futures_rx/stream_ext/
delay_every.rs

use std::{
    collections::VecDeque,
    future::Future,
    pin::Pin,
    task::{Context, Poll},
};

use futures::{
    stream::{Fuse, FusedStream},
    Stream, StreamExt,
};
use pin_project_lite::pin_project;

pin_project! {
    /// Stream for the [`delay`](RxStreamExt::delay) method.
    #[must_use = "streams do nothing unless polled"]
    pub struct DelayEvery<S: Stream, Fut, F> {
        #[pin]
        stream: Fuse<S>,
        f: F,
        delayed_events: VecDeque<S::Item>,
        delayed_event: Option<S::Item>,
        #[pin]
        current_interval: Option<Fut>,
        max_buffer_size: Option<usize>,
    }
}

impl<S: Stream, Fut, F> DelayEvery<S, Fut, F> {
    pub(crate) fn new(stream: S, f: F, max_buffer_size: Option<usize>) -> Self {
        Self {
            stream: stream.fuse(),
            f,
            delayed_events: if let Some(max_buffer_size) = &max_buffer_size {
                VecDeque::with_capacity(*max_buffer_size)
            } else {
                VecDeque::new()
            },
            delayed_event: None,
            current_interval: None,
            max_buffer_size,
        }
    }
}

impl<S: Stream, Fut, F> FusedStream for DelayEvery<S, Fut, F>
where
    F: FnMut(&S::Item) -> Fut,
    Fut: Future,
{
    fn is_terminated(&self) -> bool {
        self.stream.is_terminated()
    }
}

impl<S: Stream, Fut, F> Stream for DelayEvery<S, Fut, F>
where
    F: FnMut(&S::Item) -> Fut,
    Fut: Future,
{
    type Item = S::Item;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        let mut this = self.project();
        let mut did_push = false;

        if let Poll::Ready(Some(event)) = this.stream.poll_next(cx) {
            did_push = true;

            if let Some(max_buffer_size) = this.max_buffer_size {
                while this.delayed_events.len() >= *max_buffer_size {
                    this.delayed_events.pop_front();
                }
            }

            this.delayed_events.push_back(event);
        };

        if this.current_interval.is_none() {
            if this.delayed_events.is_empty() {
                return Poll::Ready(None);
            }

            if let Some(event) = this.delayed_events.pop_front() {
                this.current_interval.set(Some((this.f)(&event)));
                *this.delayed_event = Some(event);
            }
        }

        if let Some(interval) = this.current_interval.as_mut().as_pin_mut() {
            match interval.poll(cx) {
                Poll::Ready(_) => {
                    this.current_interval.set(None);

                    Poll::Ready(this.delayed_event.take())
                }
                Poll::Pending => {
                    cx.waker().wake_by_ref();

                    Poll::Pending
                }
            }
        } else {
            if !did_push {
                cx.waker().wake_by_ref();
            }

            Poll::Pending
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let (lower, upper) = self.stream.size_hint();
        // we know for sure that the final event (if any) will always emit,
        // any other events depend on a time interval and must be discarded.
        let lower = if lower > 0 { 1 } else { 0 };

        (lower, upper)
    }
}

#[cfg(test)]
mod test {
    use std::time::Instant;

    use futures::{executor::block_on, stream, StreamExt};
    use futures_time::{future::IntoFuture, time::Duration};

    use crate::RxExt;

    #[test]
    fn smoke() {
        block_on(async {
            let now = Instant::now();
            let all_events = stream::iter(0..=3)
                .delay_every(|_| Duration::from_millis(50).into_future(), None)
                .collect::<Vec<_>>()
                .await;

            assert_eq!(all_events, [0, 1, 2, 3]);
            assert!(now.elapsed().as_millis() >= 50 * 4);
        });
    }
}
futures_rx/stream_ext/delay_every.rs

futures_rx/stream_ext/
delay_every.rs
