Function poll_fn 

pub fn poll_fn<F, Item, Meta, CanPend, CanEnd>(
    func: F,
) -> PollFn<F, Item, Meta, CanPend, CanEnd>
where
    F: FnMut(&mut Context<'_>) -> PullStep<Item, Meta, CanPend, CanEnd>,
    Meta: Copy,
    CanPend: Toggle,
    CanEnd: Toggle,

Creates an asynchronous or synchronous pull from a closure.

The closure is called each time the pull is polled and should return a PullStep.