1use core::fmt;
2use core::pin::Pin;
3use futures_core::future::Future;
4use futures_core::ready;
5use futures_core::stream::{FusedStream, Stream};
6use futures_core::task::{Context, Poll};
7#[cfg(feature = "sink")]
8use futures_sink::Sink;
9use pin_project_lite::pin_project;
10
11pin_project! {
12 /// Stream for the [`then`](super::StreamExt::then) method.
13 #[must_use = "streams do nothing unless polled"]
14 pub struct Then<St, Fut, F> {
15 #[pin]
16 stream: St,
17 #[pin]
18 future: Option<Fut>,
19 f: F,
20 }
21}
22
23impl<St, Fut, F> fmt::Debug for Then<St, Fut, F>
24where
25 St: fmt::Debug,
26 Fut: fmt::Debug,
27{
28 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
29 f.debug_struct("Then").field("stream", &self.stream).field("future", &self.future).finish()
30 }
31}
32
33impl<St, Fut, F> Then<St, Fut, F>
34where
35 St: Stream,
36 F: FnMut(St::Item) -> Fut,
37{
38 pub(super) fn new(stream: St, f: F) -> Self {
39 Self { stream, future: None, f }
40 }
41
42 delegate_access_inner!(stream, St, ());
43}
44
45impl<St, Fut, F> FusedStream for Then<St, Fut, F>
46where
47 St: FusedStream,
48 F: FnMut(St::Item) -> Fut,
49 Fut: Future,
50{
51 fn is_terminated(&self) -> bool {
52 self.future.is_none() && self.stream.is_terminated()
53 }
54}
55
56impl<St, Fut, F> Stream for Then<St, Fut, F>
57where
58 St: Stream,
59 F: FnMut(St::Item) -> Fut,
60 Fut: Future,
61{
62 type Item = Fut::Output;
63
64 fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
65 let mut this = self.project();
66
67 Poll::Ready(loop {
68 if let Some(fut) = this.future.as_mut().as_pin_mut() {
69 let item = ready!(fut.poll(cx));
70 this.future.set(None);
71 break Some(item);
72 } else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
73 this.future.set(Some((this.f)(item)));
74 } else {
75 break None;
76 }
77 })
78 }
79
80 fn size_hint(&self) -> (usize, Option<usize>) {
81 let future_len = usize::from(self.future.is_some());
82 let (lower, upper) = self.stream.size_hint();
83 let lower = lower.saturating_add(future_len);
84 let upper = match upper {
85 Some(x) => x.checked_add(future_len),
86 None => None,
87 };
88 (lower, upper)
89 }
90}
91
92// Forwarding impl of Sink from the underlying stream
93#[cfg(feature = "sink")]
94impl<S, Fut, F, Item> Sink<Item> for Then<S, Fut, F>
95where
96 S: Sink<Item>,
97{
98 type Error = S::Error;
99
100 delegate_sink!(stream, Item);
101}
102