Register a callback that will be made any time there's a change to the Journey State for the user. Note that Nami fetches journey state at the start of each session and this is the most likely time to see a change.

@objc static func registerJourneyStateChangedHandler( _ changeHandler: NamiCustomerJourneyChangeHandler? )

public typealias NamiCustomerJourneyChangeHandler = (_ newCustomerJourneyState: CustomerJourneyState) -> ()