extension UIViewController {
 public var yy_routerAnimation : YYTransition {
  set {
   objc_setAssociatedObject(self, &YYTransitionKey.kRouterAnimationKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
  }
  get {
   guard let r = objc_getAssociatedObject(self, &YYTransitionKey.kRouterAnimationKey) as? YYTransition else {
    return YYTransition()
   }
   return r
  }
 }
}

public class YYTransition: NSObject

extension YYTransition: UIViewControllerAnimatedTransitioning, UIViewControllerTransitioningDelegate, UINavigationControllerDelegate

  return self
 }
 
 public func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
  return 转场动画所需时间
 }

 public func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
  switch self.yy_ransitionAnimationType {
  case .circle:
   break
  case .move:
   break
  case .middle:
   break
  case .tier:
   break
  }
 }

extension YYTransition {
 // 是push还是pop
 public var yy_isBack: Bool {}
 // 动画类型
 var yy_ransitionAnimationType: YYTransitionAnimationType {}
 // 源view名字 
 var yy_fromViewPath: String? { }
 // 目标view名字 
 var yy_toViewPath: String? { }
 // 句柄
 var yy_transitionContext: UIViewControllerContextTransitioning {}
}

extension YYTransition: CAAnimationDelegate {
 public func animationDidStop(_ anim: CAAnimation, finished flag: Bool) {
  yy_transitionContext.completeTransition(!yy_transitionContext.transitionWasCancelled)
  yy_transitionContext.viewController(forKey: .from)?.view.layer.mask = nil
  yy_transitionContext.viewController(forKey: .to)?.view.layer.mask = nil
 }
}

extension YYTransition {
 func maskAnimation(targetVC: UIViewController, startPath: UIBezierPath, endPath: UIBezierPath, context: UIViewControllerContextTransitioning) {
}

YYTransition+Circle
YYTransition+Move
YYTransition+Tier
YYTransition+Middle

UIViewControllerContextTransitioning 调用
public func viewController(forKey key: UITransitionContextViewControllerKey) -> UIViewController?
UIViewController调用
open func value(forKeyPath keyPath: String) -> Any?
* When requesting a snapshot, 'afterUpdates' defines whether the snapshot is representative of what's currently on screen or if you wish to include any recent changes before taking the snapshot. 
open func snapshotView(afterScreenUpdates afterUpdates: Bool) -> UIView?
open func convert(_ rect: CGRect, from view: UIView?) -> CGRect
open func insertSubview(_ view: UIView, belowSubview siblingSubview: UIView)
// This must be called whenever a transition completes (or is cancelled.)
 // Typically this is called by the object conforming to the
 // UIViewControllerAnimatedTransitioning protocol that was vended by the transitioning
 // delegate. For purely interactive transitions it should be called by the
 // interaction controller. This method effectively updates internal view
 // controller state at the end of the transition.
public func completeTransition(_ didComplete: Bool)