RxSwift subscribe to <Bool> change depend from one variable (MVVM)

Question

Start learning RxSwift (Realm courses) and have some question.

In my controller have one TextField and Button, I need to subscribe for Button.isEnable when my TextField.count > 0

In courses this problem solved next code:

In Controller set value:

userNameText.rx.text.orEmpty.bind(to: viewModel.userName).disposed(by: disposeBag)
passwordText.rx.text.orEmpty.bind(to: viewModel.passwordValue).disposed(by: disposeBag)
viewModel.isValid.map{$0}.bind(to: button.rx.isEnabled).disposed(by: disposeBag)

In model:

let userName = Variable<String>("")
    let passwordValue = Variable<String>("")
    let isValid: Observable<Bool>

    init() {
        isValid = Observable.combineLatest(self.userName.asObservable(), self.passwordValue.asObservable()) {(name, password) in
            return  name.count > 0 && password.count > 0
        }
    }

In course use Observable.combineLatest for create observable with 2 value, for which we observe.(work with 2 value)

How create Observable where we work with one value??

PS Not work like this (min 2)

isValid = Observable.combineLatest(self.passwordValue.asObservable()) {(name) in
                return  name.count > 0 && password.count > 0
            }

Answer 1

Just do:

self.isValid = self.passwordValue.asObservable().map{ password in
    password.count > 0 
}