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// Copyright 2020 CoD Technologies Corp. // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // http://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. //! Select over a list of futures. //! //! ## Usage //! //! ``` //! use async_select_all::SelectAll; //! use futures::executor::block_on; //! //! async fn inc(i: i32) -> i32 { //! i + 1 //! } //! //! fn main() { //! let futures = vec![inc(10), inc(5)]; //! let mut select_all = SelectAll::from(futures); //! let vec = block_on(async { //! let mut vec = Vec::with_capacity(select_all.len()); //! while !select_all.is_empty() { //! let val = select_all.select().await; //! vec.push(val) //! } //! vec.sort(); //! vec //! }); //! assert_eq!(vec, vec![6, 11]); //! } //! ``` use pin_project_lite::pin_project; use std::future::Future; use std::pin::Pin; use std::task::{Context, Poll}; pin_project! { struct SelectFuture<'a, F: Future> { #[pin] futures: &'a mut Vec<F> , } } impl<'a, F: Future> SelectFuture<'a, F> { #[inline] fn new(futures: &'a mut Vec<F>) -> Self { Self { futures } } } impl<'a, F: Future> Future for SelectFuture<'a, F> { type Output = F::Output; #[inline] fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { let mut ready = None; let mut this = self.project(); for (i, f) in this.futures.iter_mut().enumerate() { let p = unsafe { Pin::new_unchecked(f) }; if let Poll::Ready(output) = p.poll(cx) { ready = Some((i, Poll::Ready(output))); break; } } if let Some((id, r)) = ready { this.futures.swap_remove(id); return r; } Poll::Pending } } /// An unbounded set of futures. pub struct SelectAll<F: Future> { futures: Vec<F>, } impl<I> From<I> for SelectAll<I::Item> where I: IntoIterator, I::Item: Future, { #[inline] fn from(iter: I) -> Self { Self { futures: iter.into_iter().collect(), } } } impl<F: Future> SelectAll<F> { /// Constructs a new, empty `SelectAll`. /// The returned `SelectAll` does not contain any futures. #[allow(clippy::new_without_default)] #[inline] pub fn new() -> Self { SelectAll { futures: Vec::new(), } } /// Returns the number of futures contained in the set. /// This represents the total number of in-flight futures. #[inline] pub fn len(&self) -> usize { self.futures.len() } /// Returns true if the set contains no futures. #[inline] pub fn is_empty(&self) -> bool { self.futures.is_empty() } /// Push a future into the set. /// This function submits the given future to the set for managing. /// This function will not call poll on the submitted future. /// The caller must ensure that `SelectAll::select` is called in order to receive task notifications. #[inline] pub fn push(&mut self, future: F) { self.futures.push(future); } /// Select over a list of futures. /// /// Upon completion the item resolved will be returned. /// /// There are no guarantees provided on the order of the list with the remaining futures. /// They might be swapped around, reversed, or completely random. /// /// # Panics /// This function will panic if the `SelectAll` specified contains no items. #[inline] pub async fn select(&mut self) -> F::Output { assert!(!self.futures.is_empty()); SelectFuture::new(&mut self.futures).await } }