UITextField with shake error

TextFieldShakenErrorViewController.swift

import UIKit

class ViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var textField: UITextField!
    @IBOutlet weak var label: UILabel!

    enum Placeholder {
        case normal, empty

        var message: String {
            switch self {
            case .normal:
                return "ここにテキストを入力"
            case .empty:
                return "1文字以上を入力しましょう"
            }
        }
    }

    var placeholderState = Placeholder.normal {
        didSet {
            textField.placeholder = placeholderState.message
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        textField.delegate = self
        textField.returnKeyType = .send

        NotificationCenter.default.addObserver(self, selector: #selector(changePlaceholderIfNeeded), name: NSNotification.Name.UITextFieldTextDidChange, object: textField)
    }


    @objc func changePlaceholderIfNeeded(_ notification: NSNotification) {
        guard let field = notification.object as? UITextField else { return }
        if field.text?.count ?? 0 > 0 {
            placeholderState = .normal
        }
    }
}

extension ViewController {
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        if textField.text?.count == 0 {
            placeholderState = .empty
            textField.shake()
            return false
        } else {
            textField.resignFirstResponder()
            return true
        }
    }

    func textFieldDidEndEditing(_ textField: UITextField) {
        label.text = textField.text
    }
}

public extension UIView {

    // https://stackoverflow.com/a/38790163
    func shake(count : Float = 4, for duration : TimeInterval = 0.2, withTranslation translation : CGFloat = 5) {
        let animation : CABasicAnimation = CABasicAnimation(keyPath: "transform.translation.x")
        animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionLinear)
        animation.repeatCount = count
        animation.duration = duration/TimeInterval(animation.repeatCount)
        animation.autoreverses = true
        animation.fromValue = NSValue(cgPoint: CGPoint(x: -translation, y: self.center.y))
        animation.toValue = NSValue(cgPoint: CGPoint(x: translation, y: self.center.y))
        layer.add(animation, forKey: "shake")
    }
}