this is a mock API server demo

Server.swift

class Server {
    private init() {}
    static let shared = Server.init()
    private static let serverQueue = DispatchQueue.global(qos: .background)
    func getCountries(completion: @escaping (Array<String>) -> Void) {
        let countries = ["France", "Germany", "Spain", "Portugal"]
        let delay = Int.random(in: 1..<4)
        Self.serverQueue.asyncAfter(deadline: .now() + .seconds(delay)) {
            completion(countries)
        }
    }
    func getCapital(of country:String, completion: @escaping (String) -> Void) {
        let capitals = ["France":"Paris", "Germany":"Berlin", "Spain":"Madrid", "Portugal":"Lisbon"]
        let delay = Int.random(in: 1..<10)
        Self.serverQueue.asyncAfter(deadline: .now() + .seconds(delay)) {
            completion(capitals[country, default:"N/A"])
        }
    }
}

share from here