Function from_fn 

pub fn from_fn<F, Item, Meta, CanEnd>(func: F) -> FromFn<F, Item, Meta, CanEnd>
where
    F: FnMut() -> PullStep<Item, Meta, No, CanEnd>,
    Meta: Copy,
    CanEnd: Toggle,

Creates a synchronous pull from a closure.

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