demo_efficient_scroll.gif

README.MD

Efficient Scroll View (Vertical/Horizontal) for tik-tok like scrolling

It is a custom UIScrollView where subviews have same size as UIScrollView and only 3 subview will be created and reused efficiently.

Usage

// to reload data source
reloadAllData() 

// data source 
extension  ConnectVC: EfficientScrollViewDataSource {
    func numberOfPages() -> Int {
        return  self.podcast.liveAudiences?.count ?? 0
    }

    func onBind(view: UIView?, index: Int) -> UIView {
	 let holder: JoinReqCardView = view as? JoinReqCardView ?? JoinReqCardView.loadNib()

	 holder.podcast = podcast
	 holder.req = self.podcast.liveAudiences?[index]
	 holder.parentViewc = self
	 holder.relaodViews()
	 return holder
    }
}

Sample ViewController

let viewc = EfficientScrollViewVC()
self.show(viewc, sender: nil)

EfficientScrollView.swift

import UIKit

protocol EfficientScrollViewDataSource: class {
    func numberOfPages() -> Int
    func onBind(view: UIView?, index: Int) -> UIView
}

class EfficientScrollView: UIScrollView {
    
    weak var dataSource: EfficientScrollViewDataSource?
    private var pages: [UIView] = []
    private var totalItemCount: Int = 0
    var onPageChanged: ((Int, UIView?) -> Void)?
    
    var currentPage: Int = -1
    var vertical = true
    
    override func didMoveToSuperview() {
        super.didMoveToSuperview()
        if superview == nil {
            removeObserver(self, forKeyPath: "contentOffset", context: nil)
        } else {
            addObserver(self, forKeyPath: "contentOffset", options: [NSKeyValueObservingOptions.new], context: nil)
        }
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey: Any]?, context: UnsafeMutableRawPointer?) {
        var page = 0
        if vertical {
            page = height == 0 ? 0 : Int(round(contentOffset.y / height))
        } else {
            page = width == 0 ? 0 : Int(round(contentOffset.x / width))
        }
        
        if page != currentPage {
            currentPage = page
            load(page: page)
            load(page: page - 1)
            load(page: page + 1)
            onPageChanged?(currentPage, pages.first(where: { $0.tag == page }))
        }
        
    }
    
    private func load(page: Int, force: Bool = false) {
        if page < 0 || page >= totalItemCount { return }
        if let view = pages.first(where: { $0.tag == page }) {
            if force {
                if let v = dataSource?.onBind(view: view, index: page) {
                    if v.superview == nil { addSubview(v) }
                }
            }
        } else {
            if let view = pages.first(where: { $0.tag < currentPage - 1 || $0.tag > currentPage + 1 }) {
                view.tag = page
                if let v = dataSource?.onBind(view: view, index: page) {
                    if v.superview == nil { addSubview(v) }
                }
            } else {
                if let view = dataSource?.onBind(view: nil, index: page) {
                    view.tag = page
                    addSubview(view)
                    pages.append(view)
                }
            }
        }
        //print("pages size: \(pages.count)")
        reloadContentSizeAndPosition()
    }
    
    func reloadAllData() {
        totalItemCount = dataSource?.numberOfPages() ?? 0
        
        //if totalItemCount < 3 {
        pages.forEach({ $0.tag = -3; $0.removeFromSuperview() })
        //}
        
        if totalItemCount > 0 {
            if currentPage == -1 {
                currentPage = 0
            }
            load(page: currentPage - 1, force: true)
            load(page: currentPage, force: true)
            load(page: currentPage + 1, force: true)
        } else {
            currentPage = -1
        }
    }
    
    // MARK: size fixing
    private var height: CGFloat = 0
    private var width: CGFloat = 0
    
    override func layoutSubviews() {
        super.layoutSubviews()
        reloadContentSizeAndPosition()
    }
    
    private func reloadContentSizeAndPosition() {
        height = bounds.height
        width = bounds.width
        contentSize = vertical ? CGSize(width: width, height: CGFloat(totalItemCount) * height) :
            CGSize(width: CGFloat(totalItemCount) * width, height: height)
        pages.forEach {
            if tag >= 0 && tag < totalItemCount {
                $0.frame = vertical ? CGRect(x: 0, y: CGFloat($0.tag) * height, width: width, height: height) :
                    CGRect(x: CGFloat($0.tag) * width, y: 0, width: width, height: height)
            }
        }
    }

}

EfficientScrollViewVC.swift

import UIKit

class EfficientScrollViewVC: UIViewController {
    
    let scrollView = EfficientScrollView()
    var listShow = [Int]()
    var apiCallInProgress = false
    
    let btn = UIButton(type: .system)
     
    override func viewDidLoad() {
        super.viewDidLoad()
        
        automaticallyAdjustsScrollViewInsets = false
        scrollView.contentInsetAdjustmentBehavior = .never
        
        view.backgroundColor = .white
        view.addSubview(scrollView)
        scrollView.frame = view.bounds
        scrollView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        scrollView.dataSource = self
        scrollView.isPagingEnabled = true
        scrollView.layer.borderWidth = 3
        scrollView.layer.borderColor = UIColor.black.cgColor
        scrollView.clipsToBounds = false
        scrollView.transform = .init(scaleX: 0.3, y: 0.3)
        
        btn.setTitle("Resize", for: .normal)
        btn.sizeToFit()
        btn.addTarget(self, action: #selector(actionResize), for: .touchUpInside)
        view.addSubview(btn)
        
        callApiForData()
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        btn.frame.origin = CGPoint(x: 20, y: view.safeAreaInsets.top + 20)
    }
    
    func callApiForData() {
        if apiCallInProgress { return }
        apiCallInProgress = true
        getNextPageData { [weak self] list in
            self?.listShow.append(contentsOf: list)
            self?.scrollView.reloadAllData()
            self?.apiCallInProgress = false
        }
    }
    
    @objc func actionResize() {
        let new: CGAffineTransform = scrollView.transform == .identity ? .init(scaleX: 0.3, y: 0.3) : .identity
        UIView.animate(withDuration: 1.0) {
            self.scrollView.transform = new
        }
    }
    
    static var count = 0
    
    func getNextPageData(completion: @escaping (([Int])->Void)) {
        DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
            let pageCount = 5
            completion(Array(EfficientScrollViewVC.count...EfficientScrollViewVC.count + pageCount))
            EfficientScrollViewVC.count += (pageCount + 1)
        }
    }
}

extension EfficientScrollViewVC: EfficientScrollViewDataSource {
    func numberOfPages() -> Int {
        return listShow.count
    }
    
    func onBind(view: UIView?, index: Int) -> UIView {
        let view = (view as? SubView) ?? SubView(frame: self.view!.bounds)
        view.lbl.text = "\(listShow[index])"
        view.backgroundColor = index % 2 == 0 ? .red : .blue
        view.backgroundColor = view.backgroundColor?.withAlphaComponent(0.5)
        if index == listShow.count - 1 {
            callApiForData()
        }
        return view
    }
}

class SubView: UIView {
    
    let lbl = UILabel()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func commonInit() {
        addSubview(lbl)
        lbl.textColor = .black
        lbl.font = UIFont.systemFont(ofSize: 100)
        lbl.textAlignment = .center
        lbl.frame = self.bounds
        lbl.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
    }
}

Also are you available for contract work?

Right now I am doing full time job. So if I need to do any contract job I will definitely contact you. Thank you for showing interest.
Also added a sample view controller.