1 use super::assert_future;
2 use core::pin::Pin;
3 use futures_core::future::{Future, FusedFuture};
4 use futures_core::task::{Context, Poll};
5 use crate::future::{Either, FutureExt};
6
7 /// Future for the [`select()`] function.
8 #[must_use = "futures do nothing unless you `.await` or poll them"]
9 #[derive(Debug)]
10 pub struct Select<A, B> {
11 inner: Option<(A, B)>,
12 }
13
14 impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
15
16 /// Waits for either one of two differently-typed futures to complete.
17 ///
18 /// This function will return a new future which awaits for either one of both
19 /// futures to complete. The returned future will finish with both the value
20 /// resolved and a future representing the completion of the other work.
21 ///
22 /// Note that this function consumes the receiving futures and returns a
23 /// wrapped version of them.
24 ///
25 /// Also note that if both this and the second future have the same
26 /// output type you can use the `Either::factor_first` method to
27 /// conveniently extract out the value at the end.
28 ///
29 /// # Examples
30 ///
31 /// A simple example
32 ///
33 /// ```
34 /// # futures::executor::block_on(async {
35 /// use futures::{
36 /// pin_mut,
37 /// future::Either,
38 /// future::self,
39 /// };
40 ///
41 /// // These two futures have different types even though their outputs have the same type.
42 /// let future1 = async {
43 /// future::pending::<()>().await; // will never finish
44 /// 1
45 /// };
46 /// let future2 = async {
47 /// future::ready(2).await
48 /// };
49 ///
50 /// // 'select' requires Future + Unpin bounds
51 /// pin_mut!(future1);
52 /// pin_mut!(future2);
53 ///
54 /// let value = match future::select(future1, future2).await {
55 /// Either::Left((value1, _)) => value1, // `value1` is resolved from `future1`
56 /// // `_` represents `future2`
57 /// Either::Right((value2, _)) => value2, // `value2` is resolved from `future2`
58 /// // `_` represents `future1`
59 /// };
60 ///
61 /// assert!(value == 2);
62 /// # });
63 /// ```
64 ///
65 /// A more complex example
66 ///
67 /// ```
68 /// use futures::future::{self, Either, Future, FutureExt};
69 ///
70 /// // A poor-man's join implemented on top of select
71 ///
72 /// fn join<A, B>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
73 /// where A: Future + Unpin,
74 /// B: Future + Unpin,
75 /// {
76 /// future::select(a, b).then(|either| {
77 /// match either {
78 /// Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
79 /// Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
80 /// }
81 /// })
82 /// }
83 /// ```
select<A, B>(future1: A, future2: B) -> Select<A, B> where A: Future + Unpin, B: Future + Unpin84 pub fn select<A, B>(future1: A, future2: B) -> Select<A, B>
85 where A: Future + Unpin, B: Future + Unpin
86 {
87 assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(Select { inner: Some((future1, future2)) })
88 }
89
90 impl<A, B> Future for Select<A, B>
91 where
92 A: Future + Unpin,
93 B: Future + Unpin,
94 {
95 type Output = Either<(A::Output, B), (B::Output, A)>;
96
poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>97 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
98 let (mut a, mut b) = self.inner.take().expect("cannot poll Select twice");
99 match a.poll_unpin(cx) {
100 Poll::Ready(x) => Poll::Ready(Either::Left((x, b))),
101 Poll::Pending => match b.poll_unpin(cx) {
102 Poll::Ready(x) => Poll::Ready(Either::Right((x, a))),
103 Poll::Pending => {
104 self.inner = Some((a, b));
105 Poll::Pending
106 }
107 }
108 }
109 }
110 }
111
112 impl<A, B> FusedFuture for Select<A, B>
113 where
114 A: Future + Unpin,
115 B: Future + Unpin,
116 {
is_terminated(&self) -> bool117 fn is_terminated(&self) -> bool {
118 self.inner.is_none()
119 }
120 }
121