Function focus_changes 

pub fn focus_changes<Msg>(
    root: &dyn Widget<Msg>,
    before: Option<WidgetId>,
    after: Option<WidgetId>,
) -> Vec<Msg>

The messages a move of the focus from before to after sends (milestone 660): each widget whose subtree the focus left hears false, deepest first, then each whose subtree it entered hears true, deepest first. The widgets around both — the paths’ shared start — hear nothing; a widget no longer in the tree is not heard from.