1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
//! An unbounded set of streams

use core::fmt::{self, Debug};
use core::iter::FromIterator;
use core::pin::Pin;

use futures_core::{Poll, Stream, FusedStream};
use futures_core::task::Context;

use crate::stream::{StreamExt, StreamFuture, FuturesUnordered};

/// An unbounded set of streams
///
/// This "combinator" provides the ability to maintain a set of streams
/// and drive them all to completion.
///
/// Streams are pushed into this set and their realized values are
/// yielded as they become ready. Streams will only be polled when they
/// generate notifications. This allows to coordinate a large number of streams.
///
/// Note that you can create a ready-made `SelectAll` via the
/// `select_all` function in the `stream` module, or you can start with an
/// empty set with the `SelectAll::new` constructor.
#[must_use = "streams do nothing unless polled"]
pub struct SelectAll<St> {
    inner: FuturesUnordered<StreamFuture<St>>,
}

impl<St: Debug> Debug for SelectAll<St> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(fmt, "SelectAll {{ ... }}")
    }
}

impl<St: Stream + Unpin> SelectAll<St> {
    /// Constructs a new, empty `SelectAll`
    ///
    /// The returned `SelectAll` does not contain any streams and, in this
    /// state, `SelectAll::poll` will return `Poll::Ready(None)`.
    pub fn new() -> SelectAll<St> {
        SelectAll { inner: FuturesUnordered::new() }
    }

    /// Returns the number of streams contained in the set.
    ///
    /// This represents the total number of in-flight streams.
    pub fn len(&self) -> usize {
        self.inner.len()
    }

    /// Returns `true` if the set contains no streams
    pub fn is_empty(&self) -> bool {
        self.inner.is_empty()
    }

    /// Push a stream into the set.
    ///
    /// This function submits the given stream to the set for managing. This
    /// function will not call `poll` on the submitted stream. The caller must
    /// ensure that `SelectAll::poll` is called in order to receive task
    /// notifications.
    pub fn push(&mut self, stream: St) {
        self.inner.push(stream.into_future());
    }
}

impl<St: Stream + Unpin> Default for SelectAll<St> {
    fn default() -> SelectAll<St> {
        SelectAll::new()
    }
}

impl<St: Stream + Unpin> Stream for SelectAll<St> {
    type Item = St::Item;

    fn poll_next(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Self::Item>> {
        match self.inner.poll_next_unpin(cx) {
            Poll::Pending => Poll::Pending,
            Poll::Ready(Some((Some(item), remaining))) => {
                self.push(remaining);
                Poll::Ready(Some(item))
            }
            Poll::Ready(Some((None, _))) => {
                // FuturesUnordered thinks it isn't terminated
                // because it yielded a Some. Here we poll it
                // so it can realize it is terminated.
                let _ = self.inner.poll_next_unpin(cx);
                Poll::Ready(None)
            }
            Poll::Ready(_) => Poll::Ready(None),
        }
    }
}

impl<St: Stream + Unpin> FusedStream for SelectAll<St> {
    fn is_terminated(&self) -> bool {
        self.inner.is_terminated()
    }
}

/// Convert a list of streams into a `Stream` of results from the streams.
///
/// This essentially takes a list of streams (e.g. a vector, an iterator, etc.)
/// and bundles them together into a single stream.
/// The stream will yield items as they become available on the underlying
/// streams internally, in the order they become available.
///
/// Note that the returned set can also be used to dynamically push more
/// futures into the set as they become available.
pub fn select_all<I>(streams: I) -> SelectAll<I::Item>
    where I: IntoIterator,
          I::Item: Stream + Unpin
{
    let mut set = SelectAll::new();

    for stream in streams {
        set.push(stream);
    }

    set
}

impl<St: Stream + Unpin> FromIterator<St> for SelectAll<St> {
    fn from_iter<T: IntoIterator<Item = St>>(iter: T) -> Self {
        select_all(iter)
    }
}