Qt Accessibility: Notifying Assistive Technologies about Table Data Changes

Purpose

• This enables these assistive tools to keep their representation of the data synchronized with the actual content, providing a seamless experience for users with disabilities.
• In Qt applications with tables, lists, or trees, QAccessibleTableModelChangeEvent serves as a notification mechanism to inform assistive technologies (screen readers, for example) about changes in the underlying data model.

Functionality

• This event object encapsulates details about the specific change, including:
  • The type of change (ModelChangeType enum):
    • ModelReset: The entire model has been reset, invalidating previous information.
    • DataChanged: Cell data has been modified within a specific range (without adding/removing cells).
    • RowsInserted: New rows have been inserted.
    • ColumnsInserted: New columns have been inserted.
    • RowsRemoved: Rows have been removed.
    • ColumnsRemoved: Columns have been removed.
  • Affected row/column indices (firstRow, lastRow, firstColumn, lastColumn).
• When a change occurs in the data model (e.g., adding/removing rows or columns, modifying cell content), an instance of QAccessibleTableModelChangeEvent is created.

Usage

1. #include <QAccessibleTableModelChangeEvent>
   

2. Create the Event Object
   

   • You don't typically create QAccessibleTableModelChangeEvent objects directly in your application code.
   • Qt's accessibility framework handles this internally when changes occur in the data model.

3. Accessibility Notification
   

   • The framework emits the event through the QAccessible::updateAccessibility() signal.
   • Assistive technologies listening to this signal receive the event object, enabling them to update their representation of the table based on the change type and affected cells.

Example (Illustrative)

// (Assuming you have a QAbstractTableModel subclass for your table data)

void MyTableModel::insertRows(int row, int count) {
    // ... (data insertion logic)

    // Notify accessibility framework about the change
    QAccessibleTableModelChangeEvent event(this, QAccessibleTableModelChangeEvent::RowsInserted);
    event.setFirstRow(row);
    event.setLastRow(row + count - 1);
    QAccessible::updateAccessibility(&event);
}

---

#include <QCoreApplication>
#include <QAbstractTableModel>
#include <QAccessibleTableModelChangeEvent>
#include <QAccessible>
#include <QString>
#include <QDebug>

// A simple table model subclass
class MyTableModel : public QAbstractTableModel {
    Q_OBJECT

public:
    MyTableModel(QObject* parent = nullptr) : QAbstractTableModel(parent), data(QStringList() << "Item 1" << "Item 2" << "Item 3") {}

    int rowCount(const QModelIndex& parent = QModelIndex()) const override { return data.size(); }
    int columnCount(const QModelIndex& parent = QModelIndex()) const override { return 1; }
    QVariant data(const QModelIndex& index, int role = Qt::DisplayRole) const override {
        if (role == Qt::DisplayRole && index.isValid()) {
            return this->data.at(index.row());
        }
        return QVariant();
    }

    void addItem(const QString& item) {
        beginInsertRows(QModelIndex(), rowCount(), rowCount());
        data.append(item);
        endInsertRows();
    }

private:
    QStringList data;

signals:
    void dataChanged(const QModelIndex& topLeft, const QModelIndex& bottomRight);
};

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

    // Create the table model
    MyTableModel model;

    // Simulate adding a new item
    model.addItem("Item 4");

    // Connect the dataChanged signal to a slot that emits an accessibility notification
    QObject::connect(&model, &MyTableModel::dataChanged, &model, &MyTableModel::onDataChanged);

    return app.exec();
}

// Slot to emit the accessibility notification
void MyTableModel::onDataChanged(const QModelIndex& topLeft, const QModelIndex& bottomRight) {
    QAccessibleTableModelChangeEvent event(this, QAccessibleTableModelChangeEvent::RowsInserted);
    event.setFirstRow(topLeft.row());
    event.setLastRow(bottomRight.row());
    QAccessible::updateAccessibility(&event);

    qDebug() << "Accessibility notified about data change (new item added)";
}

1. MyTableModel
   This class inherits from QAbstractTableModel and implements the required methods (rowCount, columnCount, data) to provide data for the table.
2. addItem Function
   This function demonstrates how the model's data can be modified. It simulates adding a new item by appending it to the data list and emitting the dataChanged signal.
3. dataChanged Signal
   This signal is emitted whenever the model's data changes.
4. onDataChanged Slot
   This slot is connected to the dataChanged signal. When the signal is emitted, it creates a QAccessibleTableModelChangeEvent object, specifying the type of change (RowsInserted) and the affected row (topLeft.row()). Finally, it calls QAccessible::updateAccessibility to notify assistive technologies about the change.
5. Debug Output
   The qDebug statement is for illustrative purposes, showing that the accessibility notification has been sent.

---

1. • If you have a more granular approach to data changes (e.g., notifying about individual cell modifications), you could create custom signals in your table model class.
   • These signals would provide details about the specific change (cell index, new value) and could be connected to slots in your application logic that handle accessibility updates.
   • You might need to implement your own logic to translate these custom signals into a format suitable for assistive technologies, potentially using lower-level Qt accessibility APIs like QAccessible::setValue or QAccessible::text.

2. Model-View-Accessibility (MVA) Pattern
   

   • Qt offers the MVA pattern for accessibility, where a separate accessibility model is used to represent the data for assistive technologies.
   • This might be an option if you have complex table structures or require more control over the information presented to assistive tools.
   • However, implementing the MVA pattern can be more involved compared to using QAccessibleTableModelChangeEvent.

Important Considerations

• If your needs are relatively simple (notifying about row/column insertions/removals or cell value changes), QAccessibleTableModelChangeEvent is generally the recommended approach for Qt applications.
• Using custom approaches might require more development effort and testing to guarantee proper accessibility.
• When considering alternatives, ensure you maintain compatibility with Qt's accessibility framework and the assistive technologies your application targets.