Infinite Scrolling Final

ViewController.swift

//
//  ViewController.swift
//  Example
//
//  Created by John DeLong on 5/11/16.
//  Copyright © 2016 delong. All rights reserved.
//

import UIKit

extension Date {
    func dateFromDays(_ days: Int) -> Date {
        return (Calendar.current as NSCalendar).date(byAdding: .day, value: days, to: self, options: [])!
    }
}

class ViewController: UIViewController {

    @IBOutlet weak var tableView: UITableView!

    let cellBuffer: CGFloat = 2
    let cellHeight: CGFloat = 44
    let daysToAdd = 30

    let dateFormatter = DateFormatter()

    lazy var days: [Date] = {
        let beginDate = Date().dateFromDays(-29)
        let endDate = Date().dateFromDays(30)
        return self.generateDays(beginDate, endDate: endDate)
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        self.tableView.delegate = self
        self.tableView.dataSource = self

        self.dateFormatter.dateFormat = "MM-dd-yyyy"
    }

    func generateDays(_ beginDate: Date, endDate: Date) -> [Date] {
        var dates: [Date] = []
        var date = beginDate

        while date.compare(endDate) != .orderedDescending {
            dates.append(date)
            date = date.dateFromDays(1)
        }
        return dates
    }
}

extension ViewController: UITableViewDataSource {
    func numberOfSections(in tableView: UITableView) -> Int {
        return 1
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return self.days.count
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = UITableViewCell()
        cell.textLabel!.text = self.dateFormatter.string(from: self.days[(indexPath as NSIndexPath).row])
        return cell
    }
}

extension ViewController: UITableViewDelegate {

    func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
        return self.cellHeight;
    }

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let top: CGFloat = 0
        let bottom: CGFloat = scrollView.contentSize.height - scrollView.frame.size.height
        let buffer: CGFloat = self.cellBuffer * self.cellHeight
        let scrollPosition = scrollView.contentOffset.y

        // Reached the bottom of the list
        if scrollPosition > bottom - buffer {
            // Add more dates to the bottom
            let lastDate = self.days.last!
            let additionalDays = self.generateDays(lastDate.dateFromDays(1), endDate: lastDate.dateFromDays(self.daysToAdd))
            self.days.append(contentsOf: additionalDays)
            self.days.removeFirst(self.daysToAdd)

            // Update the tableView and contentOffset
            self.tableView.reloadData()
            self.tableView.contentOffset.y -= CGFloat(self.daysToAdd) * self.cellHeight
        }
        // Reach the top of the list
        else if scrollPosition < top + buffer {
            // Add more dates to the top
            let firstDate = self.days.first!
            let additionalDates = self.generateDays(firstDate.dateFromDays(-self.daysToAdd), endDate: firstDate.dateFromDays(-1))
            self.days.insert(contentsOf: additionalDates, at: 0)
            self.days.removeLast(self.daysToAdd)

            // Update the tableView and contentOffset
            tableView.reloadData()
            self.tableView.contentOffset.y += CGFloat(self.daysToAdd) * self.cellHeight
        }
    }
}

For infinite scroll up, how would you calculate the contentOffset for the tableview, if the newly added cells have dynamic height?

When run this code with paging enabled (each cell occupying whole page), the offset change applied at end of data manipulation is lost. So when the code comes back to compare scroll again, the data change situation will be triggered again, making scroll jump by numToAdd each time. If we separate the data changes and apply only one each time, be it remove or append, the offset change will be carried over and scroll will work perfectly.

Run this with Xcode 10.2, Swift 5. Not sure whether the author or anybody else has seen similar problem.