Function pattern_switcher 

pub fn pattern_switcher<State, P, V>(
    key: impl Into<ChildKey>,
    pattern: P,
    child: V,
) -> PatternSwitcherView<State, P>
where
    State: 'static,
    P: TransitionPattern + Clone + 'static,
    V: View<State>,

Stage a keyed child switch under pattern: whenever key changes identity, the previous child animates out and child animates in. See the module docs.