[Swift3] UIPageViewControllerの使い方

この記事は、【決定版】UIPageViewControllerの使い方（Swift）のコードをSwift3で書き直したものです。
StoryBoardの設定などは上記の記事で確認してください。

swift3

import UIKit

class PageViewController: UIPageViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        self.setViewControllers([getFirst()], direction: .forward, animated: true, completion: nil)
        self.dataSource = self
        
    }
    
    func getFirst() -> FirstViewController {
        return storyboard!.instantiateViewController(withIdentifier: "FirstViewController") as!FirstViewController
    }
    func getSecond() -> SecondViewController {
        return storyboard!.instantiateViewController(withIdentifier: "SecondViewController") as! SecondViewController
    }
    func getThird() -> ThirdViewController{
        return storyboard!.instantiateViewController(withIdentifier: "ThirdViewController") as!ThirdViewController
    }
}

    extension PageViewController : UIPageViewControllerDataSource {
        
        func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
            if viewController.isKind(of: ThirdViewController.self) {
                // 3 -> 2
                return getSecond()
            } else if viewController.isKind(of: SecondViewController.self) {
                // 2 -> 1
                return getFirst()
            }else{
                // 1 -> end of the road
                return nil
            }
        }
        
        func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
            if viewController.isKind(of: FirstViewController.self) {
                // 1 -> 2
                return getSecond()
            } else if viewController.isKind(of: SecondViewController.self) {
                // 2 -> 3
                return getThird()
            }else{
                // 3 -> end of the road
                return nil
            }
        }
    }