how text length of a UITextView can be fixed?

Replace

 - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string

with

- (BOOL)textView:(UITextView *)textView shouldChangeTextInRange:(NSRange)range replacementText:(NSString *)text

which is answered here

Swift 4,

I have written an extension of UITextView in swift 4, for making the code reusable. Also works fine with copy paste.

    private var maxLengths = [UITextView: Int]()

extension UITextView : UITextViewDelegate {

  @IBInspectable var maxLength: Int {

    get {

      guard let length = maxLengths[self]
        else {
          return Int.max
      }
      return length
    }
    set {
      maxLengths[self] = newValue
      self.delegate = self
    }
  }

  @objc func limitLength(textView: UITextView) {
    guard let prospectiveText = textView.text,
      prospectiveText.count > maxLength
      else {
        return
    }

    let selection = selectedTextRange
    let maxCharIndex = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)
    text = String(prospectiveText[..<maxCharIndex])
    selectedTextRange = selection

  }

  public func textViewDidChange(_ textView: UITextView) {
    limitLength(textField:textView)
  }

  public func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
    scrollToBottom()
    return true
  }

  public func textViewShouldEndEditing(_ textView: UITextView) -> Bool {
    scrollToBottom()
    return true
  }

  func scrollToBottom() {
    let location = text.count - 1
    let bottom = NSMakeRange(location, 1)
    self.scrollRangeToVisible(bottom)
  }

}

Set max length value in storyboard,