Understanding QTreeWidget::sortColumn() in Qt Widgets

Purpose

• The QTreeWidget::sortColumn() function in Qt Widgets retrieves the column index that is currently being used to sort the items within the QTreeWidget.

Return Value

• If the tree widget is not currently sorted, the function returns -1.
• It returns an integer representing the column index (starting from 0 for the first column).

Usage

#include <QTreeWidget>

void checkSortColumn(QTreeWidget* treeWidget) {
  int sortColumn = treeWidget->sortColumn();
  if (sortColumn != -1) {
    // The tree widget is sorted by the column at index sortColumn
    QString columnName = treeWidget->headerItem()->text(sortColumn);
    qDebug() << "Tree widget is sorted by column:" << columnName;
  } else {
    qDebug() << "Tree widget is not currently sorted.";
  }
}

In this example:

1. We get the sort column index using treeWidget->sortColumn().
2. If the index is not -1 (meaning it's sorted), we retrieve the column header text using treeWidget->headerItem()->text(sortColumn).
3. We then print information about the current sort state.

• For more advanced sorting behavior, you can create custom comparators using lambdas or slots connected to the sortingEnabledChanged() signal.
• To sort the QTreeWidget items, use the sortItems(int column, Qt::SortOrder order) function. This function takes the column index and the desired sort order (ascending or descending) as arguments.

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Example 1: Sorting by a Specific Column

This code sorts the QTreeWidget by the second column (index 1) in ascending order:

#include <QTreeWidget>
#include <Qt>

void sortBySecondColumn(QTreeWidget* treeWidget) {
  treeWidget->sortItems(1, Qt::AscendingOrder); // Sort by second column (index 1)
}

Example 2: Checking Sort Column and Order

This code retrieves the current sort column and order, and displays them if the tree widget is sorted:

#include <QTreeWidget>
#include <Qt>

void checkSortState(QTreeWidget* treeWidget) {
  int sortColumn = treeWidget->sortColumn();
  if (sortColumn != -1) {
    QString columnName = treeWidget->headerItem()->text(sortColumn);
    Qt::SortOrder order = treeWidget->sortIndicator();
    QString orderString = (order == Qt::AscendingOrder) ? "Ascending" : "Descending";
    qDebug() << "Tree widget is sorted by column:" << columnName << ", order:" << orderString;
  } else {
    qDebug() << "Tree widget is not currently sorted.";
  }
}

Example 3: Sorting Using a Custom Comparator

This code demonstrates sorting by a custom criterion (e.g., sorting by the second digit of each item's first column):

#include <QTreeWidget>
#include <Qt>

bool customComparator(const QTreeWidgetItem* item1, const QTreeWidgetItem* item2) {
  QString text1 = item1->text(0);
  QString text2 = item2->text(0);
  return text1[1] < text2[1]; // Sort by second digit of first column
}

void sortBySecondDigit(QTreeWidget* treeWidget) {
  treeWidget->sortItems(0, Qt::AscendingOrder, customComparator); // Sort by first column (index 0) using custom comparator
}

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1. • The QTreeWidget class emits the sortingEnabledChanged(bool) signal whenever the sorting state changes (enabled or disabled). You can connect a slot to this signal and check the new sorting state without directly querying the current column.

   void handleSortingChange(bool enabled) {
       if (enabled) {
           // Get sorting details using other methods (e.g., header visibility)
       } else {
           // Sorting disabled
       }
   }
   
   // Connect the signal to your slot
   connect(treeWidget, &QTreeWidget::sortingEnabledChanged, this, &handleSortingChange);
   

2. Manual Inspection
   

   • If you only need to know if the tree is sorted or not, you can visually inspect the header items. A sorted column typically displays a sort indicator (arrow) next to the header text. However, this approach is not reliable for programmatic checks.

3. Custom Logic
   

   • In some scenarios, you might have additional information about the sorting state stored elsewhere in your application logic. You can use this information instead of relying on QTreeWidget::sortColumn(). This is suitable for more complex sorting scenarios where you have control over the sorting process.

Choosing the Right Alternative

The best alternative depends on your specific requirements:

• For simple checks when sorting is enabled/disabled, the sortingEnabledChanged signal is a good choice.

Remember

• Alternatives have their own use cases and might require additional logic in some scenarios.
• QTreeWidget::sortColumn() is the most direct approach to retrieve the current sorted column.