A repeating GCD timer that can run on a background queue

RepeatingTimer.swift

import Foundation

/// RepeatingTimer mimics the API of DispatchSourceTimer but in a way that prevents
/// crashes that occur from calling resume multiple times on a timer that is
/// already resumed (noted by https://github.com/SiftScience/sift-ios/issues/52
class RepeatingTimer {
  // MARK: State
  private enum State {
    case suspended
    case resumed
  }
  
  // MARK: Properties
  let timeInterval: TimeInterval
  var eventHandler: (() -> Void)?

  private var state: State = .suspended
  private lazy var timer: DispatchSourceTimer = {
    let t = DispatchSource.makeTimerSource()
    t.schedule(
      deadline: .now() + self.timeInterval, 
      repeating: self.timeInterval
    )
    t.setEventHandler(
      handler: { [weak self] in self?.eventHandler?() }
    )
    return t
  }()

  // MARK: Initializer
  init(timeInterval: TimeInterval, eventHandler: (() -> Void)?) {
    self.timeInterval = timeInterval
    self.eventHandler = eventHandler
  }

  deinit {
    self.timer.setEventHandler {}
    self.timer.cancel()
    /*
     If the timer is suspended, calling cancel without resuming
     triggers a crash. This is documented here https://forums.developer.apple.com/thread/15902
     */
    self.resume()
    self.eventHandler = nil
  }

  func resume() {
    guard self.state != .resumed { return }
    self.state = .resumed
    self.timer.resume()
  }

  func suspend() {
    guard self.state != .suspended { return }
    self.state = .suspended
    self.timer.suspend()
  }
}