func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController?
{
  // 第一次回調(diào)索取背面的controller
  // 第二次回調(diào)索取正面的controller
}
func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController?
{
  // 第一次回調(diào)索取背面的controller
  // 第二次回調(diào)索取正面的controller
}

// 對(duì)輸入的controller進(jìn)行反向截圖
func grabViewController(viewController: DUAPageViewController) -> Void {
  self.index = viewController.index
  self.chapterBelong = viewController.chapterBelong
  let rect = viewController.view.bounds
  UIGraphicsBeginImageContextWithOptions(rect.size, true, 0.0)
  let context = UIGraphicsGetCurrentContext()
  let transform = CGAffineTransform(a: -1.0, b: 0.0, c: 0.0, d: 1.0, tx: rect.size.width, ty: 0.0)
  context?.concatenate(transform)
  viewController.view.layer.render(in: context!)
  self.backImage = UIGraphicsGetImageFromCurrentImageContext()
  UIGraphicsEndImageContext()
 }

@objc func handleTapGes(gesture: UITapGestureRecognizer) -> Void {
  let hitPoint = gesture.location(in: gesture.view)
  let curController = self.childViewControllers.first!
  
  if hitPoint.x < gesture.view!.frame.size.width/3 {
//   滑向上一個(gè)controller
   let lastController = self.delegate?.translationController(translationController: self, controllerBefore: curController)
   if lastController != nil {
    self.delegate?.translationController(translationController: self, willTransitionTo: lastController!)
    self.setViewController(viewController: lastController!, direction: .right, animated: allowAnimating, completionHandler: {(complete) in
     self.delegate?.translationController(translationController: self, didFinishAnimating: complete, previousController: curController, transitionCompleted: complete)
    })
   }
   
  }
  if hitPoint.x > gesture.view!.frame.size.width*2/3 {
//   滑向下一個(gè)controller
   let nextController: UIViewController? = self.delegate?.translationController(translationController: self, controllerAfter: self.childViewControllers.first!)
   if nextController != nil {
    self.delegate?.translationController(translationController: self, willTransitionTo: nextController!)
    self.setViewController(viewController: nextController!, direction: .left, animated: allowAnimating, completionHandler: {(complete) in

     self.delegate?.translationController(translationController: self, didFinishAnimating: complete, previousController: curController, transitionCompleted: complete)
    })
   }
   
  }
  
 }
// 該方法模仿UIpageviewcontroller，切換到某一個(gè)controller
func setViewController(viewController: UIViewController, direction: translationControllerNavigationDirection, animated: Bool, completionHandler: ((Bool) -> Void)?) -> Void {
  if animated == false {
   // 直接添加child controller ，略
  }else {
   let oldController = self.childViewControllers.first
   self.addController(controller: viewController)
   
   var newVCEndTransform: CGAffineTransform
   var oldVCEndTransform: CGAffineTransform
   viewController.view.transform = .identity
   if direction == .left {
    viewController.view.transform = CGAffineTransform(translationX: screenWidth, y: 0)
    newVCEndTransform = .identity
    oldController?.view.transform = .identity
    oldVCEndTransform = CGAffineTransform(translationX: -screenWidth, y: 0)
   }else {
    viewController.view.transform = CGAffineTransform(translationX: -screenWidth, y: 0)
    newVCEndTransform = .identity
    oldController?.view.transform = .identity
    oldVCEndTransform = CGAffineTransform(translationX: screenWidth, y: 0)
   }
   
   UIView.animate(withDuration: animationDuration, animations: {
    oldController?.view.transform = oldVCEndTransform
    viewController.view.transform = newVCEndTransform
   }, completion: { (complete) in
    if complete {
     self.removeController(controller: oldController!)
    }
    if completionHandler != nil {
     completionHandler!(complete)
    }
    
   })
  }
 }