Customizing Selection Behavior in Qt ListWidget with QListWidget::setSelectionModel()

Purpose

• A selection model manages how items in a view (like QListWidget) can be selected. It determines the selection behavior, such as single selection, multiple selection, or no selection at all.
• This function allows you to replace the default selection model associated with a QListWidget instance.

How it Works

1. • You first need to create a QItemSelectionModel object. This object encapsulates the selection logic.
   • You can either use the default constructor or provide the underlying data model (if applicable) as an argument.

2. Set the Selection Model
   

   • Call setSelectionModel() on your QListWidget instance, passing the newly created QItemSelectionModel object as an argument.
   • This replaces the widget's internal selection model with the one you provide.

Example

#include <QApplication>
#include <QListWidget>
#include <QItemSelectionModel>

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

    QListWidget listWidget;
    QStringList items;
    items << "Item 1" << "Item 2" << "Item 3";
    listWidget.addItems(items);

    // Create a new selection model allowing single selection
    QItemSelectionModel* newSelectionModel = new QItemSelectionModel(&listWidget);
    newSelectionModel->setSelectionMode(QItemSelectionModel::SingleSelection);

    // Set the new selection model on the list widget
    listWidget.setSelectionModel(newSelectionModel);

    listWidget.show();

    return app.exec();
}

In this example:

• Finally, we call setSelectionModel() on the QListWidget to replace its default selection model.
• We create a new QItemSelectionModel object and set its selection mode to single selection.
• We create a QListWidget and add some items.

Key Points

• Remember to manage the memory of the created QItemSelectionModel object using deleteLater() or a similar mechanism if you dynamically allocate it.
• You can use this function to customize the selection behavior, such as allowing multiple selections or no selection at all.
• By default, QListWidget uses a single selection model.

Additional Considerations

• If you're using a custom data model with your QListWidget, you'll need to provide that model to the QItemSelectionModel constructor for proper functionality.
• Changing the selection model might affect how signals like itemSelectionChanged() are emitted, as the new model might have different behavior.

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Multiple Selection

#include <QApplication>
#include <QListWidget>
#include <QItemSelectionModel>

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

    QListWidget listWidget;
    QStringList items;
    items << "Item 1" << "Item 2" << "Item 3";
    listWidget.addItems(items);

    // Create a new selection model allowing multiple selection with Ctrl/Shift click
    QItemSelectionModel* newSelectionModel = new QItemSelectionModel(&listWidget);
    newSelectionModel->setSelectionMode(QItemSelectionModel::MultiSelection);
    newSelectionModel->setSelectionBehavior(QItemSelectionModel::SelectItems); // Select using Ctrl/Shift

    // Set the new selection model on the list widget
    listWidget.setSelectionModel(newSelectionModel);

    listWidget.show();

    return app.exec();
}

In this example, we create a selection model that allows multiple selections using the Ctrl or Shift key modifiers.

No Selection

#include <QApplication>
#include <QListWidget>
#include <QItemSelectionModel>

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

    QListWidget listWidget;
    QStringList items;
    items << "Item 1" << "Item 2" << "Item 3";
    listWidget.addItems(items);

    // Create a new selection model that disallows any selection
    QItemSelectionModel* newSelectionModel = new QItemSelectionModel(&listWidget);
    newSelectionModel->setSelectionMode(QItemSelectionModel::NoSelection);

    // Set the new selection model on the list widget
    listWidget.setSelectionModel(newSelectionModel);

    listWidget.show();

    return app.exec();
}

This example creates a selection model that prevents any items from being selected in the list widget.

#include <QApplication>
#include <QListWidget>
#include <QItemSelectionModel>

class CustomSelectionModel : public QItemSelectionModel {
    Q_OBJECT

public:
    CustomSelectionModel(QObject* parent = nullptr) : QItemSelectionModel(parent) {}

protected:
    // Override selection to only allow even-numbered items
    virtual bool select(const QModelIndex &index, QItemSelectionModel::SelectionFlags command) const override {
        if (index.row() % 2 != 0) { // Only allow even-numbered items
            return false;
        }
        return QItemSelectionModel::select(index, command);
    }
};

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

    QListWidget listWidget;
    QStringList items;
    items << "Item 1" << "Item 2" << "Item 3" << "Item 4";
    listWidget.addItems(items);

    // Create a custom selection model that only allows even-numbered items
    CustomSelectionModel* newSelectionModel = new CustomSelectionModel(&listWidget);
    newSelectionModel->setSelectionMode(QItemSelectionModel::SingleSelection);

    // Set the new selection model on the list widget
    listWidget.setSelectionModel(newSelectionModel);

    listWidget.show();

    return app.exec();
}

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Using QListWidget::setSelectionMode()

• Code example:
• It offers options like single selection, multiple selection, and no selection, similar to what you can achieve with a custom selection model.
• This built-in function allows you to modify the selection mode of the default selection model associated with QListWidget.

QListWidget listWidget;
listWidget.setSelectionMode(QListWidget::SingleSelection); // or MultiSelection, NoSelection

Handling User Interaction Events

• Implement your desired selection logic within the slots connected to these signals.
• These signals include:
  • itemClicked(QListWidgetItem* item): Emitted when an item is clicked.
  • itemActivated(QListWidgetItem* item): Emitted when an item is double-clicked or activated by another means.
  • currentItemChanged(QListWidgetItem* current, QListWidgetItem* previous): Emitted when the current item changes.
• If you only need to react to specific user interactions that cause selection changes, you can connect to signals emitted by QListWidget.

Manual Management with QListWidget::setCurrentItem() and QListWidget::setSelected()

• However, this approach can become cumbersome for complex selection scenarios.
• For very basic selection control, you can directly use functions like:
  • QListWidget::setCurrentItem(QListWidgetItem* item): Sets the current item (also selects it by default unless selection mode is NoSelection).
  • QListWidget::setSelected(QListWidgetItem* item, bool selected): Sets the selected state of an individual item.

• For simpler cases, modifying the default selection mode with setSelectionMode() or handling interaction events might be sufficient.
• If you need a high degree of customization over selection behavior, creating a custom selection model with QListWidget::setSelectionModel() provides the most flexibility.