// Paste this code into a Swift playground.

// @objc is required for this to work
// Try removing @objc and see what happens.
@objc protocol ServiceType {
    func performService() -> String
}

class ServiceResolver {
    private init() {}
    
    private static var services = [String: Any]()
    
    static func resolve<R>(proto: Protocol) -> R {
        return services[NSStringFromProtocol(proto)]! as! R
    }
    
    static func register(proto: Protocol, instance: Any) {
        // NSStringFromProtocol works ONLY with Objective-C protocols.
        services[NSStringFromProtocol(proto)] = instance
    }
}

// To conform to an @objc protocol,
// a class must descend from NSObject
class Service: NSObject, ServiceType {
    func performService() -> String {
        return "I'm doing it!"
    }
}

// To conform to an @objc protocol,
// a class must descend from NSObject
class MockService: NSObject, ServiceType {
    func performService() -> String {
        return "I'm mocking you!"
    }
}

// .self is required here due to a pecularity of Swift. If there's more than one parameter,
// you have to use .self
ServiceResolver.register(ServiceType.self, instance: MockService())
// We don't need .self here because there's only one parameter, though
// you can add it if you wish.
let service: ServiceType = ServiceResolver.resolve(ServiceType)