//
//  PageViewController.swift
//  UIPageViewController
//
//  Created by Apple on 2019/4/14.
//  Copyright © 2019 Apple. All rights reserved.
//

import UIKit
import Foundation

class PageViewController: UIPageViewController, UIPageViewControllerDelegate, UIPageViewControllerDataSource {

    lazy var VCArr: [UIViewController] = {
        return [self.VCInstance(name: "FirstVC"),
                self.VCInstance(name: "SecondVC"),
                self.VCInstance(name: "ThirdVC")]
    }()


    override func viewDidLoad() {
        super.viewDidLoad()
        self.dataSource = self
        self.delegate = self
        if let firstVC = VCArr.first{
            setViewControllers([firstVC], direction: .forward, animated: true, completion: nil)
        }
        // Do any additional setup after loading the view.
    }

    private func VCInstance(name:String) -> UIViewController{
        return UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: name)
    }


    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = VCArr.firstIndex(of: viewController) else{
            return nil
        }

        let previousIndex = viewControllerIndex - 1

        guard previousIndex >= 0 else{
            return VCArr.last
        }
//        不知道什麼用途
//        guard VCArr.count > previousIndex else{
//            return  nil
//        }

        return VCArr[previousIndex]
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = VCArr.firstIndex(of: viewController) else{
            return nil
        }

        let nextIndex = viewControllerIndex + 1

        guard nextIndex < VCArr.count else{
            return VCArr.first
        }
//        不知道什麼用途
//        guard VCArr.count > nextIndex else{
//            return nil
//        }

        return VCArr[nextIndex]
    }

    func presentationCount(for pageViewController: UIPageViewController) -> Int {
        return VCArr.count
    }

    func presentationIndex(for pageViewController: UIPageViewController) -> Int {
        guard let firstViewController = viewControllers?.first, let firstViewControllerIndex = VCArr.firstIndex(of: firstViewController) else{
            return 0
        }

        return firstViewControllerIndex
    }



    /*
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        // Get the new view controller using segue.destination.
        // Pass the selected object to the new view controller.
    }
    */

}