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sharplet/Always.swift

Last active Dec 2, 2020
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A Combine publisher that repeatedly emits the same value as long as there is demand
// Copyright (c) 2019–20 Adam Sharp and thoughtbot, inc.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
import Combine
/// A publisher that repeatedly sends the same value as long as there
/// is demand.
public struct Always<Output>: Publisher {
public typealias Failure = Never
public let output: Output
public init(_ output: Output) {
self.output = output
}
public func receive<S: Subscriber>(subscriber: S) where S.Input == Output, S.Failure == Failure {
let subscription = Subscription(output: output, subscriber: subscriber)
subscriber.receive(subscription: subscription)
}
}
private extension Always {
final class Subscription<S: Subscriber> where S.Input == Output, S.Failure == Failure {
private let output: Output
private var subscriber: S?
init(output: Output, subscriber: S) {
self.output = output
self.subscriber = subscriber
}
}
}
extension Always.Subscription: Cancellable {
func cancel() {
subscriber = nil
}
}
extension Always.Subscription: Subscription {
func request(_ demand: Subscribers.Demand) {
var demand = demand
while let subscriber = subscriber, demand > 0 {
demand -= 1
demand += subscriber.receive(output)
}
}
}
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