Function animate 

pub fn animate(
    clock: FrameClock,
    timing: Timing,
    step: impl FnMut(Sample) + 'static,
) -> FrameHandle

Call step(sample) once per display frame of clock until timing is done, then stop.

Elapsed time is measured from the first frame delivered, not from this call, so a transition started while the app is busy begins on screen rather than part-way through. The final call has sample.done == true and the exact end value; after it the subscription ends and the window requests no more frames on this transition’s behalf. Keep the handle, or call FrameHandle::in_scope: dropping it cancels the transition where it stands.