Stateless Control

Stepper.swift

/// Example interface of a control that does not manage its own state.
public class Stepper {

    /// Enum is usually a convenient type to represent action.
    public enum Action {
        case increase
        case decrease
    }

    /// On the output is an action or intent describing what to do with the state.
    /// Control never updates its own state. It only sends intents to model/logic layer.
    public let action: Signal<Action>

    /// On the input is the binding the updates displayed state.
    /// Model/logic layer binds a signal to this property that represents a state to be displayed.
    public let value: Bond<Int>

    /// Internally control probably holds the state for rendering/optimization purposes, but
    /// it never changes the state on its own (as a result of action happening).
    private var _value: Int
}


func test() {
    let stepper = Stepper()

    stepper.action
        .reduce(4) { value, action in  // initial value is 4
            switch action {
            case .decrease:
                return value - 2       // on .decrease, subtract 2
            case .increase:
                return value + 2       // on .increase, add 2
            }
        }
        .bind(to: stepper.value)
}