't directly target the middle line, it often removes both the main frame border and any middle line that might be drawn within it.
• We create a QTabWidget and set it as the central widget of a QWidget.

Creating a Custom Style (Advanced)

For more granular control over the appearance, you can create a custom style that inherits from QProxyStyle and overrides the drawing functions for specific widget parts. This is an advanced technique and requires a deeper understanding of Qt's styling system. However, it allows you to modify individual aspects like the middle line width or color.

#include <QProxyStyle>
#include <QPainter>
#include <QStyleOption> // For QStyleOptionTabWidgetFrame

class MyCustomStyle : public QProxyStyle {
public:
  // Override drawing functions for specific widget parts (e.g., drawControl)
  void drawControl(ControlElement element, const QStyleOption *option, QPainter *painter) override {
    if (element == CE_TabWidgetFrame) {
      // Cast option to QStyleOptionTabWidgetFrame and access midLineWidth
      const auto* tabWidgetFrameOption = static_cast<const QStyleOptionTabWidgetFrame*>(option);
      int midLineWidth = tabWidgetFrameOption->midLineWidth;

      // Draw the frame (potentially with custom logic for the middle line)
      // ...
    } else {
      // Call the base class implementation for other elements
      QProxyStyle::drawControl(element, option, painter);
    }
  }
};

In this example:

• We can then draw the frame with custom logic for the middle line, potentially using midLineWidth to adjust its width.
• If it is, we cast the option pointer to QStyleOptionTabWidgetFrame to access the midLineWidth property.
• Inside the drawControl function, we check if the element being drawn is CE_TabWidgetFrame.
• The drawControl function is overridden to handle drawing specific widget parts.
• We create a MyCustomStyle class that inherits from QProxyStyle.

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Using Stylesheets for Different Effects

• Customizing Background Colors
  Use background-color and border-color properties in stylesheets to set specific colors for the frame and background, potentially creating a contrasting effect that mimics a raised or sunken appearance.
• Creating a Shadow Effect
  You can use stylesheet properties like box-shadow to create a shadow effect around the QTabWidget, which can visually separate it from the background.
• Removing the Frame
  You can achieve this using border: none; in a stylesheet, as shown in the previous example code.

Creating a Custom Style for More Control

• If stylesheets aren't sufficient for your needs, you can create a custom style that inherits from QProxyStyle. This allows you to override various drawing functions and implement custom logic for the frame's appearance. For example, you could:
  • Draw a custom border instead of relying on the default frame style.
  • Use drawing primitives like lines or rectangles to create specific visual effects within the frame area.

Choosing the Right Approach

• Custom Styles
  Use them for more granular control over the appearance, but be aware of the increased complexity involved.
• Stylesheets
  These are generally the preferred approach for basic customization because they are easier to implement and maintain.

• If you need a more complex visual effect, creating a custom widget class that inherits from QTabWidget might be an option, but this is an advanced technique.
• Qt's styling system provides many other properties that can be used in conjunction with border and background-color to achieve various visual effects. Exploring the available stylesheet properties can be helpful.