compile_time_stubbing.swift

import UIKit

class User {
    
}

enum ResultType<T> {
    case success(T)
    case error(Error)
}

class DAO {
    
}

class UserDAO: DAO {
    
    func getAll(completion: @escaping (ResultType<[User]>) -> Void) {
    #if TEST
        print("Stubbed data from the mocks")
    #else
        print("Real data from the server")
    #endif
    }
}

let dao: UserDAO = UserDAO()
dao.getAll(completion: { _ in })

//Depending if the flag is set or not at compile time, the result will be diferent