UIScheduler.swift

import Combine

// `UIScheduler` eagerly executes actions on the current queue if
// it is the main thread, avoiding unnecessary async behaviour.
// If the current queue is not the main thread, then the action is
// then scheduled on the main queue asynchonously.
//
// `DispatchQueue` schedulers execute all actions asynchronously.
//
struct UIScheduler: Scheduler {
  static let shared = UIScheduler()
  
  private let queue: DispatchQueue

  private init(queue: DispatchQueue = .main) {
    queue.setSpecific(key: queueKey, value: queueIdentifier)
    self.queue = queue
  }
    
  var now: DispatchQueue.SchedulerTimeType { queue.now }

  var minimumTolerance: DispatchQueue.SchedulerTimeType.Stride { queue.minimumTolerance }

  func schedule(options: DispatchQueue.SchedulerOptions?, _ action: @escaping () -> Void) {
    if queue.getSpecific(key: queueKey) == queueIdentifier {
      action()
    } else {
      queue.schedule(action)
    }
  }

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

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

  private let queueKey = DispatchSpecificKey<UInt8>()
  private let queueIdentifier: UInt8 = 0
}