PageViewControllerSegmentedAdapter.swift

import UIKit

final class PageViewControllerSegmentedAdapter: NSObject {
	private unowned let pageViewController: UIPageViewController
	private unowned let segmentControl: UISegmentedControl
	private let viewControllers: [UIViewController]
	
	private var selectedIndex: Int
	
	init(pageViewController: UIPageViewController, segmentControl: UISegmentedControl, viewControllers: [UIViewController], at selectedIndex: Int = 0) {
		assert(segmentControl.numberOfSegments == viewControllers.count)
		
		self.pageViewController = pageViewController
		self.segmentControl = segmentControl
		self.viewControllers = viewControllers
		self.selectedIndex = selectedIndex
		
		super.init()
		
		pageViewController.delegate = self
		pageViewController.dataSource = self
		
		if viewControllers.count > selectedIndex {
			let targetViewController = viewControllers[selectedIndex]
			pageViewController.setViewControllers([targetViewController], direction: .forward, animated: false)
		}
		
		segmentControl.selectedSegmentIndex = selectedIndex
		segmentControl.addTarget(self, action: #selector(segmentControlValueChanged), for: .valueChanged)
	}
	
	@objc private func segmentControlValueChanged() {
		let selectedIndex = self.selectedIndex
		
		let selectedSegmentIndex = segmentControl.selectedSegmentIndex
		if selectedSegmentIndex > selectedIndex {
			let nextIndex = selectedIndex + 1
			for index in nextIndex...selectedSegmentIndex {
				setViewController(at: index, direction: .forward)
			}
		} else if selectedSegmentIndex < selectedIndex {
			let previousIndex = selectedIndex - 1
			for index in (selectedSegmentIndex...previousIndex).reversed() {
				setViewController(at: index, direction: .reverse)
			}
		}
	}
	
	private func setViewController(at index: Int, direction: UIPageViewController.NavigationDirection) {
		pageViewController.setViewControllers([viewControllers[index]], direction: direction, animated: true) { [weak self] completed in
			guard let me = self else {
				return
			}
			if completed {
				me.selectedIndex = index
			}
		}
	}
}

// MARK: - UIPageViewControllerDelegate
extension PageViewControllerSegmentedAdapter: UIPageViewControllerDelegate {
	public func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
		guard let index = self.viewControllers.firstIndex(of: viewController), index > 0 else {
			return nil
		}
		return self.viewControllers[index &- 1]
	}
	
	public func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
		guard var index = self.viewControllers.firstIndex(of: viewController) else {
			return nil
		}
		index = index &+ 1
		guard index < self.viewControllers.count else {
			return nil
		}
		return self.viewControllers[index]
	}
}

// MARK: - UIPageViewControllerDataSource
extension PageViewControllerSegmentedAdapter: UIPageViewControllerDataSource {
	func pageViewController(_ pageViewController: UIPageViewController,
							didFinishAnimating finished: Bool,
							previousViewControllers: [UIViewController],
							transitionCompleted completed: Bool) {
		guard let viewControllers = pageViewController.viewControllers,
			  let lastViewController = viewControllers.last,
			  let index = self.viewControllers.firstIndex(of: lastViewController) else {
				return
		}
		
		if finished, completed {
			self.selectedIndex = index
			self.segmentControl.selectedSegmentIndex = self.selectedIndex
		}
	}
}