63 lines
1.4 KiB
Rust
63 lines
1.4 KiB
Rust
use crate::Stream;
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use core::pin::Pin;
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use core::task::{Context, Poll};
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/// Stream for the [`once`](fn@once) function.
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#[derive(Debug)]
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#[must_use = "streams do nothing unless polled"]
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pub struct Once<T> {
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value: Option<T>,
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}
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impl<I> Unpin for Once<I> {}
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/// Creates a stream that emits an element exactly once.
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///
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/// The returned stream is immediately ready and emits the provided value once.
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///
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/// # Examples
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///
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/// ```
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/// use tokio_stream::{self as stream, StreamExt};
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///
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/// # #[tokio::main(flavor = "current_thread")]
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/// # async fn main() {
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/// // one is the loneliest number
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/// let mut one = stream::once(1);
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///
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/// assert_eq!(Some(1), one.next().await);
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///
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/// // just one, that's all we get
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/// assert_eq!(None, one.next().await);
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/// # }
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/// ```
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pub fn once<T>(value: T) -> Once<T> {
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Once { value: Some(value) }
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}
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impl<T> Stream for Once<T> {
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type Item = T;
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fn poll_next(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<T>> {
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#[cfg(feature = "rt")]
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{
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use tokio::task::coop;
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let coop = std::task::ready!(coop::poll_proceed(_cx));
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coop.made_progress();
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}
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Poll::Ready(self.value.take())
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}
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fn size_hint(&self) -> (usize, Option<usize>) {
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if self.value.is_some() {
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(1, Some(1))
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} else {
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(0, Some(0))
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}
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}
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}
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