Merge pull request #349 from k-nasa/add_future_delay

Add future::delay

src/future/future.rs

@@ -1,3 +1,11 @@
+cfg_unstable! {
+    mod delay;
+
+    use std::time::Duration;
+
+    use delay::DelayFuture;
+}
+
 extension_trait! {
     use std::pin::Pin;
     use std::ops::{Deref, DerefMut};
@@ -99,6 +107,28 @@ extension_trait! {
     }
 
     pub trait FutureExt: std::future::Future {
+        /// Returns a Future that delays execution for a specified time.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// # async_std::task::block_on(async {
+        /// use async_std::prelude::*;
+        /// use async_std::future;
+        /// use std::time::Duration;
+        ///
+        /// let a = future::ready(1).delay(Duration::from_millis(2000));
+        /// dbg!(a.await);
+        /// # })
+        /// ```
+        #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+        #[cfg(any(feature = "unstable", feature = "docs"))]
+        fn delay(self, dur: Duration) -> impl Future<Output = Self::Output> [DelayFuture<Self>]
+        where
+            Self: Future + Sized
+        {
+            DelayFuture::new(self, dur)
+        }
     }
 
     impl<F: Future + Unpin + ?Sized> Future for Box<F> {

src/future/future/delay.rs

@@ -0,0 +1,43 @@
+use std::pin::Pin;
+use std::time::Duration;
+
+use futures_timer::Delay;
+use pin_project_lite::pin_project;
+
+use crate::future::Future;
+use crate::task::{Context, Poll};
+
+pin_project! {
+    #[doc(hidden)]
+    #[derive(Debug)]
+    pub struct DelayFuture<F> {
+        #[pin]
+        future: F,
+        #[pin]
+        delay: Delay,
+    }
+}
+
+impl<F> DelayFuture<F> {
+    pub fn new(future: F, dur: Duration) -> DelayFuture<F> {
+        let delay = Delay::new(dur);
+
+        DelayFuture { future, delay }
+    }
+}
+
+impl<F: Future> Future for DelayFuture<F> {
+    type Output = F::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.project();
+
+        match this.delay.poll(cx) {
+            Poll::Pending => Poll::Pending,
+            Poll::Ready(_) => match this.future.poll(cx) {
+                Poll::Ready(v) => Poll::Ready(v),
+                Poll::Pending => Poll::Pending,
+            },
+        }
+    }
+}