Swift Access Constraint from different View contro

Question

Hello I'm trying to access my constraint from my second view controller to my initial view controller, but it always give an error, Im accessing my constraint from a pageviewontroller class, to automatically adjust the height of my second page.

found nil while unwrapping optional value

 func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {

    guard let viewControllerIndex = orderedViewControllers.index(of: viewController) else {
        return nil
    }
    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    let vc = storyboard.instantiateViewController(withIdentifier: "MainNeedPage") as! NeedDetailsController;
    vc.bottomContainerHeight.constant = CGFloat(50)
    vc.containerHeight.constant = CGFloat(30)

    UIView.animate(withDuration: 0.5, animations: {
        vc.view.layoutIfNeeded()
    })
    let nextIndex = viewControllerIndex + 1
    let orderedViewControllersCount = orderedViewControllers.count

    guard orderedViewControllersCount != nextIndex else {

         return nil
    }
    guard orderedViewControllersCount > nextIndex else {
        return nil
    }
    return orderedViewControllers[nextIndex]
}

Sh_Khan Answer worked, but when I want to get back to the first page of the pageviewcontroller it doesn't get back to it's original size. Here's what i've tried

 func pageViewController(_ pageViewController: UIPageViewController, willTransitionTo pendingViewControllers: [UIViewController]) {

    let pageContentViewController = pageViewController.viewControllers![0]
    var page = orderedViewControllers.index(of: pageContentViewController)!

   if(page == 1) {     
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let vc = storyboard.instantiateViewController(withIdentifier: "MainNeedPage") as! NeedDetailsController

        NeedDetailsController.status.viewstat = "true"
        NeedDetailsController.status.bottomContainerHeightCon = CGFloat(50)
        NeedDetailsController.status.containerHeightCon = CGFloat(30)
        vc.view.layoutIfNeeded()

   } else if(page == 0) {
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let vc = storyboard.instantiateViewController(withIdentifier: "MainNeedPage") as! NeedDetailsController;

        NeedDetailsController.status.viewstat = "false"
        NeedDetailsController.status.bottomContainerHeightCon = CGFloat(50)
        NeedDetailsController.status.containerHeightCon = CGFloat(30)
        vc.view.layoutSubviews()
    }
}

and on my initial controller

override func viewDidLayoutSubviews() {
    print("statusx: \(NeedDetailsController.status.viewstat)")
    if(NeedDetailsController.status.viewstat == "true"){
        print("statusy: \(NeedDetailsController.status.viewstat)")
        NeedDetailsController.status.viewstat = "false"
        self.bottomContainerHeight.constant = 0
        self.containerHeight.constant = 0
        UIView.animate(withDuration: 1) {
            self.view.layoutIfNeeded()
        }

    } else if(NeedDetailsController.status.viewstat == "false") {
        print("statusz: \(NeedDetailsController.status.viewstat)")
        setupview()
        NeedDetailsController.status.viewstat = "old"
        self.bottomContainerHeight.constant = 50
        self.containerHeight.constant = 30
        UIView.animate(withDuration: 1) {
            self.view.layoutIfNeeded()
        }   
    }   
}

Answer 1

You can't access a property whose view controller is not loaded

 vc.bottomContainerHeight.constant = CGFloat(50)
 vc.containerHeight.constant = CGFloat(30)

try to add two variables to that class

  var bottomContainerHeightCon:CGFloat!

  var containerHeightCon:CGFloat!

set them like this

    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    let vc = storyboard.instantiateViewController(withIdentifier: "MainNeedPage") as! NeedDetailsController;
    vc.bottomContainerHeightCon = CGFloat(50)
    vc.containerHeightCon = CGFloat(30)

and in viewDidLayoutSubviews

apply the variables to the constraints

 override func viewDidLayoutSubviews
 {
    if(once){
         once = false
         self.bottomContainerHeight = bottomContainerHeightCon
         self.containerHeight = containerHeightCon
         self.view.layoutIfNeeded()

      }
  }