How to make a loop appending 20 -20 rows in a tableview when scrolling?

Create a flag variable to check where is the table now on what cell // var showMore : Bool = false i suggest to convert your array to more friendly type such as String

 func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {

if allUserArray.count > 100 && showMore != true {
    return 20
}else if allUserArray.count > 100 && showMore == true {
    return allUserArray.count 
}else {
    return 20
}



func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
     if indexpath.row == 20 
    // append new content if any
    // update flag : showMore = true
    // reload table
}

I would suggest you use Bond framework to bind your data. It is built on top of RxSwift. So, in your case, you just need a Mutable Observable Array and bind that array with your tableView. let userArray = MutableObservableArray(allUserArray)

userArray.bind(to: collectionView, cellType: UserCell.self) { (cell, name) in
cell.titleLabel.text = name

}

Call a function from "willDisplay" and change the values in your array. With this approach, you only need to maintain an array and it will automatically shows up in your tableView.

https://github.com/DeclarativeHub/Bond

I'll elaborate the pagination in tableView using an example. You can go through it and then implement it in your project.

1. Let's assume you've a String array that contains around 500 elements.

var allUsers = [String](repeating: "User", count: 500)

2. Create another String array that initially contains first 10 elements from allUsers array.

var dataSource = [String]()

override func viewDidLoad() {
    super.viewDidLoad()
    self.addElements() //explained next..
}

The dataSource array will act as the actual dataSource for the tableView.

3. Let's maintain an offset that'll keep track of the index from which we need to add next 10 elements, i.e.

var nextOffset = 0

func addElements() {
    let newOffset = nextOffset+10
    for i in nextOffset..<newOffset {
        dataSource.append(allUsers[i])
    }
    nextOffset = newOffset
}

addElements() method simply add next 10 elements starting from the nextOffset.

4. The UITableViewDataSource methods look like,

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

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath)
    cell.textLabel?.text = "\(dataSource[indexPath.row]) : \(indexPath.row)"
    return cell
}

5. Now implement UITableViewDelegate method - tableView(_:willDisplay:forRowAt:)

func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    let currentCount = self.dataSource.count
    if currentCount < 500 && indexPath.row == (currentCount-1) { //last row
        self.addData()
        self.setUpLoaderView(toShow: true)
    } else {
        self.setUpLoaderView(toShow: false)
    }
}

In the above methods, I've checked if the cell is being displayed for the last row. If yes, then we add more data to it using addData() method and show a loaderView at bottom using setUpLoaderView(toShow:) method.

6. Here is the definition of both the methods,

func addData() {
    DispatchQueue.main.asyncAfter(deadline: .now() + 10) {
        self.addElements()
        self.tableView.reloadData()
    }
}

func setUpLoaderView(toShow: Bool) {
    if toShow {
        self.tableView.tableFooterView?.isHidden = false
        self.tableView.tableFooterView = self.loaderMoreView
    } else {
        self.tableView.tableFooterView = UIView()
    }
}

In addData() I've added a delay of 10 seconds as per your requirement. So the loader will be visible for 10 seconds before the tableView is updated with new data.

7. loaderMoreView is simply a UIActivityIndicatorView that is set as the tableFooterView, like:

private lazy var loaderMoreView: UIView = {
    let loaderView = UIActivityIndicatorView(style: .whiteLarge)
    loaderView.color = UIColor.gray
    loaderView.startAnimating()
    return loaderView
}()