1 | use super::assert_future; |
2 | use core::pin::Pin; |
3 | use futures_core::future::{FusedFuture, Future}; |
4 | use futures_core::task::{Context, Poll}; |
5 | |
6 | /// Future for the [`lazy`] function. |
7 | #[derive (Debug)] |
8 | #[must_use = "futures do nothing unless you `.await` or poll them" ] |
9 | pub struct Lazy<F> { |
10 | f: Option<F>, |
11 | } |
12 | |
13 | // safe because we never generate `Pin<&mut F>` |
14 | impl<F> Unpin for Lazy<F> {} |
15 | |
16 | /// Creates a new future that allows delayed execution of a closure. |
17 | /// |
18 | /// The provided closure is only run once the future is polled. |
19 | /// |
20 | /// # Examples |
21 | /// |
22 | /// ``` |
23 | /// # futures::executor::block_on(async { |
24 | /// use futures::future; |
25 | /// |
26 | /// let a = future::lazy(|_| 1); |
27 | /// assert_eq!(a.await, 1); |
28 | /// |
29 | /// let b = future::lazy(|_| -> i32 { |
30 | /// panic!("oh no!" ) |
31 | /// }); |
32 | /// drop(b); // closure is never run |
33 | /// # }); |
34 | /// ``` |
35 | pub fn lazy<F, R>(f: F) -> Lazy<F> |
36 | where |
37 | F: FnOnce(&mut Context<'_>) -> R, |
38 | { |
39 | assert_future::<R, _>(Lazy { f: Some(f) }) |
40 | } |
41 | |
42 | impl<F, R> FusedFuture for Lazy<F> |
43 | where |
44 | F: FnOnce(&mut Context<'_>) -> R, |
45 | { |
46 | fn is_terminated(&self) -> bool { |
47 | self.f.is_none() |
48 | } |
49 | } |
50 | |
51 | impl<F, R> Future for Lazy<F> |
52 | where |
53 | F: FnOnce(&mut Context<'_>) -> R, |
54 | { |
55 | type Output = R; |
56 | |
57 | fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<R> { |
58 | Poll::Ready((self.f.take().expect(msg:"Lazy polled after completion" ))(cx)) |
59 | } |
60 | } |
61 | |