BondBinding.swift

numberTextField.reactive.text.bidirectionalBind(to: viewModel.phoneNumber).dispose(in: reactive.bag)
numberTextField.reactive.textColor.bind(signal: viewModel.isValid.map({ [unowned self] (isValid) in return isValid ? self.view.tintColor : UIColor.red})).dispose(in: reactive.bag)

self.navigationItem.rightBarButtonItem.reactive.isEnabled.bind(signal: viewModel.isValid).dispose(in: reactive.bag)

BondVM.swift

class NumberSelectionViewModel {
    let selectedCountry = Observable<CountryDefinition?>(nil)
    let phoneNumber = Observable<String?>(nil)
    let isValid: Signal<Bool, Property<Any>.Error>
    
    init() {
        isValid = combineLatest(self.selectedCountry, self.phoneNumber) {
            (country: CountryDefinition?, number: String?) in if let number = number, let parsedNumber = try? NBPhoneNumberUtil.sharedInstance().parse(number, defaultRegion: country?.isoCode ?? "de") {
                return NBPhoneNumberUtil.sharedInstance().isValidNumber(parsedNumber)
            } else {
                return false
            }
        }
    }
}

RxSwift.swift

class NumberSelectionViewModel {
    let selectedCountry = Variable<CountryDefinition?>(nil)
    let phoneNumber = Variable<String?>(nil)
    let isValid : Observable<Bool>
    
    init() {        
        isValid = Observable.combineLatest(selectedCountry.asObservable(), phoneNumber.asObservable()) {
            (country: CountryDefinition?, number: String?)->Bool in if let number = number, let parsedNumber = try? NBPhoneNumberUtil.sharedInstance().parse(number, defaultRegion: country?.isoCode ?? "de") {
                return NBPhoneNumberUtil.sharedInstance().isValidNumber(parsedNumber)
            } else {
                return false
            }
        }
    }
}

RxSwiftBinding.swift

numberTextField.rx.text.bindTo(viewModel.phoneNumber).disposed(by: disposeBag)

viewModel.isValid.bindTo(navigationItem.rightButtonItem.rx.isEnabled).disposed(by: disposeBag)
viewModel.isValid.asObservable().map({[unowned self] (isValid) in return isValid ? self.view.tintColor : UIColor.red}).bindTo(numberTextField.rx.textColor).disposed(by: disposeBag)