Creating New Models

Inserting and Removing Rows

It is possible to change the number of rows and columns in a model. In the string list model it only makes sense to change the number of rows, so we only reimplement the functions for inserting and removing rows. These are declared in the class definition:

     bool insertRows(int position, int rows, const QModelIndex &index = QModelIndex());
     bool removeRows(int position, int rows, const QModelIndex &index = QModelIndex());

Since rows in this model correspond to strings in a list, the insertRows() function inserts a number of empty strings into the string list before the specified position. The number of strings inserted is equivalent to the number of rows specified.

The parent index is normally used to determine where in the model the rows should be added. In this case, we only have a single top-level list of strings, so we just insert empty strings into that list.

 bool StringListModel::insertRows(int position, int rows, const QModelIndex &parent)
 {
     beginInsertRows(QModelIndex(), position, position+rows-1);

     for (int row = 0; row < rows; ++row) {
         stringList.insert(position, "");
     }

     endInsertRows();
     return true;
 }

The model first calls the beginInsertRows() function to inform other components that the number of rows is about to change. The function specifies the row numbers of the first and last new rows to be inserted, and the model index for their parent item. After changing the string list, it calls endInsertRows() to complete the operation and inform other components that the dimensions of the model have changed, returning true to indicate success.

The function to remove rows from the model is also simple to write. The rows to be removed from the model are specified by the position and the number of rows given. We ignore the parent index to simplify our implementation, and just remove the corresponding items from the string list.

 bool StringListModel::removeRows(int position, int rows, const QModelIndex &parent)
 {
     beginRemoveRows(QModelIndex(), position, position+rows-1);

     for (int row = 0; row < rows; ++row) {
         stringList.removeAt(position);
     }

     endRemoveRows();
     return true;
 }

The beginRemoveRows() function is always called before any underlying data is removed, and specifies the first and last rows to be removed. This allows other components to access the data before it becomes unavailable. After the rows have been removed, the model emits endRemoveRows() to finish the operation and let other components know that the dimensions of the model have changed.