class SignupTextField: UITextField {
    @IBOutlet private var validStateConstraints: [NSLayoutConstraint]
    @IBOutlet private var invalidStateConstraints: [NSLayoutConstraint]

    var isValid: Bool {
        get {
            let validStateConstraints = Set(self.validStateConstraints)
            let constraints = Set(self.constraints)
            let isValid = validStateConstraints.isSubsetOf(constraints)
            return isValid
        }
        set {
            guard newValue != isValid else {
                return
            }
            let all = [validStateConstraints, invalidStateConstraints].flatten().map{$0}
            let newStateConstraints = newValue ? validStateConstraints : invalidStateConstraints
            removeConstraints(all)
            addConstraints(newStateConstraints)
        }
    }
}