1 | use crate::future::{Either, TryFutureExt}; |
2 | use core::pin::Pin; |
3 | use futures_core::future::{Future, TryFuture}; |
4 | use futures_core::task::{Context, Poll}; |
5 | |
6 | /// Future for the [`try_select()`] function. |
7 | #[must_use = "futures do nothing unless you `.await` or poll them" ] |
8 | #[derive (Debug)] |
9 | pub struct TrySelect<A, B> { |
10 | inner: Option<(A, B)>, |
11 | } |
12 | |
13 | impl<A: Unpin, B: Unpin> Unpin for TrySelect<A, B> {} |
14 | |
15 | type EitherOk<A, B> = Either<(<A as TryFuture>::Ok, B), (<B as TryFuture>::Ok, A)>; |
16 | type EitherErr<A, B> = Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::Error, A)>; |
17 | |
18 | /// Waits for either one of two differently-typed futures to complete. |
19 | /// |
20 | /// This function will return a new future which awaits for either one of both |
21 | /// futures to complete. The returned future will finish with both the value |
22 | /// resolved and a future representing the completion of the other work. |
23 | /// |
24 | /// Note that this function consumes the receiving futures and returns a |
25 | /// wrapped version of them. |
26 | /// |
27 | /// Also note that if both this and the second future have the same |
28 | /// success/error type you can use the `Either::factor_first` method to |
29 | /// conveniently extract out the value at the end. |
30 | /// |
31 | /// # Examples |
32 | /// |
33 | /// ``` |
34 | /// use futures::future::{self, Either, Future, FutureExt, TryFuture, TryFutureExt}; |
35 | /// |
36 | /// // A poor-man's try_join implemented on top of select |
37 | /// |
38 | /// fn try_join<A, B, E>(a: A, b: B) -> impl TryFuture<Ok=(A::Ok, B::Ok), Error=E> |
39 | /// where A: TryFuture<Error = E> + Unpin + 'static, |
40 | /// B: TryFuture<Error = E> + Unpin + 'static, |
41 | /// E: 'static, |
42 | /// { |
43 | /// future::try_select(a, b).then(|res| -> Box<dyn Future<Output = Result<_, _>> + Unpin> { |
44 | /// match res { |
45 | /// Ok(Either::Left((x, b))) => Box::new(b.map_ok(move |y| (x, y))), |
46 | /// Ok(Either::Right((y, a))) => Box::new(a.map_ok(move |x| (x, y))), |
47 | /// Err(Either::Left((e, _))) => Box::new(future::err(e)), |
48 | /// Err(Either::Right((e, _))) => Box::new(future::err(e)), |
49 | /// } |
50 | /// }) |
51 | /// } |
52 | /// ``` |
53 | pub fn try_select<A, B>(future1: A, future2: B) -> TrySelect<A, B> |
54 | where |
55 | A: TryFuture + Unpin, |
56 | B: TryFuture + Unpin, |
57 | { |
58 | super::assert_future::<Result<EitherOk<A, B>, EitherErr<A, B>>, _>(TrySelect { |
59 | inner: Some((future1, future2)), |
60 | }) |
61 | } |
62 | |
63 | impl<A: Unpin, B: Unpin> Future for TrySelect<A, B> |
64 | where |
65 | A: TryFuture, |
66 | B: TryFuture, |
67 | { |
68 | type Output = Result<EitherOk<A, B>, EitherErr<A, B>>; |
69 | |
70 | fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { |
71 | let (mut a: A, mut b: B) = self.inner.take().expect(msg:"cannot poll Select twice" ); |
72 | match a.try_poll_unpin(cx) { |
73 | Poll::Ready(Err(x: ::Error)) => Poll::Ready(Err(Either::Left((x, b)))), |
74 | Poll::Ready(Ok(x: ::Ok)) => Poll::Ready(Ok(Either::Left((x, b)))), |
75 | Poll::Pending => match b.try_poll_unpin(cx) { |
76 | Poll::Ready(Err(x: ::Error)) => Poll::Ready(Err(Either::Right((x, a)))), |
77 | Poll::Ready(Ok(x: ::Ok)) => Poll::Ready(Ok(Either::Right((x, a)))), |
78 | Poll::Pending => { |
79 | self.inner = Some((a, b)); |
80 | Poll::Pending |
81 | } |
82 | }, |
83 | } |
84 | } |
85 | } |
86 | |