How to sanitise data with bidirectional binding with Bond

Question

I'm quite new to FRP and decided to get started with Bond and ReactiveKit as it seemed lightweight enough to start gradually applying it to my apps and my head.

I have a setup where I have a view, which has an observable State and I have a view model with another observable property of type State. I want my view to be dull and unaware of the semantics, so I want my view model to validate the state, transform it and send back to the view.

The view contains a couple of text fields and a segmented control. When the segmented control is 0, I want only the first text field to be visible, otherwise — both fields. Whenever the user enters something or taps the segmented control, the observable state object is updated. Here is the State struct:

enum ValueType {
    case text
    case number
}

struct State {
    var name: String?
    var unit: String?
    var valueType: ValueType = .text
    var showsUnitTextField: Bool = true
}

Here is the best I could come up with:

View model:

override init() {
    super.init()
    self.bind()
}

let inputState: Observable<State> = Observable<State>(State())

var outputState: Observable<State> = Observable<State>(State())

private func bind() {
    inputState.map(self.sanitizeState(_:)).bind(to: outputState)
}

private func sanitizeState(_ state: State) -> State {
    var newState = state
    newState.showsUnitTextField = state.valueType == .number
    return newState
}

View controller:

private func bind() {
    myView.reactive.state.bind(to: viewModel.inputState)
    viewModel.outputState.bind(to: myView.reactive.state)
}

Basically I'm having two observables, one receives the updates and the other one sends the transformed value back to the view. This solutions seems to introduce a heavy boilerplate and I'm looking for a better one. Does anyone have any idea?

P.S. Other scenarios where this could come handy is when I want to have some constraints on the user input (only letters or only digits), or maybe I want to format the input in a fancy way.