//

class PagesViewController: UIViewController {
	
	override func viewDidLoad() {
		super.viewDidLoad()
		
		// a UIView to hold the page view controller
		let pvcContainer = UIView()
		pvcContainer.backgroundColor = .gray
		
		[pvcContainer].forEach { v in
			v.translatesAutoresizingMaskIntoConstraints = false
			view.addSubview(v)
		}
		
		let g = view.safeAreaLayoutGuide
		NSLayoutConstraint.activate([
			
			pvcContainer.topAnchor.constraint(equalTo: g.topAnchor, constant: 100.0),
			pvcContainer.centerXAnchor.constraint(equalTo: g.centerXAnchor),
			pvcContainer.widthAnchor.constraint(equalToConstant: 200.0),
			pvcContainer.heightAnchor.constraint(equalTo: pvcContainer.widthAnchor, multiplier: 1.5),
			
		])
		
		let pvc = MyPageViewController(transitionStyle: .scroll, navigationOrientation: .horizontal)
		addChild(pvc)

		pvc.view.translatesAutoresizingMaskIntoConstraints = false
		pvcContainer.addSubview(pvc.view)
		
		NSLayoutConstraint.activate([
			
			pvc.view.topAnchor.constraint(equalTo: pvcContainer.topAnchor),
			pvc.view.leadingAnchor.constraint(equalTo: pvcContainer.leadingAnchor),
			pvc.view.trailingAnchor.constraint(equalTo: pvcContainer.trailingAnchor),
			pvc.view.bottomAnchor.constraint(equalTo: pvcContainer.bottomAnchor),
			
		])
		
		pvc.didMove(toParent: self)
		
	}
	
}

class MyPageViewController: UIPageViewController {
	
	// so we can get the index of the current page
	var currentIndex: Int {
		guard let vc = viewControllers?.first else { return 0 }
		return pageViewControllers.firstIndex(of: vc) ?? 0
	}
	
	let colors: [UIColor] = [
		.systemRed,
		.systemGreen,
		.systemBlue,
		.systemYellow,
		.green,
		.blue,
	]
	
	var pageViewControllers: [UIViewController] = [UIViewController]()
	
	override init(transitionStyle style: UIPageViewController.TransitionStyle, navigationOrientation: UIPageViewController.NavigationOrientation, options: [UIPageViewController.OptionsKey : Any]? = nil) {
		super.init(transitionStyle: style, navigationOrientation: navigationOrientation, options: options)
	}
	
	required init?(coder: NSCoder) {
		super.init(coder: coder)
	}
	
	override func viewDidLoad() {
		super.viewDidLoad()
		
		dataSource = self
		
		// instantiate all "pages"
		
		for i in 0..<colors.count {
			let vc = ExamplePageVC()
			vc.theLabel.text = "Page: \(i)"
			vc.view.backgroundColor = colors[i]
			pageViewControllers.append(vc)
		}
		
		setViewControllers([pageViewControllers[0]], direction: .forward, animated: false, completion: nil)
	}
	
}

extension MyPageViewController: UIPageViewControllerDataSource {
	func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
		
		guard let viewControllerIndex = pageViewControllers.firstIndex(of: viewController) else { return nil }
		
		let previousIndex = viewControllerIndex - 1
		
		// if we are at the First controller,
		//	return nil
		guard previousIndex >= 0 else { return nil }
		
		return pageViewControllers[previousIndex]
	}
	
	func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
		
		guard let viewControllerIndex = pageViewControllers.firstIndex(of: viewController) else { return nil }
		
		let nextIndex = viewControllerIndex + 1
		
		// if we are at the Last controller,
		//	return nil
		guard nextIndex < pageViewControllers.count else { return nil }
		
		return pageViewControllers[nextIndex]
	}
	
}

class ExamplePageVC: UIViewController {
	
	let theLabel: UILabel = {
		let v = UILabel()
		v.translatesAutoresizingMaskIntoConstraints = false
		v.backgroundColor = UIColor(white: 0.9, alpha: 1.0)
		v.textAlignment = .center
		return v
	}()
	
	override func viewDidLoad() {
		super.viewDidLoad()
		view.backgroundColor = .blue
		
		view.addSubview(theLabel)
		NSLayoutConstraint.activate([
			theLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor),
			theLabel.centerYAnchor.constraint(equalTo: view.centerYAnchor),
			theLabel.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.8),
			theLabel.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 0.8),
		])
		
	}
	
}