blob: 9d0f238b91bba48fe52866e380db73fb6150e123 [file] [log] [blame]
use std::mem;
use futures::{Future, IntoFuture, Poll};
pub(crate) trait Started: Future {
fn started(&self) -> bool;
}
pub(crate) fn lazy<F, R>(func: F) -> Lazy<F, R>
where
F: FnOnce() -> R,
R: IntoFuture,
{
Lazy {
inner: Inner::Init(func),
}
}
// FIXME: allow() required due to `impl Trait` leaking types to this lint
#[allow(missing_debug_implementations)]
pub(crate) struct Lazy<F, R: IntoFuture> {
inner: Inner<F, R::Future>
}
enum Inner<F, R> {
Init(F),
Fut(R),
Empty,
}
impl<F, R> Started for Lazy<F, R>
where
F: FnOnce() -> R,
R: IntoFuture,
{
fn started(&self) -> bool {
match self.inner {
Inner::Init(_) => false,
Inner::Fut(_) |
Inner::Empty => true,
}
}
}
impl<F, R> Future for Lazy<F, R>
where
F: FnOnce() -> R,
R: IntoFuture,
{
type Item = R::Item;
type Error = R::Error;
fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
match self.inner {
Inner::Fut(ref mut f) => return f.poll(),
_ => (),
}
match mem::replace(&mut self.inner, Inner::Empty) {
Inner::Init(func) => {
let mut fut = func().into_future();
let ret = fut.poll();
self.inner = Inner::Fut(fut);
ret
},
_ => unreachable!("lazy state wrong"),
}
}
}