this.swift

struct User {}
enum ResultType<T> {}

enum Auth {
    case email(email: String, password: String)
    case facebook
    
    func login(completion: ResultType<User> -> Void) {
        switch self {
        case .email(let email, let password):
            EmailAuth.login(email, password: password, completion: completion)
        case .facebook:
            FacebookAuth.login(completion)
        }
    }
    
    func logout() {
        switch self {
        case .email:
            EmailAuth.logout()
        case .facebook:
            FacebookAuth.logout()
        }
    }
}

// MARK: Email
private struct EmailAuth {
    
    static func login(email: String, password: String, completion: (ResultType<User>) -> Void) {
        
    }
    
    static func logout() {
        
    }
}

// MARK: Facebook
private struct FacebookAuth {
    
    static func login(completion: (ResultType<User>) -> Void) {
        
    }
    
    static func logout() {
        
    }
}