Basic implementation of a UISearchController reusing the same type of UIViewController as the base and results.

BasicSearch.swift

import UIKit

protocol Searchable {
    func update(text: String?)
}

class BaseSearchResultUpdating: NSObject, UISearchResultsUpdating {
    func updateSearchResults(for searchController: UISearchController) {
        guard let searchable = searchController.searchResultsController as? Searchable else {
            fatalError("The searchResultsController is does not conform to Searchable.")
        }
        searchable.update(text: searchController.searchBar.text)
    }
}

class TableViewController: UITableViewController, Searchable {
    
    var elements = ["A" ,"B", "C", "D"]
    var resultsController: TableViewController?
    var baseSearchResultUpdating: BaseSearchResultUpdating?
    
    init(searchEnabled: Bool) {
        super.init(style: .plain)
        if searchEnabled {
            setSearch()
        }
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    func setSearch() {
        resultsController = TableViewController(searchEnabled: false)
        let searchController = UISearchController(searchResultsController: resultsController)
        
        baseSearchResultUpdating = BaseSearchResultUpdating()
        searchController.searchResultsUpdater = baseSearchResultUpdating
        searchController.searchBar.placeholder = "Search char"
        searchController.obscuresBackgroundDuringPresentation = true
        definesPresentationContext = true
        
        searchController.searchBar.sizeToFit()
        
        if #available(iOS 11.0, *) {
            navigationItem.searchController = searchController
            navigationItem.hidesSearchBarWhenScrolling = true
        } else {
            tableView.tableHeaderView = searchController.searchBar
        }
    }
    
    override func numberOfSections(in tableView: UITableView) -> Int {
        return 1
    }
    
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return elements.count
    }
    
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        var cell = tableView.dequeueReusableCell(withIdentifier: "cell")
        if cell == nil {
            cell = UITableViewCell(style: .default, reuseIdentifier: "cell")
        }
        
        cell?.textLabel?.text = elements[indexPath.row]
        return cell!
    }
    
    func update(text: String?) {
        guard let text = text, !text.isEmpty else { return }
        elements = elements.filter { $0.contains(text) }
        tableView.reloadData()
    }
}