Swift - RootPageViewController, add buttons from different ViewControllers - ios

Question

I've got my RootPageViewController set up and all I need is to add UIButtons. I've already added the button and its function from the UIViewController which is to be displayed first.
How am I able to also add UIButtons from f.e. the third UIViewController and assign them a function?
Just like with the button from the secondViewController which is displayed first: secondViewController.buttonAdd.action = #selector(addData(_:))
If I solely use let's say firstViewController.buttonBack.action = #selector(backData(_:)) - I'll get an error saying:
Thread 1: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value
I assume thats because, there's no if let firstViewController ...
But let's say I'd add:
if let firstViewController = viewControllerList.first as? TimelineViewController {
self.setViewControllers([firstViewController], direction: .forward, animated: true, completion: nil)
firstViewController.buttonBack.action = #selector(backData(_:))
}
In this case the order which view will be shown initially and which next etc. will be mixed up.
I'm open for suggestion and better approaches, I just want this to work,
RootPageViewController:
import UIKit
class RootPageViewController: UIPageViewController, UIPageViewControllerDataSource {
lazy var viewControllerList:[UIViewController] = {
let sb = UIStoryboard(name: "Main", bundle: nil)
let vc1 = sb.instantiateViewController(withIdentifier: "timelineView")
let vc2 = sb.instantiateViewController(withIdentifier: "mainView")
let vc3 = sb.instantiateViewController(withIdentifier: "addView")
return [vc1, vc2, vc3]
}()
override func viewDidLoad() {
super.viewDidLoad()
self.dataSource = self
if let secondViewController = viewControllerList[1] as? MainViewController {
self.setViewControllers([secondViewController], direction: .forward, animated: true, completion: nil)
secondViewController.buttonAdd.action = #selector(addData(_:))
}
}
#objc func addData(_ sender: Any) {
if let thirdViewController = viewControllerList.last {
self.setViewControllers([thirdViewController], direction: .forward, animated: true, completion: nil)
}
}
#objc func backData(_ sender: Any) {
let secondViewController = viewControllerList[1]
self.setViewControllers([secondViewController], direction: .forward, animated: true, completion: nil)
}
func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
guard let vcIndex = viewControllerList.firstIndex(of: viewController) else { return nil }
let previousIndex = vcIndex - 1
guard previousIndex >= 0 else { return nil }
guard viewControllerList.count > previousIndex else { return nil }
return viewControllerList[previousIndex]
}
func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
guard let vcIndex = viewControllerList.firstIndex(of: viewController) else { return nil }
let nextIndex = vcIndex + 1
guard viewControllerList.count != nextIndex else { return nil }
guard viewControllerList.count > nextIndex else { return nil }
return viewControllerList[nextIndex]
}
}

Answer 1

The solution was to call this in any UIViewControllers button action:
// get parent view controller
let parentVC = self.parent as! UserDetailsPVC
// change page of PageViewController
parentVC.setViewControllers([parentVC.pages[1]], direction: .forward, animated: true, completion: nil)