1 | use crate::{Iter, Stream}; |
2 | |
3 | use core::option; |
4 | use core::pin::Pin; |
5 | use core::task::{Context, Poll}; |
6 | |
7 | /// Stream for the [`once`](fn@once) function. |
8 | #[derive(Debug)] |
9 | #[must_use = "streams do nothing unless polled" ] |
10 | pub struct Once<T> { |
11 | iter: Iter<option::IntoIter<T>>, |
12 | } |
13 | |
14 | impl<I> Unpin for Once<I> {} |
15 | |
16 | /// Creates a stream that emits an element exactly once. |
17 | /// |
18 | /// The returned stream is immediately ready and emits the provided value once. |
19 | /// |
20 | /// # Examples |
21 | /// |
22 | /// ``` |
23 | /// use tokio_stream::{self as stream, StreamExt}; |
24 | /// |
25 | /// #[tokio::main] |
26 | /// async fn main() { |
27 | /// // one is the loneliest number |
28 | /// let mut one = stream::once(1); |
29 | /// |
30 | /// assert_eq!(Some(1), one.next().await); |
31 | /// |
32 | /// // just one, that's all we get |
33 | /// assert_eq!(None, one.next().await); |
34 | /// } |
35 | /// ``` |
36 | pub fn once<T>(value: T) -> Once<T> { |
37 | Once { |
38 | iter: crate::iter(Some(value)), |
39 | } |
40 | } |
41 | |
42 | impl<T> Stream for Once<T> { |
43 | type Item = T; |
44 | |
45 | fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> { |
46 | Pin::new(&mut self.iter).poll_next(cx) |
47 | } |
48 | |
49 | fn size_hint(&self) -> (usize, Option<usize>) { |
50 | self.iter.size_hint() |
51 | } |
52 | } |
53 | |