Qt ListWidget: Programmatically Highlighting Rows using setCurrentRow()

Purpose

• This highlights the chosen row, indicating it's the one in focus.
• Sets the currently selected row (item) in a QListWidget.

Functionality

• If row is out of bounds (less than 0 or greater than or equal to the number of rows), the behavior depends on Qt's configuration:
  • In some cases, no row is selected.
  • In others, an error might be thrown. It's best to check the row index validity before using setCurrentRow().
• Takes an integer argument row that specifies the index of the row to be set as current.
  • Row indices in QListWidget start from 0 (the first row).

Selection Behavior

• The selection mode of the QListWidget determines how setCurrentRow() affects selection:
  • If the selection mode allows single selection (SingleSelection or SinglePassSelection), the specified row becomes the only selected item.
  • If multiple selection is allowed (MultiSelection or ExtendedSelection), the row is added to the current selection (unless it's already selected).

Code Example

#include <QApplication>
#include <QtWidgets>

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

    QListWidget listWidget;
    listWidget.addItem("Item 1");
    listWidget.addItem("Item 2");
    listWidget.addItem("Item 3");

    // Set the second row (index 1) as current (selected)
    listWidget.setCurrentRow(1);

    listWidget.show();

    return app.exec();
}

• Validate the row index before using setCurrentRow() to avoid potential errors or unexpected behavior.
• Be mindful of the selection mode to control how setCurrentRow() influences selection.
• Use setCurrentRow() when you want to programmatically highlight and potentially select a specific row in your QListWidget.

---

Selecting the Last Added Item

#include <QApplication>
#include <QtWidgets>

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

    QListWidget listWidget;
    listWidget.addItem("Item 1");
    listWidget.addItem("Item 2");

    // Add a new item and set it as current (assuming new items are added to the end)
    QListWidgetItem* newItem = new QListWidgetItem("Item 3");
    listWidget.addItem(newItem);
    listWidget.setCurrentRow(listWidget.count() - 1); // Get the last row index

    listWidget.show();

    return app.exec();
}

Handling Out-of-Bounds Row Index

#include <QApplication>
#include <QtWidgets>

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

    QListWidget listWidget;
    listWidget.addItem("Item 1");
    listWidget.addItem("Item 2");

    int desiredRow = 3; // This row doesn't exist

    // Check if the row index is valid before setting
    if (desiredRow >= 0 && desiredRow < listWidget.count()) {
        listWidget.setCurrentRow(desiredRow);
    } else {
        // Handle the case of invalid row (e.g., print an error message)
        qDebug() << "Invalid row index: " << desiredRow;
    }

    listWidget.show();

    return app.exec();
}

#include <QApplication>
#include <QtWidgets>

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

    QListWidget listWidget;
    listWidget.setSelectionMode(QListWidget::MultiSelection); // Allow multiple selection
    listWidget.addItem("Item 1");
    listWidget.addItem("Item 2");
    listWidget.addItem("Item 3");

    // Select the first row and keep existing selections (using QItemSelectionModel::Select)
    listWidget.setCurrentRow(0, QItemSelectionModel::Select);

    // Alternatively, deselect all other items and select only the second row (using QItemSelectionModel::ClearAndSelect)
    listWidget.setCurrentRow(1, QItemSelectionModel::ClearAndSelect);

    listWidget.show();

    return app.exec();
}

---

1. • This method takes a QListWidgetItem pointer as an argument instead of a row index.
   • It sets the specified item as the current item, effectively selecting it.
   • Use this if you already have a reference to the item you want to select.

   QListWidgetItem* myItem = listWidget.item(1); // Get the second item
   listWidget.setCurrentItem(myItem);
   

2. QListWidget::selectionModel()->setCurrentIndex(const QModelIndex& index, QItemSelectionModel::SelectionCommand command)
   

   • This approach leverages the QItemSelectionModel associated with the QListWidget.
   • It takes a QModelIndex as an argument, which you can obtain using listWidget.model()->index(row, 0) (assuming single column).
   • The command argument specifies how to handle selection:
     • QItemSelectionModel::Select: Selects the specified item.
     • QItemSelectionModel::ClearAndSelect: Deselects all other items and selects the specified item.
     • QItemSelectionModel::Deselect: Deselects the specified item.
   • This method provides more fine-grained control over selection behavior.

   QModelIndex index = listWidget.model()->index(1, 0); // Get the second item's index
   listWidget.selectionModel()->setCurrentIndex(index, QItemSelectionModel::Select);
   

3. Signals and Slots
   

   • You can connect to the itemClicked(QListWidgetItem* item) signal of the QListWidget.
   • When a user clicks on an item, the slot function is triggered, allowing you to perform actions based on the clicked item.
   • Inside the slot function, you can use other selection methods like setCurrentItem() or manipulate the selection model directly.

   connect(listWidget, &QListWidget::itemClicked, this, &MyClass::handleItemClick);
   
   void MyClass::handleItemClick(QListWidgetItem* item) {
       // Perform selection logic based on the clicked item
   }
   

The choice between these alternatives depends on your specific needs and coding style.

• Signals and slots provide a reactive approach based on user interactions.
• QListWidget::selectionModel()->setCurrentIndex() offers more control over selection behavior, especially when dealing with complex scenarios.
• QListWidget::setCurrentItem() is convenient when you have a reference to the item you want to select.