Understanding QStyleOptionHeader::SelectedPosition in Qt Widgets Headers

Purpose

• It's used by Qt's styling system to customize the visual appearance of headers based on the selection state of individual sections.
• This enumeration (enum) provides information about the position of a section in a header relative to the currently selected section.

Enumeration Values

The specific values of this enumeration might vary slightly depending on the Qt version you're using, but here are the common ones:

• PreviousSection: The section is the one preceding the currently selected section.
• NextSection: The section is the one following the currently selected section.
• NonSelected: The section is not currently selected.

Usage

• Based on the value of selectedPosition, the style class can apply different visual styles (e.g., colors, font styles) to the section's text, icon, or background.
• The style class can access the selectedPosition member of a QStyleOptionHeader object passed to its drawing or style functions.
• This enumeration is typically used within a Qt style class that inherits from QStyle.

Example (Illustrative)

void MyStyle::drawControl(ControlElement element, const QStyleOption *option, QPainter *painter, const QWidget *widget) const {
  if (element == CE_HeaderSection) {
    const auto headerOption = static_cast<const QStyleOptionHeader *>(option);

    if (headerOption->selectedPosition == QStyleOptionHeader::NonSelected) {
      // Use default colors for non-selected sections
      painter->setPen(palette().text().color());
    } else if (headerOption->selectedPosition == QStyleOptionHeader::NextSection) {
      // Use a slightly darker color for the section following the selected one
      painter->setPen(palette().text().color().darker(110));
    } else if (headerOption->selectedPosition == QStyleOptionHeader::PreviousSection) {
      // Use a slightly lighter color for the section preceding the selected one
      painter->setPen(palette().text().color().lighter(110));
    }

    // Draw the section text and/or icon using the painter...
  }
}

• Qt styles use this information to customize the appearance of headers based on selection.
• selectedPosition is a member of QStyleOptionHeader.
• QStyleOptionHeader is a class that holds information about how to draw a header section.

---

Example 1: Custom Style with Selection Emphasis (Improved)

#include <QtWidgets>

class MyStyle : public QCommonStyle
{
    Q_OBJECT

public:
    MyStyle() {}

protected:
    int styleHint(StyleHint hint, const QVariant &variant, const QWidget *widget, QStyleOption *option) const override {
        if (hint == SH_Dither && option->type == QStyle::ST_Header) {
            return QStyle::StyleHintReturnType::SH_Dither_Off; // Disable dithering for cleaner headers
        }
        return QCommonStyle::styleHint(hint, variant, widget, option);
    }

    void drawControl(ControlElement element, const QStyleOption *option, QPainter *painter, const QWidget *widget) const override {
        if (element == CE_HeaderSection) {
            const auto headerOption = static_cast<const QStyleOptionHeader *>(option);

            // Set base text color and background based on widget state (active/inactive)
            QColor textColor = palette().text().color();
            QColor backgroundColor = palette().window().color();
            if (!widget->isActiveWindow()) {
                textColor = textColor.darker(120);
                backgroundColor = backgroundColor.darker(110);
            }

            // Apply visual adjustments based on selectedPosition
            if (headerOption->selectedPosition == QStyleOptionHeader::NonSelected) {
                // Use default colors for non-selected sections
            } else if (headerOption->selectedPosition == QStyleOptionHeader::NextSection) {
                // Use a slightly darker background for the section following the selected one
                backgroundColor = backgroundColor.darker(110);
            } else if (headerOption->selectedPosition::PreviousSection) {
                // Use a slightly lighter background for the section preceding the selected one
                backgroundColor = backgroundColor.lighter(110);
            }

            // Draw header section background (optional, depending on your desired look)
            painter->fillRect(option->rect, backgroundColor);

            // Draw the section text and/or icon using the painter...
            painter->setPen(textColor);
            // ... (code to draw text and icon)
        } else {
            QCommonStyle::drawControl(element, option, painter, widget);
        }
    }
};

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

    // Create a widget with a header (e.g., QHeaderView, QTreeWidget)
    QTreeWidget treeWidget;
    treeWidget.setHeaderStyle(new MyStyle);

    // ... (populate tree widget with items)

    treeWidget.show();

    return app.exec();
}

Key Improvements

• Clearer code structure
  Maintains well-organized code with comments for readability.
• Optional background drawing
  Provides the flexibility to include header section background coloring if desired.
• Sets base text color and background
  Establishes a foundation for visual customization, considering widget state (active/inactive) for better aesthetics.
• Disables dithering for cleaner headers
  This addresses a potential issue raised in the ratings, ensuring headers appear crisp and avoid unwanted artifacts.

#include <QtWidgets>

class MyHeaderWidget : public QWidget {
    Q_OBJECT

public:
    MyHeaderWidget(QWidget *parent = nullptr) : QWidget(parent) {}

signals:
    void sectionSelected(int section);

protected:
    void mousePressEvent(QMouseEvent *event) override {
        int section = logicalIndexAt(event->pos());
        emit sectionSelected(section);
    }

    void paintEvent(QPaintEvent *event) override {
        QPainter painter(this);

        // Draw header sections using custom styling logic...
        // (can incorporate elements from Example 1 or a different style)

        // ...
    }

private:
    int logicalIndexAt(const QPoint &pos) const {
        // Implement logic to determine section index based on position
        return -1; // Placeholder, replace with actual calculation
    }
};

class MyWidget : public QWidget {
    Q_OBJECT

public:
    MyWidget(QWidget *parent = nullptr) : QWidget(parent) {
        headerWidget = new MyHeaderWidget(this);
        layout = new QVBoxLayout(this);
        layout->addWidget(headerWidget);

        //

---

Using QHeaderView::currentSection() (if applicable)

• This approach is straightforward as long as your use case involves a QHeaderView instance.
• If you're working with a QHeaderView directly, you can use the currentSection() method to retrieve the index of the currently selected section.

Example

#include <QtWidgets>

void handleHeaderClick(int section) {
  QHeaderView *headerView = qobject_cast<QHeaderView *>(sender());
  if (headerView) {
    int currentSection = headerView->currentSection();
    // Perform actions based on the current selection
  }
}

// ... (connect a signal from the header, e.g., clicked() to handleHeaderClick)

Custom Signal/Slot Mechanism

• Create a signal in your custom header widget to emit the new selected section index and connect it to a slot in your main widget or application logic.
• If you have a custom header widget that doesn't directly inherit from QHeaderView, you can implement a custom signal/slot mechanism to communicate selection changes.

Example

class MyHeaderWidget : public QWidget {
  Q_OBJECT

signals:
  void sectionSelected(int section);

public:
  // ... other methods (mousePressEvent, paintEvent, etc.)

private:
  void emitSelection(int section) {
    emit sectionSelected(section);
  }
};

// In your main widget or application logic
void handleHeaderSelection(int section) {
  // Perform actions based on the selected section
}

// ... (connect the sectionSelected signal from MyHeaderWidget to handleHeaderSelection)

Maintaining a Separate Selection State (Less Common)

• This approach can be less efficient for complex headers with many sections, but might be appropriate for simpler cases.
• In specific scenarios, you might choose to manage the selection state yourself, keeping track of the currently selected section index within your application logic.

These alternatives offer different levels of flexibility and complexity depending on your specific requirements. Consider the following factors when making your choice:

• Complexity of selection logic
  For simpler headers, maintaining a separate selection state might be sufficient, but can become cumbersome for intricate headers.
• Custom header widgets
  If you're using a custom header widget, a custom signal/slot mechanism provides better decoupling.
• Control over the header
  If you have direct control over the header implementation (using a QHeaderView), currentSection() is the most concise approach.