Search Controller in Table View Controller in iOS 11 with Swift 4. From: https://www.raywenderlich.com/113772/uisearchcontroller-tutorial

search-controller.swift

// The code bellow must be written in your Table View Controller
// Remember to use your own classes and properties when creating your own search.

// 1. Create a new property in your class
// The Search Controller is the responsible to do the "searching magic"
let searchController = UISearchController(searchResultsController: nil)

// 2. At the viewDidLoad() add those initializations
searchController.searchResultsUpdater = self // You will get an error here for now, but it will vanish at step number 5
searchController.dimsBackgroundDuringPresentation = false
definesPresentationContext = true
tableView.tableHeaderView = searchController.searchBar

// 3. Create a propertie in your class to store the results of your filtering
var filteredWords = [String]()

// 4. Create the filter method to filter the objects in your array based on the text the user input
func filterContent(for searchText: String, scope: String = "All") {
  filteredWords = words.filter({ word in
    return word.lowercased().contains(searchText.lowercased())
  })

  tableView.reloadData()
}

// 5. Create an extension of your class to implementthe UISearchResultsUpdating protocol
//   This protocol is required by the searchController, so it knows what to do when the user
//   types the text in the search bar.
extension SuaClasseTableViewController: UISearchResultsUpdating {
    func updateSearchResults(for searchController: UISearchController) {
        filterContent(for: searchController.searchBar.text!)
    }
}


// 6. Update the method tableView(_:numberOfRowsInSection:)
// so it returns correctly the number of rows when the user is searching
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
  if searchController.isActive && searchController.searchBar.text != "" {
    return filteredWords.count
  }
  return words.count
}

// 7. Update the method tableView(_:cellForRowAtIndexPath:)
// so it uses the correct array to display the information
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
  let cell = tableView.dequeueReusableCell(withIdentifier: "cellIdentifier", for: indexPath)

  let word: String
        
  if searchController.isActive && searchController.searchBar.text != "" {
    word = filteredWords[indexPath.row]
  } else {
    word = words[indexPath.row]
  }
        
  cell.textLabel?.text = word
  
  return cell
}

// Now your table view have a beautiful and functional search bar!
// Remember to use your own classes and properties when creating your own search.