Function actix_service::apply_fn

pub fn apply_fn<T, F, R, In, Out, Err, U>(
    service: U, 
    f: F
) -> Apply<T, F, R, In, Out, Err> where
    T: Service<Error = Err>,
    F: FnMut(In, &mut T) -> R,
    R: Future<Output = Result<Out, Err>>,
    U: IntoService<T>, 

Apply tranform function to a service.