Function part_views 

pub fn part_views<'a>(
    name: &str,
    widget: &'a mut Box<dyn Widget>,
    slot: &str,
    key: &str,
    of: &str,
) -> Option<&'a mut dyn SlotViews>

The views its slot slot holds for the part key, in the part’s own view slot of, built for an instance of name, to change by key in place: None when the widget changes them only by being built again.