1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
|
use std::pin::Pin;
use std::task::{Context, Poll};
use pin_project_lite::pin_project;
use smol::future::Future;
/// Returns the result of the future that completes first, preferring future1
/// if both are ready.
///
/// # Examples
///
/// ```
/// use std::future;
///
/// use karyon_core::async_util::{select, Either};
///
/// async {
/// let fut1 = future::pending::<String>();
/// let fut2 = future::ready(0);
/// let res = select(fut1, fut2).await;
/// assert!(matches!(res, Either::Right(0)));
/// // ....
/// };
///
/// ```
///
pub fn select<T1, T2, F1, F2>(future1: F1, future2: F2) -> Select<F1, F2>
where
F1: Future<Output = T1>,
F2: Future<Output = T2>,
{
Select { future1, future2 }
}
pin_project! {
#[derive(Debug)]
pub struct Select<F1, F2> {
#[pin]
future1: F1,
#[pin]
future2: F2,
}
}
/// The return value from the [`select`] function, indicating which future
/// completed first.
#[derive(Debug)]
pub enum Either<T1, T2> {
Left(T1),
Right(T2),
}
// Implement the Future trait for the Select struct.
impl<T1, T2, F1, F2> Future for Select<F1, F2>
where
F1: Future<Output = T1>,
F2: Future<Output = T2>,
{
type Output = Either<T1, T2>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.project();
if let Poll::Ready(t) = this.future1.poll(cx) {
return Poll::Ready(Either::Left(t));
}
if let Poll::Ready(t) = this.future2.poll(cx) {
return Poll::Ready(Either::Right(t));
}
Poll::Pending
}
}
#[cfg(test)]
mod tests {
use super::{select, Either};
use smol::Timer;
use std::future;
#[test]
fn test_async_select() {
smol::block_on(async move {
let fut = select(Timer::never(), future::ready(0 as u32)).await;
assert!(matches!(fut, Either::Right(0)));
let fut1 = future::pending::<String>();
let fut2 = future::ready(0);
let res = select(fut1, fut2).await;
assert!(matches!(res, Either::Right(0)));
let fut1 = future::ready(0);
let fut2 = future::pending::<String>();
let res = select(fut1, fut2).await;
assert!(matches!(res, Either::Left(_)));
});
}
}
|
