Stepper.swift

private var subjectContext: UInt8 = 0

/// a Stepper has only one purpose: emit Steps that correspond to
/// specific navigation states.
/// The state changes lead to navigation actions in the context of
/// a specific Flow
public protocol Stepper: Synchronizable {

    /// the Rx Obsersable that will trigger new Steps
    var steps: Observable<Step> { get }
}

public extension Stepper {

    /// The step in which to publish new Steps
    public var step: BehaviorSubject<Step> {
        return self.synchronized {
            if let subject = objc_getAssociatedObject(self, &subjectContext)
                             as? BehaviorSubject<Step> {
                return subject
            }
            let newSubject = BehaviorSubject<Step>(value: NoStep())
            objc_setAssociatedObject(self,
                                     &subjectContext,
                                     newSubject,
                                     .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            return newSubject
        }
    }

    /// the Rx Obsersable that will trigger new Steps
    public var steps: Observable<Step> {
        return self.step.asObservable()
    }
}