import UIKit

class ExpandedLabelViewController: UIViewController {

  @IBOutlet weak var myLabel: UILabel!
  @IBOutlet weak var lblHeight: NSLayoutConstraint!
  @IBOutlet weak var btn: UIButton!

  var isLabelAtMaxHeight = false
  override func viewDidLoad()
  {
    super.viewDidLoad()
    myLabel.text = "bla bla blabla bla blabla bla blabla bla blabla bla blabla bla bla"

  }

  @IBAction func btnAction(_ sender: Any)
  {
    if isLabelAtMaxHeight {
        btn.setTitle("Read more", for: .normal)
        isLabelAtMaxHeight = false
        lblHeight.constant = 70
    }
    else {
        btn.setTitle("Read less", for: .normal)
        isLabelAtMaxHeight = true
        lblHeight.constant = getLabelHeight(text: "bla bla bla vvvbla bla blabla bla blabla bla blabla bla blabla bla blabla bla blabla bla blabla bla blabla bla blabla bla blabla bla bla", width: view.bounds.width, font: myLabel.font)
    }
  }
  func getLabelHeight(text: String, width: CGFloat, font: UIFont) -> CGFloat {

    let lbl = UILabel(frame: .zero)
    lbl.frame.size.width = width
    lbl.font = font
    lbl.numberOfLines = 0
    lbl.text = text
    lbl.sizeToFit()
    lbl.adjustsFontSizeToFitWidth = true
    return lbl.frame.size.height

  }
}

2018-05-27 12:28:53.274735+0500 testingControllers[8035:240013] -[UILabel setConstant:]: unrecognized selector sent to instance 0x7fbd0f513e60
2018-05-27 12:28:53.291991+0500 testingControllers[8035:240013] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[UILabel setConstant:]: unrecognized selector sent to instance 0x7fbd0f513e60'