Function spring 

pub fn spring<T>(
    id: impl Into<TransitionId>,
    target: T,
    policy: Spring,
    window: &mut Window,
    cx: &mut App,
) -> T::Output
where
    T: SpringTarget,

Returns the current value for a spring travelling toward target.

State is keyed by id exactly as transition keys its own. The first value is adopted immediately; later target changes preserve both the current position and the current velocity, so an interrupted spring is redirected rather than restarted.

Call this while rendering an element, where GPUI keyed element state is available. A channel id must identify one value within that element.