UIScheduler.swift

/// MIT License
///
/// Copyright (c) 2020 Point-Free, 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
import Dispatch

/// A scheduler that executes its work on the main queue as soon as possible.
///
/// This scheduler is inspired by the
/// [equivalent](https://github.com/ReactiveCocoa/ReactiveSwift/blob/58d92aa01081301549c48a4049e215210f650d07/Sources/Scheduler.swift#L92)
/// scheduler in the [ReactiveSwift](https://github.com/ReactiveCocoa/ReactiveSwift) project.
///
/// If `UIScheduler.shared.schedule` is invoked from the main thread then the unit of work will be
/// performed immediately. This is in contrast to `DispatchQueue.main.schedule`, which will incur
/// a thread hop before executing since it uses `DispatchQueue.main.async` under the hood.
///
/// This scheduler can be useful for situations where you need work executed as quickly as
/// possible on the main thread, and for which a thread hop would be problematic, such as when
/// performing animations.
@available(iOS 13, macOS 10.15, tvOS 13, watchOS 6, *)
public struct UIScheduler: Scheduler {
    public typealias SchedulerOptions = Never
    public typealias SchedulerTimeType = DispatchQueue.SchedulerTimeType

    /// The shared instance of the UI scheduler.
    ///
    /// You cannot create instances of the UI scheduler yourself. Use only the shared instance.
    public static let shared = Self()

    public var now: SchedulerTimeType { DispatchQueue.main.now }
    public var minimumTolerance: SchedulerTimeType.Stride { DispatchQueue.main.minimumTolerance }

    public func schedule(options _: SchedulerOptions? = nil, _ action: @escaping () -> Void) {
        if DispatchQueue.getSpecific(key: key) == value {
            action()
        } else {
            DispatchQueue.main.schedule(action)
        }
    }

    public func schedule(
        after date: SchedulerTimeType,
        tolerance: SchedulerTimeType.Stride,
        options _: SchedulerOptions? = nil,
        _ action: @escaping () -> Void
    ) {
        DispatchQueue.main.schedule(after: date, tolerance: tolerance, options: nil, action)
    }

    public func schedule(
        after date: SchedulerTimeType,
        interval: SchedulerTimeType.Stride,
        tolerance: SchedulerTimeType.Stride,
        options _: SchedulerOptions? = nil,
        _ action: @escaping () -> Void
    ) -> Cancellable {
        DispatchQueue.main.schedule(
            after: date, interval: interval, tolerance: tolerance, options: nil, action
        )
    }

    private init() { _ = setSpecific }
}

private let key = DispatchSpecificKey<UInt8>()
private let value: UInt8 = 0
private var setSpecific: () = { DispatchQueue.main.setSpecific(key: key, value: value) }()