this one with an enum to force type safety. really messy though.

ServiceAvailability.swift

import UIKit

enum ServiceId {
    case service(id: String)
    func id() -> String {
        switch self {
        case .service(let id): return id
        }
    }
}


class Service {
    let id: ServiceId
    
    init(id: String) {
        self.id = ServiceId.service(id: id)
    }
}

class ServiceAvailability {
    func checkAvailabilityOf(serviceId: ServiceId, handler: (ServiceId, Bool) -> ()) {
        handler(serviceId, true)
    }
}

let service = Service(id: "myServiceId")

let availability = ServiceAvailability()

availability.checkAvailabilityOf(serviceId: service.id) { id, available in
    print("service \(id.id()) is available? \(available)")
}