Implemented StreamExt::throttle

examples/throttle.rs

@@ -0,0 +1,27 @@
+//! Spawns a timed task which gets throttled.
+
+fn main() {
+    #[cfg(feature = "unstable")]
+    {
+        use async_std::prelude::*;
+        use async_std::task;
+
+        task::block_on(async {
+            use async_std::stream;
+            use std::time::Duration;
+
+            // emit value every 1 second
+            let s = stream::interval(Duration::from_secs(1)).enumerate();
+
+            // throttle for 2 seconds
+            let s = s.throttle(Duration::from_secs(2));
+
+            s.for_each(|(n, _)| {
+                dbg!(n);
+            })
+            .await;
+            // => 0 .. 1 .. 2 .. 3
+            // with a pause of 2 seconds between each print
+        })
+    }
+}

src/stream/stream/mod.rs

@@ -123,11 +123,13 @@ cfg_unstable! {
     pub use flatten::Flatten;
     pub use flat_map::FlatMap;
     pub use timeout::{TimeoutError, Timeout};
+    pub use throttle::Throttle;
 
     mod merge;
     mod flatten;
     mod flat_map;
     mod timeout;
+    mod throttle;
 }
 
 extension_trait! {
@@ -313,6 +315,56 @@ extension_trait! {
             TakeWhile::new(self, predicate)
         }
 
+        #[doc = r#"
+            Limit the amount of items yielded per timeslice in a stream.
+
+            This stream does not drop any items, but will only limit the rate at which items pass through.
+            # 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();
+
+            // emit value every 5 milliseconds
+            let s = stream::interval(Duration::from_millis(5))
+                .enumerate()
+                .take(3);
+
+            // throttle for 10 milliseconds
+            let mut s = s.throttle(Duration::from_millis(10));
+
+            assert_eq!(s.next().await, Some((0, ())));
+            let duration_ms = start.elapsed().as_millis();
+            assert!(duration_ms >= 5);
+
+            assert_eq!(s.next().await, Some((1, ())));
+            let duration_ms = start.elapsed().as_millis();
+            assert!(duration_ms >= 15);
+
+            assert_eq!(s.next().await, Some((2, ())));
+            let duration_ms = start.elapsed().as_millis();
+            assert!(duration_ms >= 25);
+
+            assert_eq!(s.next().await, None);
+            let duration_ms = start.elapsed().as_millis();
+            assert!(duration_ms >= 35);
+            #
+            # }) }
+            ```
+        "#]
+        #[cfg(all(feature = "default", feature = "unstable"))]
+        #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+        fn throttle(self, d: Duration) -> Throttle<Self>
+        where
+            Self: Sized,
+        {
+            Throttle::new(self, d)
+        }
+
         #[doc = r#"
             Creates a stream that yields each `step`th element.
 

src/stream/stream/throttle.rs

@@ -0,0 +1,70 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::time::{Duration, Instant};
+
+use futures_timer::Delay;
+use pin_project_lite::pin_project;
+
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+pin_project! {
+    /// A stream that only yields one element once every `duration`.
+    ///
+    /// This `struct` is created by the [`throttle`] method on [`Stream`]. See its
+    /// documentation for more.
+    ///
+    /// [`throttle`]: trait.Stream.html#method.throttle
+    /// [`Stream`]: trait.Stream.html
+    #[doc(hidden)]
+    #[allow(missing_debug_implementations)]
+    pub struct Throttle<S> {
+        #[pin]
+        stream: S,
+        duration: Duration,
+        #[pin]
+        blocked: bool,
+        #[pin]
+        delay: Delay,
+    }
+}
+
+impl<S: Stream> Throttle<S> {
+    pub(super) fn new(stream: S, duration: Duration) -> Self {
+        Throttle {
+            stream,
+            duration,
+            blocked: false,
+            delay: Delay::new(Duration::default()),
+        }
+    }
+}
+
+impl<S: Stream> Stream for Throttle<S> {
+    type Item = S::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<S::Item>> {
+        let mut this = self.project();
+        if *this.blocked {
+            let d = this.delay.as_mut();
+            if d.poll(cx).is_ready() {
+                *this.blocked = false;
+            } else {
+                return Poll::Pending;
+            }
+        }
+
+        match this.stream.poll_next(cx) {
+            Poll::Pending => {
+                cx.waker().wake_by_ref(); // Continue driving even though emitting Pending
+                Poll::Pending
+            }
+            Poll::Ready(None) => Poll::Ready(None),
+            Poll::Ready(Some(v)) => {
+                *this.blocked = true;
+                this.delay.reset(Instant::now() + *this.duration);
+                Poll::Ready(Some(v))
+            }
+        }
+    }
+}