Implements 2 finger swipe using scroll event tracking

SwipeableViewController.swift

class SwipeableViewController: NSViewController {
  
  private var isHandlingScrollEvent = false
  private var swipeLeftExecuted = false
  private var swipeRightExecuted = false
  private var scrollDeltaX: CGFloat = 0
  
  override func wantsScrollEventsForSwipeTracking(on axis: NSEvent.GestureAxis) -> Bool {
    return axis == .horizontal
  }
  
  /// Informs the receiver that the user has begun a swipe gesture.
  ///
  /// NOTE!
  /// Works with 3 finger swipe gesture (strange, I know).
  ///
  /// Works only when System preferences -> Trackpad -> More Gestures, preferences are set to the following:
  /// 1. Swipe between pages:
  ///     - Swipe with two or three fingers, or
  ///     - Swipe with three fingers
  /// 2. Swipe between full-screen apps:
  ///     - Swipe left or right with four fingers
  override func swipe(with event: NSEvent) {
    swipe(deltaX: event.deltaX)
  }
  
  /// This implements 2 finger swipe using scroll event tracking.
  override func scrollWheel(with event: NSEvent) {
    if !NSEvent.isSwipeTrackingFromScrollEventsEnabled {
      super.scrollWheel(with: event)
      return
    }
    
    switch event.phase {
    case .began:
      isHandlingScrollEvent = true
      swipeLeftExecuted = false
      swipeRightExecuted = false
      scrollDeltaX = 0
    case .changed:
      guard isHandlingScrollEvent else { break }
      
      let directionChanged = scrollDeltaX.sign != event.scrollingDeltaX.sign
      
      guard !directionChanged else {
        scrollDeltaX = event.scrollingDeltaX
        break
      }
      
      scrollDeltaX += event.scrollingDeltaX
      
      // throttle
      guard abs(scrollDeltaX) > 50 else { break }
      
      let flippedScrollDelta = scrollDeltaX * -1
      let swipedLeft = flippedScrollDelta > 0
      
      switch (swipedLeft, swipeLeftExecuted, swipeRightExecuted) {
      case (true, false, _): // swiped left
        swipeLeftExecuted = true
        swipeRightExecuted = false // allow swipe back (right)
      case (false, _, false): // swiped right
        swipeLeftExecuted = false // allow swipe back (left)
        swipeRightExecuted = true
      default:
        super.scrollWheel(with: event)
        return
      }
      swipe(deltaX: flippedScrollDelta)
      return
    case .ended,
         .cancelled,
         .mayBegin:
      isHandlingScrollEvent = false
    default:
      break
    }
    
    super.scrollWheel(with: event)
  }
  
  private func swipe(deltaX: CGFloat) {
    guard deltaX != 0 else { return }
    
    let swipedLeft = deltaX > 0
    
    if swipedLeft {
      // swipe left action
    } else {
      // swipe right action
    }
  }
}