添加到UILabel的点击手势识别器不起作用

在viewDidLoad中初始化tapRecognizer应该可以做到，因为您在初始化视图之前就将自己定位为

class ViewController: UIViewController, MFMailComposeViewControllerDelegate {

@IBOutlet weak var tokenDisplay: UILabel!
var tapGestureRecognizer:UITapGestureRecognizer!

override func viewDidLoad() {
    super.viewDidLoad()
    tapGestureRecognizer = UITapGestureRecognizer(target:self, action:      #selector(onUserClickingSendToken(_:)))
    tapGestureRecognizer.numberOfTapsRequired = 1
    tokenDisplay.isUserInteractionEnabled = true
    tokenDisplay.addGestureRecognizer(tapGestureRecognizer)
}

func onUserClickingSendToken(_ sender: Any)
{
  ....