futures_util/future/future/
fuse.rs

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1use core::pin::Pin;
2use futures_core::future::{FusedFuture, Future};
3use futures_core::task::{Context, Poll};
4use pin_project_lite::pin_project;
5
6pin_project! {
7    /// Future for the [`fuse`](super::FutureExt::fuse) method.
8    #[derive(Debug)]
9    #[must_use = "futures do nothing unless you `.await` or poll them"]
10    pub struct Fuse<Fut> {
11        #[pin]
12        inner: Option<Fut>,
13    }
14}
15
16impl<Fut> Fuse<Fut> {
17    pub(super) fn new(f: Fut) -> Self {
18        Self { inner: Some(f) }
19    }
20}
21
22impl<Fut: Future> Fuse<Fut> {
23    /// Creates a new `Fuse`-wrapped future which is already terminated.
24    ///
25    /// This can be useful in combination with looping and the `select!`
26    /// macro, which bypasses terminated futures.
27    ///
28    /// # Examples
29    ///
30    /// ```
31    /// # futures::executor::block_on(async {
32    /// use futures::channel::mpsc;
33    /// use futures::future::{Fuse, FusedFuture, FutureExt};
34    /// use futures::select;
35    /// use futures::stream::StreamExt;
36    /// use futures::pin_mut;
37    ///
38    /// let (sender, mut stream) = mpsc::unbounded();
39    ///
40    /// // Send a few messages into the stream
41    /// sender.unbounded_send(()).unwrap();
42    /// sender.unbounded_send(()).unwrap();
43    /// drop(sender);
44    ///
45    /// // Use `Fuse::terminated()` to create an already-terminated future
46    /// // which may be instantiated later.
47    /// let foo_printer = Fuse::terminated();
48    /// pin_mut!(foo_printer);
49    ///
50    /// loop {
51    ///     select! {
52    ///         _ = foo_printer => {},
53    ///         () = stream.select_next_some() => {
54    ///             if !foo_printer.is_terminated() {
55    ///                 println!("Foo is already being printed!");
56    ///             } else {
57    ///                 foo_printer.set(async {
58    ///                     // do some other async operations
59    ///                     println!("Printing foo from `foo_printer` future");
60    ///                 }.fuse());
61    ///             }
62    ///         },
63    ///         complete => break, // `foo_printer` is terminated and the stream is done
64    ///     }
65    /// }
66    /// # });
67    /// ```
68    pub fn terminated() -> Self {
69        Self { inner: None }
70    }
71}
72
73impl<Fut: Future> FusedFuture for Fuse<Fut> {
74    fn is_terminated(&self) -> bool {
75        self.inner.is_none()
76    }
77}
78
79impl<Fut: Future> Future for Fuse<Fut> {
80    type Output = Fut::Output;
81
82    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Fut::Output> {
83        match self.as_mut().project().inner.as_pin_mut() {
84            Some(fut) => fut.poll(cx).map(|output| {
85                self.project().inner.set(None);
86                output
87            }),
88            None => Poll::Pending,
89        }
90    }
91}
futures_util/future/future/fuse.rs

futures_util/future/future/
fuse.rs
