class RegisterViewModel {

    var username: String? {
        didSet {
            emitValidity()
        }
    }

    var password: String? {
        didSet {
            emitValidity()
        }
    }

    fileprivate func emitValidity() {
        let isValid = username?.isEmpty == false && password?.isEmpty == false
        isValidListener?(isValid)
    }

    // use a hook to check for form valid
    var isValidListener: ((Bool) -> ())?

}