gtk:custom-filter-set-filter-func (filter &optional func)

filter -- a gtk:custom-filter object
func -- an optional gtk:custom-filter-func callback function to filter items, or the default value nil to unset the function

Sets (or unsets) the function used for filtering items. If func is nil, the custom filter matches all items. If the custom filter function changes its filtering behavior, the gtk:filter-changed function needs to be called.