zoomable and swipeable image viewer

gistfile1.txt

//
//  MyPaveViewController.swift
//  zzz
//
//  Created by Yan Cheng Cheok on 22/08/2021.
//

import UIKit

class MyPageViewController: UIPageViewController {
    let imageNames = [
        "0",
        "1",
        "2",
        "3",
        "4"
    ]
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        dataSource = self
        delegate = self
        
        let imageViewController = ImageViewController()
        
        imageViewController.postInit(imageName: "0")
        
        setViewControllers([imageViewController], direction: .forward, animated: true, completion: nil)
    }
}

extension MyPageViewController: UIPageViewControllerDataSource, UIPageViewControllerDelegate {
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        
        let imageable = viewController as! Imageable
        
        let imageName = imageable.imageName
        
        let index = imageNames.firstIndex(of: imageName)
        
        if let index = index {
            if index <= 0 {
                return nil
            }
            
            let imageViewController = ImageViewController()
            let newIndex = index - 1
            imageViewController.postInit(imageName: imageNames[newIndex])
            return imageViewController
        } else {
            return nil
        }
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        let imageable = viewController as! Imageable
        
        let imageName = imageable.imageName
        
        let index = imageNames.firstIndex(of: imageName)
        
        if let index = index {
            if index >= imageNames.count-1 {
                return nil
            }
            
            let imageViewController = ImageViewController()
            let newIndex = index + 1
            imageViewController.postInit(imageName: imageNames[newIndex])
            return imageViewController
        } else {
            return nil
        }
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        
        guard let viewControllers = self.viewControllers else { return }
        guard let imageViewController = viewControllers.first as? ImageViewController else { return }

        imageViewController.becomeCurrentPage()
    }
}

//
//  ImageViewController.swift
//  zzz
//
//  Created by Yan Cheng Cheok on 22/08/2021.
//

import UIKit

class ImageViewController: UIViewController, Imageable {
    var imageName: String {
        return _imageName
    }
    
    private var _imageName: String!
    
    private var firstTime = false
    
    private lazy var imageScrollView: ImageScrollView = {
        let imageScrollView = ImageScrollView()
        imageScrollView.imageContentMode = .aspectFit
        imageScrollView.initialOffset = .center
        return imageScrollView
    }()
    
    func postInit(imageName: String) {
        self._imageName = imageName
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        if firstTime {
            return
        }
        
        firstTime = true
        
        // viewDidLayoutSubviews is the only place where imageScrollView.display will work correctly.
        imageScrollView.display(image: UIImage(named: imageName)!)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        imageScrollView.setup()
        
        imageScrollView.translatesAutoresizingMaskIntoConstraints = false
        
        self.view.addSubview(imageScrollView)
        
        NSLayoutConstraint.activate([
            imageScrollView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
            imageScrollView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor),
            imageScrollView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            imageScrollView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor)
        ])
    }
    
    func becomeCurrentPage() {
        // Ensure the image is in zoom-out state.
        imageScrollView.refresh()
    }
}

//
//  Imageable.swift
//  zzz
//
//  Created by Yan Cheng Cheok on 22/08/2021.
//

import Foundation

protocol Imageable {
    var imageName: String { get }
}