QStandardItemModel header with widget and text

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Enhancing QStandardItemModel Headers with Widgets and Text

Hero image for QStandardItemModel header with widget and text

Learn how to customize QStandardItemModel headers by combining standard text with interactive widgets like QCheckBoxes for advanced UI control in Qt applications.

The QStandardItemModel is a powerful and flexible model for presenting data in Qt's item view classes (e.g., QTableView, QListView, QTreeView). While it natively supports displaying text in headers, there are scenarios where you might need more interactive or visually rich headers. This article explores how to combine standard text with custom widgets, specifically QCheckBox, within the header section of a QStandardItemModel.

Understanding QStandardItemModel Headers

By default, QStandardItemModel uses QStandardItem objects to store data for both cells and headers. For headers, you typically set text using setHeaderData(). However, if you want to embed a widget like a QCheckBox directly into a header section, you need to leverage QHeaderView's ability to render custom widgets. This involves creating a custom delegate or directly setting a widget on the header view.

flowchart TD
    A[QStandardItemModel] --> B{setHeaderData()}
    B --> C[Text-only Header]
    A --> D[QHeaderView]
    D --> E{Custom Widget in Header?}
    E -->|Yes| F[Create QWidget for Header]
    F --> G[Set Widget on QHeaderView]
    G --> H[Interactive Header (e.g., CheckBox)]
    E -->|No| C

Flowchart for customizing QStandardItemModel headers

Implementing a CheckBox in a Header

To place a QCheckBox in a header, you generally follow these steps: subclass QHeaderView to override its paintSection method, or more commonly, create a custom widget that contains both the QCheckBox and a QLabel for the text, then set this widget as the header's section widget. The latter approach is often simpler for interactive elements.

#include <QApplication>
#include <QTableView>
#include <QStandardItemModel>
#include <QCheckBox>
#include <QHBoxLayout>
#include <QLabel>
#include <QHeaderView>

class CheckableHeaderWidget : public QWidget {
    Q_OBJECT
public:
    CheckableHeaderWidget(const QString& text, Qt::Orientation orientation, QWidget* parent = nullptr)
        : QWidget(parent),
          m_checkBox(new QCheckBox(this)),
          m_label(new QLabel(text, this))
    {
        QHBoxLayout* layout = new QHBoxLayout(this);
        layout->setContentsMargins(0, 0, 0, 0);
        layout->setSpacing(2);

        if (orientation == Qt::Horizontal) {
            layout->addWidget(m_checkBox);
            layout->addWidget(m_label);
            layout->addStretch();
        } else { // Vertical header, might need different layout
            layout->addWidget(m_label);
            layout->addWidget(m_checkBox);
            layout->addStretch();
        }

        connect(m_checkBox, &QCheckBox::stateChanged, this, &CheckableHeaderWidget::checkBoxStateChanged);
    }

    QCheckBox* checkBox() const { return m_checkBox; }
    QLabel* label() const { return m_label; }

signals:
    void checkBoxStateChanged(int state);

private:
    QCheckBox* m_checkBox;
    QLabel* m_label;
};

class CustomHeaderView : public QHeaderView {
    Q_OBJECT
public:
    CustomHeaderView(Qt::Orientation orientation, QWidget* parent = nullptr)
        : QHeaderView(orientation, parent) {}

    void setHeaderWidget(int logicalIndex, CheckableHeaderWidget* widget) {
        if (logicalIndex >= 0 && logicalIndex < count()) {
            setIndexWidget(model()->index(0, logicalIndex), widget);
        }
    }

protected:
    // This method is often overridden for custom painting, but for embedding widgets,
    // setIndexWidget is more direct. We keep it here for demonstration of potential.
    void paintSection(QPainter* painter, const QRect& rect, int logicalIndex) const override {
        QHeaderView::paintSection(painter, rect, logicalIndex);
        // If we were doing custom drawing *around* a widget, this is where it would go.
        // For a full widget, setIndexWidget is generally preferred.
    }
};

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

    QStandardItemModel model(4, 3);
    for (int row = 0; row < 4; ++row) {
        for (int col = 0; col < 3; ++col) {
            model.setItem(row, col, new QStandardItem(QString("Item %0,%1").arg(row).arg(col)));
        }
    }

    // Set standard header data for other columns
    model.setHeaderData(0, Qt::Horizontal, "Column 0");
    model.setHeaderData(2, Qt::Horizontal, "Column 2");

    QTableView view;
    view.setModel(&model);

    CustomHeaderView* header = new CustomHeaderView(Qt::Horizontal, &view);
    view.setHorizontalHeader(header);

    // Create and set the custom widget for the first column header
    CheckableHeaderWidget* checkableHeader = new CheckableHeaderWidget("Select All", Qt::Horizontal, header);
    header->setIndexWidget(model.index(0, 1), checkableHeader); // Use setIndexWidget for QHeaderView

    // Connect the checkbox signal to a slot (e.g., to select/deselect all items)
    QObject::connect(checkableHeader, &CheckableHeaderWidget::checkBoxStateChanged, [&](int state) {
        qDebug() << "Checkbox in header changed state to:" << state;
        // Example: Iterate through the model and update items based on checkbox state
        for (int row = 0; row < model.rowCount(); ++row) {
            QStandardItem* item = model.item(row, 1); // Assuming column 1 is affected
            if (item) {
                item->setCheckState(static_cast<Qt::CheckState>(state));
            }
        }
    });

    view.resize(400, 300);
    view.show();

    return a.exec();
}

#include "main.moc"

C++ code for embedding a QCheckBox and text in a QStandardItemModel header using a custom QWidget and QHeaderView.

💡

While setIndexWidget is convenient for placing widgets directly, remember that QHeaderView reuses widgets. If you have many custom header widgets, you might need to manage their state carefully or consider a custom delegate for more complex scenarios to avoid performance issues.

Alternative: Using a Custom Delegate for Header Painting

For more fine-grained control over how header sections are drawn, including drawing a checkbox and text directly without embedding a full QWidget, you can create a custom QStyledItemDelegate and set it on the QHeaderView. This approach gives you full control over the painting process within the paint() method, allowing you to draw text, a checkbox, and any other custom elements.

#include <QApplication>
#include <QTableView>
#include <QStandardItemModel>
#include <QStyledItemDelegate>
#include <QPainter>
#include <QStyleOptionButton>
#include <QMouseEvent>

class CheckBoxHeaderDelegate : public QStyledItemDelegate {
    Q_OBJECT
public:
    CheckBoxHeaderDelegate(QObject* parent = nullptr) : QStyledItemDelegate(parent) {}

    void paint(QPainter* painter, const QStyleOptionViewItem& option, const QModelIndex& index) const override {
        QStyleOptionViewItem opt = option;
        initStyleOption(&opt, index);

        // Draw background and standard header elements
        opt.widget->style()->drawControl(QStyle::CE_HeaderSection, &opt, painter, opt.widget);

        // Draw checkbox
        QStyleOptionButton checkBoxOption;
        checkBoxOption.rect = QRect(opt.rect.x() + 5, opt.rect.y() + (opt.rect.height() - 16) / 2, 16, 16);
        checkBoxOption.state = QStyle::State_Enabled;
        if (index.data(Qt::CheckStateRole).toInt() == Qt::Checked) {
            checkBoxOption.state |= QStyle::State_On;
        } else {
            checkBoxOption.state |= QStyle::State_Off;
        }
        opt.widget->style()->drawControl(QStyle::CE_CheckBox, &checkBoxOption, painter, opt.widget);

        // Draw text next to checkbox
        QRect textRect = opt.rect;
        textRect.setLeft(checkBoxOption.rect.right() + 5);
        opt.widget->style()->drawItemText(painter, textRect, Qt::AlignVCenter, opt.palette, opt.state & QStyle::State_Enabled, opt.text);
    }

    bool editorEvent(QEvent* event, QAbstractItemModel* model, const QStyleOptionViewItem& option, const QModelIndex& index) override {
        if (event->type() == QEvent::MouseButtonRelease) {
            QMouseEvent* mouseEvent = static_cast<QMouseEvent*>(event);
            QStyleOptionButton checkBoxOption;
            checkBoxOption.rect = QRect(option.rect.x() + 5, option.rect.y() + (option.rect.height() - 16) / 2, 16, 16);

            if (checkBoxOption.rect.contains(mouseEvent->pos())) {
                int currentState = index.data(Qt::CheckStateRole).toInt();
                int newState = (currentState == Qt::Checked) ? Qt::Unchecked : Qt::Checked;
                model->setData(index, newState, Qt::CheckStateRole);
                return true;
            }
        }
        return QStyledItemDelegate::editorEvent(event, model, option, index);
    }
};

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

    QStandardItemModel model(4, 3);
    for (int row = 0; row < 4; ++row) {
        for (int col = 0; col < 3; ++col) {
            model.setItem(row, col, new QStandardItem(QString("Item %0,%1").arg(row).arg(col)));
        }
    }

    // Set header data for the column that will have the checkbox
    model.setHeaderData(0, Qt::Horizontal, "Select All");
    model.setHeaderData(0, Qt::Horizontal, Qt::Unchecked, Qt::CheckStateRole); // Initial check state
    model.setHeaderData(1, Qt::Horizontal, "Column 1");
    model.setHeaderData(2, Qt::Horizontal, "Column 2");

    QTableView view;
    view.setModel(&model);

    // Set the custom delegate for the horizontal header
    view.horizontalHeader()->setItemDelegateForColumn(0, new CheckBoxHeaderDelegate(&view));

    // Connect a slot to react to header checkbox state changes
    QObject::connect(&model, &QStandardItemModel::headerDataChanged, [&](Qt::Orientation orientation, int first, int last) {
        if (orientation == Qt::Horizontal && first == 0 && last == 0) {
            int state = model.headerData(0, Qt::Horizontal, Qt::CheckStateRole).toInt();
            qDebug() << "Header checkbox state changed to:" << state;
            // Example: Update all items in the column
            for (int row = 0; row < model.rowCount(); ++row) {
                QStandardItem* item = model.item(row, 0);
                if (item) {
                    item->setCheckState(static_cast<Qt::CheckState>(state));
                }
            }
        }
    });

    view.resize(400, 300);
    view.show();

    return a.exec();
}

#include "main.moc"

C++ code for implementing a QCheckBox and text in a QStandardItemModel header using a custom QStyledItemDelegate.

⚠️

When using a custom delegate, ensure you handle mouse events correctly in editorEvent() to toggle the checkbox state. Without this, the checkbox will be drawn but not interactive.

Choosing the Right Approach

Both setIndexWidget() on QHeaderView and using a custom QStyledItemDelegate have their advantages:

setIndexWidget() (Custom Widget Approach):
- Pros: Simpler to implement for interactive widgets, leverages standard QWidget functionality, easier to manage complex layouts within the header.
- Cons: QHeaderView reuses widgets, which can lead to unexpected behavior if not managed carefully. Can be less performant if many unique widgets are used.
Custom QStyledItemDelegate:
- Pros: Offers maximum control over painting, potentially better performance for many similar custom headers as it only paints, not creates widgets. Ideal for drawing custom indicators or complex text layouts.
- Cons: More complex to implement, requires manual handling of painting and event processing (e.g., mouse clicks for checkboxes).

For a simple checkbox and text combination, the custom widget approach with setIndexWidget() is often sufficient and easier to maintain. For highly customized drawing or performance-critical scenarios with many headers, a delegate might be preferred.