Using Observables with ViewModels

The Observable and Binding protocols help to reduce the boilerplate that's necessary when binding a particular view or control to the viewModel's properties.

Let's suppose that we have a simple form for registering users, with their name, password, and age. It can be represented with the following ViewModel:

class SignupViewModel {
    var username = Observable<String?>(nil)
    var email = Observable<String?>(nil)
    var age = Observable<Double>(0.0)
}

extension SignupViewModel  {
    var description: String {
        return """
username: \(username.value ?? "")
email: \(email.value ?? "")
age: \(age.value)
"""
    }
}

With the ViewModel configured, we can take a look at SignupViewController. It is quite simple, with a few text fields and a stepper to set the age:

class SignupViewController: UIViewController {
    @IBOutlet var ageStepper: UIStepper!
    @IBOutlet var ageLabel: UILabel!
    @IBOutlet var usernameField: UITextField!
    ...