Displaying sub-categories in sidebar when parent category selected

Question

What I'm trying to achieve in WP 3.1.x using the wp_nav_menu function, is to have a menu such as:

-- Category 1
-- Category 2
-- Category 3
-- Category 4
-- Category 5
-- Category 6

But when you're on 'Category 2' it displays as:

-- Category 1
-- Category 2
  -- Sub Category 1
  -- Sub Category 2
  -- Sub Category 3
-- Category 3
-- Category 4
-- Category 5
-- Category 6

PS. I'm not looking for CSS hide/show functionality, that can be achieved easily.

PPS. Is a possible solution for this extending the Walker_Nav_Menu class?

Answer 1

If you subclass the Walker_Nav_Menu class you can override the display_element function and conditionally remove an item's children before they are looped through.

if (!$element->current && !$element->current_item_ancestor && isset( $children_elements[$id])) {
   unset( $children_elements[ $id ] );
}

This would remove the children of any item that is not current, so you would need to check the item's type if you just want to do this for categories. The complete walker would look something like this: https://gist.github.com/954627