When dragging a row in a QTableWidget, how can I find out what row index it was dragged FROM and TO?

Question

I'm trying to keep some array data synchronized with the contents of a QTableWidget. I'd like to enable drag and drop reordering (moving items within the table, as opposed to copying), but it's not clear to me how, when the drop event is fired, I can find out what index the item was dragged FROM. Hence, I have no way of knowing what object to move within the list I'm synchronizing with. How can I get the original row index of the item being dragged?

Answer 1

Encode the from index in QMimeData and store it in the QDrag object with setMimeData(). When the drop event occurs, extract the data from the QDropEvent with mimeData().

Answer 2

QDropEvent has a source() function that will give you the widget that started the drag drop event. Then do a qobject_cast<QTableWidget> on the source. Once you verify the pointer, call QTableWidget::findItems to get the row of the item.

So something like this:

void dropEvent ( QDropEvent * event ) {
  if (event) {
    QTableWidget* table = qobject_cast<QTableWidget*>(event->source());
    if (table) {
      QString item = ""// Decode MIME data here.
      Qt::MatchFlag someFlag = Qt::MatchExactly; // Check documentation for match type.
      QList<QTableWidgetItem *> items = table->findItems(item, someFlag)
      // If you don't have repeats, items[0] is what you want.
      int initialRow = table->row(items[0]);
    }
  }
}

I tend to use model/view classes so this might be a little off, but it should work.

Answer 3

Step 1. Override the QTableWidget::mimeData function. Call the base class implementation, then stuff your own custom MIME type into the QMimeData, and return it.

Step 2. Override the QTableWidget::dropEvent function. If your MIME data is in the QMimeData, accept the drop and extract your data. Use the QTableWidget::indexAt to find what row/column the drop went into.