The TreeModelSort is a model which implements the TreeSortable interface. It does not hold any data itself, but rather is created with a child model and proxies its data. It has identical column types to this child model, and the changes in the child are propagated. The primary purpose of this model is to provide a way to sort a different model without modifying it. Note that the sort function used by TreeModelSort is not guaranteed to be stable.

The use of this is best demonstrated through an example. In the following sample code we create two TreeView widgets each with a view of the same data. As the model is wrapped here by a TreeModelSort, the two GtkTreeViews can each sort their view of the data without affecting the other. By contrast, if we simply put the same model in each widget, then sorting the first would sort the second.

## Using a TreeModelSort

C code

{
  GtkTreeView *tree_view1;
  GtkTreeView *tree_view2;
  GtkTreeModel *sort_model1;
  GtkTreeModel *sort_model2;
  GtkTreeModel *child_model;

  // get the child model
  child_model = get_my_model ();

  // Create the first tree
  sort_model1 = gtk_tree_model_sort_new_with_model (child_model);
  tree_view1 = gtk_tree_view_new_with_model (sort_model1);

  // Create the second tree
  sort_model2 = gtk_tree_model_sort_new_with_model (child_model);
  tree_view2 = gtk_tree_view_new_with_model (sort_model2);

  // Now we can sort the two models independently
  gtk_tree_sortable_set_sort_column_id (GTK_TREE_SORTABLE (sort_model1),
                                        COLUMN_1, GTK_SORT_ASCENDING);
  gtk_tree_sortable_set_sort_column_id (GTK_TREE_SORTABLE (sort_model2),
                                        COLUMN_1, GTK_SORT_DESCENDING);
}

To demonstrate how to access the underlying child model from the sort model, the next example will be a callback for the TreeSelection changed signal. In this callback, we get a string from COLUMN_1 of the model. We then modify the string, find the same selected row on the child model, and change the row there.

Accessing the child model of in a selection changed callback

C code

void
selection_changed (GtkTreeSelection *selection, gpointer data)
{
  GtkTreeModel *sort_model = NULL;
  GtkTreeModel *child_model;
  GtkTreeIter sort_iter;
  GtkTreeIter child_iter;
  char *some_data = NULL;
  char *modified_data;

  // Get the current selected row and the model.
  if (! gtk_tree_selection_get_selected (selection,
                                         &sort_model,
                                         &sort_iter))
    return;

  // Look up the current value on the selected row and get
  // a new value to change it to.
  gtk_tree_model_get (GTK_TREE_MODEL (sort_model), &sort_iter,
                      COLUMN_1, &some_data,
                      -1);

  modified_data = change_the_data (some_data);
  g_free (some_data);

  // Get an iterator on the child model, instead of the sort model.
  gtk_tree_model_sort_convert_iter_to_child_iter (GTK_TREE_MODEL_SORT (sort_model),
                                                  &child_iter,
                                                  &sort_iter);

  // Get the child model and change the value of the row. In this
  // example, the child model is a GtkListStore. It could be any other
  // type of model, though.
  child_model = gtk_tree_model_sort_get_model (GTK_TREE_MODEL_SORT (sort_model));
  gtk_list_store_set (GTK_LIST_STORE (child_model), &child_iter,
                      COLUMN_1, &modified_data,
                      -1);
  g_free (modified_data);
}

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