시스템에서 몇몇의 component 들이 한번 사용 될 수 있음
initializer call 은 비용이 많이 듬
추가적인 copy 를 방지하고 싶음
lazy instantiation 과 thread safety 케어할 필요가 있음

Singleton Implemention

class SingletonDatabase
{
    var captials = [String: Int]()
    static var instanceCount = 0
    
    // singleton
    static let instance = SingletonDatabase()
    
    private init()
    {
        type(of: self).instanceCount += 1
        print("initializing database")
        
        let path = "/Users/hong/capitals"
        if let text = try? String(contentsOfFile: path, encoding: .utf8)
        {
            let string = text.components(separatedBy: .newlines)
                .filter { !$0.isEmpty }
                .map { $0.trimmingCharacters(in: .whitespaces) }
            for i in 0..<string.count/2
            {
                captials[string[i*2]] = Int(string[i*2+1])!
            }
        }
    }
    
    func getPopulation(name: String) -> Int
    {
        return captials[name]!
    }
}

func main()
{
    let db = SingletonDatabase.instance
    var city = "Seoul"
    print("\(city) has population \(db.getPopulation(name: city))")
    print(SingletonDatabase.instanceCount)
    
    let db2 = SingletonDatabase.instance
    city = "Tokyo"
    print("\(city) has population \(db.getPopulation(name: city))")
    print(SingletonDatabase.instanceCount)

}

Monostate

class CEO : CustomStringConvertible
{
    private static var _name = ""
    private static var _age = 0
    
    var name: String
    {
        get { return type(of: self)._name }
        set(value) { type(of: self)._name = value }
    }
    
    var age : Int
    {
        get { return type(of: self)._age }
        set(value) { type(of: self)._age = value }
    }
    
    public var description: String
    {
        return "\(name) is \(age) years old"
    }
    
}

func main()
{
    var ceo = CEO()
    ceo.name = "Kang"
    ceo.age = 88
    
    var ceo2 = CEO()
    ceo.age = 20
    
    print(ceo)
    print(ceo2)
}

안 좋은 방식 ... 비추천

정리

• 안전한 싱글턴을 만들기는 쉬움, static property 와 그 값을 리턴하면 되니
• 하지만 테스트하기 어려움
• 싱글턴을 이용하기보단, protocol 을 이용해서 의존성을 테스트함