Stream::delay

src/stream/stream/delay.rs

@@ -0,0 +1,48 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::time::Duration;
+
+use pin_project_lite::pin_project;
+
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+pin_project! {
+    #[doc(hidden)]
+    #[allow(missing_debug_implementations)]
+    pub struct Delay<S> {
+        #[pin]
+        stream: S,
+        #[pin]
+        delay: futures_timer::Delay,
+        delay_done: bool,
+    }
+}
+
+impl<S> Delay<S> {
+    pub(super) fn new(stream: S, dur: Duration) -> Self {
+        Delay {
+            stream,
+            delay: futures_timer::Delay::new(dur),
+            delay_done: false,
+        }
+    }
+}
+
+impl<S> Stream for Delay<S>
+where
+    S: Stream,
+{
+    type Item = S::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let this = self.project();
+
+        if !*this.delay_done {
+            futures_core::ready!(this.delay.poll(cx));
+            *this.delay_done = true;
+        }
+
+        this.stream.poll_next(cx)
+    }
+}

src/stream/stream/mod.rs

@@ -130,6 +130,7 @@ cfg_unstable! {
     pub use flat_map::FlatMap;
     pub use timeout::{TimeoutError, Timeout};
     pub use throttle::Throttle;
+    pub use delay::Delay;
 
     mod count;
     mod merge;
@@ -138,6 +139,7 @@ cfg_unstable! {
     mod partition;
     mod timeout;
     mod throttle;
+    mod delay;
     mod unzip;
 }
 
@@ -573,6 +575,47 @@ extension_trait! {
             Enumerate::new(self)
         }
 
+        #[doc = r#"
+            Creates a stream that is delayed before it starts yielding items.
+
+            # Examples
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::prelude::*;
+            use async_std::stream;
+            use std::time::{Duration, Instant};
+
+            let start = Instant::now();
+            let mut s = stream::from_iter(vec![0u8, 1, 2]).delay(Duration::from_millis(200));
+
+            assert_eq!(s.next().await, Some(0));
+            // The first time will take more than 200ms due to delay.
+            assert!(start.elapsed().as_millis() >= 200);
+
+            assert_eq!(s.next().await, Some(1));
+            // There will be no delay after the first time.
+            assert!(start.elapsed().as_millis() <= 210);
+
+            assert_eq!(s.next().await, Some(2));
+            assert!(start.elapsed().as_millis() <= 210);
+
+            assert_eq!(s.next().await, None);
+            assert!(start.elapsed().as_millis() <= 210);
+            #
+            # }) }
+            ```
+        "#]
+        #[cfg(any(feature = "unstable", feature = "docs"))]
+        #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+        fn delay(self, dur: std::time::Duration) -> Delay<Self>
+        where
+            Self: Sized,
+        {
+            Delay::new(self, dur)
+        }
+
         #[doc = r#"
             Takes a closure and creates a stream that calls that closure on every element of this stream.