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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift Async Algorithms open source project
//
// Copyright (c) 2022 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
//
//===----------------------------------------------------------------------===//
/// A channel for sending elements from one task to another with back pressure.
///
/// The `AsyncChannel` class is intended to be used as a communication type between tasks,
/// particularly when one task produces values and another task consumes those values. The back
/// pressure applied by `send(_:)` via the suspension/resume ensures that
/// the production of values does not exceed the consumption of values from iteration. This method
/// suspends after enqueuing the event and is resumed when the next call to `next()`
/// on the `Iterator` is made, or when `finish()` is called from another Task.
/// As `finish()` induces a terminal state, there is no more need for a back pressure management.
/// This function does not suspend and will finish all the pending iterations.
public final class AsyncChannel<Element: Sendable>: AsyncSequence, Sendable {
public typealias Element = Element
public typealias AsyncIterator = Iterator
let storage: ChannelStorage<Element, Never>
public init() {
self.storage = ChannelStorage()
}
/// Sends an element to an awaiting iteration. This function will resume when the next call to `next()` is made
/// or when a call to `finish()` is made from another task.
/// If the channel is already finished then this returns immediately.
/// If the task is cancelled, this function will resume without sending the element.
/// Other sending operations from other tasks will remain active.
public func send(_ element: Element) async {
await self.storage.send(element: element)
}
/// Immediately resumes all the suspended operations.
/// All subsequent calls to `next(_:)` will resume immediately.
public func finish() {
self.storage.finish()
}
public func makeAsyncIterator() -> Iterator {
Iterator(storage: self.storage)
}
public struct Iterator: AsyncIteratorProtocol {
let storage: ChannelStorage<Element, Never>
public mutating func next() async -> Element? {
// Although the storage can throw, its usage in the context of an `AsyncChannel` guarantees it cannot.
// There is no public way of sending a failure to it.
try! await self.storage.next()
}
}
}
@available(*, unavailable)
extension AsyncChannel.Iterator: Sendable { }
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