Вы можете использовать в swift 4 ниже.

1. создайте проект в swift 4
2. перетащите текстовое поле и соединение в ViewController
3. это будет выше, как изображение

4. вставить код ViewController

   class ViewController: UIViewController,UITextFieldDelegate {
   
     var myarray = ["Enamul Haque", "Ariful", "Mohsin"]
     var autoCompleteCharacterCount = 0
     var timer = Timer()
   
    @IBOutlet weak var txtUserId: UITextField!
   
   
    override func viewDidLoad() {
       super.viewDidLoad()
   
        txtUserId.delegate = self
      }
   
   
    //Setp 2
   
    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool { //1
        var subString = (textField.text!.capitalized as NSString).replacingCharacters(in: range, with: string) // 2
   subString = formatSubstring(subString: subString)
   
      if subString.count == 0 { // 3 when a user clears the textField
        resetValues()
      } else {
         searchAutocompleteEntriesWIthSubstring(substring: subString) //4
       }
        return true
     }
     func formatSubstring(subString: String) -> String {
      let formatted = String(subString.dropLast(autoCompleteCharacterCount)).lowercased().capitalized //5
       return formatted
       }
   
   
   
    func resetValues() {
      autoCompleteCharacterCount = 0
     txtUserId.text = ""
     }
       //Step 3
      func searchAutocompleteEntriesWIthSubstring(substring: String) {
      let userQuery = substring
       let suggestions = getAutocompleteSuggestions(userText: substring) //1
   
      if suggestions.count > 0 {
            timer = .scheduledTimer(withTimeInterval: 0.01, repeats: false, block: { (timer) in //2
              let autocompleteResult =   self.formatAutocompleteResult(substring: substring, possibleMatches: suggestions) // 3
              self.putColourFormattedTextInTextField(autocompleteResult: autocompleteResult, userQuery : userQuery) //4
             self.moveCaretToEndOfUserQueryPosition(userQuery: userQuery) //5
         })
     } else {
          timer = .scheduledTimer(withTimeInterval: 0.01, repeats: false, block: { (timer) in //7
            self.txtUserId.text = substring
        })
          autoCompleteCharacterCount = 0
       }
      }
   
     //step 4
      func getAutocompleteSuggestions(userText: String) -> [String]{
       var possibleMatches: [String] = []
      for item in myarray { //2
       let myString:NSString! = item as NSString
       let substringRange :NSRange! = myString.range(of: userText)
   
       if (substringRange.location == 0)
       {
           possibleMatches.append(item)
       }
     }
      return possibleMatches
    }
   
         func putColourFormattedTextInTextField(autocompleteResult: String, userQuery : String) {
         let colouredString: NSMutableAttributedString = NSMutableAttributedString(string: userQuery + autocompleteResult)
     colouredString.addAttribute(NSAttributedStringKey.foregroundColor, value: UIColor.red, range: NSRange(location: userQuery.count,length:autocompleteResult.count))
       self.txtUserId.attributedText = colouredString
   }
     func moveCaretToEndOfUserQueryPosition(userQuery : String) {
        if let newPosition = self.txtUserId.position(from: self.txtUserId.beginningOfDocument, offset: userQuery.count) {
          self.txtUserId.selectedTextRange = self.txtUserId.textRange(from: newPosition, to: newPosition)
      }
       let selectedRange: UITextRange? = txtUserId.selectedTextRange
       txtUserId.offset(from: txtUserId.beginningOfDocument, to: (selectedRange?.start)!)
    }
        func formatAutocompleteResult(substring: String, possibleMatches: [String]) -> String {
       var autoCompleteResult = possibleMatches[0]
     autoCompleteResult.removeSubrange(autoCompleteResult.startIndex..<autoCompleteResult.index(autoCompleteResult.startIndex, offsetBy: substring.count))
     autoCompleteCharacterCount = autoCompleteResult.count
     return autoCompleteResult
      }
   
   
   }