Implementation of infinite scroll

InfinitePagingScrollViewController.swift

import UIKit

protocol ScrollingContent: Comparable {}

protocol ScrollingContentProvider {
    associatedtype Content: ScrollingContent
    func previous(of content: Content) -> Content
    func next(of content: Content) -> Content
}

protocol ScrollingContentView: UIViewController {
    associatedtype Content: ScrollingContent
    func set(content: Content)
}

class InfinitePagingScrollViewController<ContentView: ScrollingContentView, ContentProvider: ScrollingContentProvider>: UIViewController, UIScrollViewDelegate where ContentView.Content == ContentProvider.Content {
    private var didLayoutSubviews: Bool = false
    private let scrollView = UIScrollView()
    private var contentViews: [ContentView] = (1...3).map { _ in ContentView() }

    private var middleContent: ContentView.Content
    private let contentProvider: ContentProvider

    init(middleContent: ContentView.Content, contentProvider: ContentProvider) {
        self.middleContent = middleContent
        self.contentProvider = contentProvider

        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .systemBackground

        scrollView.isPagingEnabled = true
        scrollView.showsVerticalScrollIndicator = false
        scrollView.showsHorizontalScrollIndicator = false
        scrollView.delegate = self

        view.addSubview(scrollView)
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            scrollView.frameLayoutGuide.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            scrollView.frameLayoutGuide.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
            scrollView.frameLayoutGuide.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
            scrollView.frameLayoutGuide.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor)
        ])

        contentViews.forEach { scrollView.addSubview($0.view) }
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        if !didLayoutSubviews {
            didLayoutSubviews = true

            contentViews[0].set(content: contentProvider.previous(of: middleContent))
            contentViews[1].set(content: middleContent)
            contentViews[2].set(content: contentProvider.next(of: middleContent))

            layoutContents()
            scrollView.contentSize = CGSize(width: scrollView.bounds.width * 3, height: scrollView.bounds.height)
            scrollView.contentOffset = CGPoint(x: scrollView.bounds.width, y: 0)
        }
    }

    func layoutContents() {
        for (index, vc) in contentViews.enumerated() {
            vc.view.frame = CGRect(origin: .init(x: scrollView.bounds.width * CGFloat(index), y: 0), size: scrollView.bounds.size)
        }
    }

    // MARK: - UIScrollViewDelegate

    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        let page = targetContentOffset.pointee.x / scrollView.bounds.width - 1
        guard page != 0 else { return }

        if page > 0 {
            middleContent = contentProvider.next(of: middleContent)

            let view = contentViews.remove(at: 0)
            view.set(content: contentProvider.next(of: middleContent))
            contentViews.append(view)
        } else {
            middleContent = contentProvider.previous(of: middleContent)

            let view = contentViews.remove(at: 2)
            view.set(content: contentProvider.previous(of: middleContent))
            contentViews.insert(view, at: 0)
        }

        layoutContents()

        scrollView.contentOffset.x -= scrollView.bounds.width * CGFloat(page)
        targetContentOffset.pointee = CGPoint(x: scrollView.bounds.width, y: 0)
    }
}