Singleton Pattern in Swift

singleton.swift

//
//  UserSingleton.swift
//  
//
//  Created by Jean-David Morgenstern-Peirolo on 7/30/18.
//

import Foundation

final class UserSingleton {
    
    // A static type property (constant or variable) is guaranteed to be lazily initialized only once, even when accessed across multiple threads simultaneously
    public static let sharedInstance: UserSingleton = {
        let instance = UserSingleton()
        return instance
    }()
    
    private init() {}
    
    private var _count: Int = 0
    
    private var _likes: Int = 0
    
    private let serialQueue = DispatchQueue(label: "UserSingletonSerialQueue")
    
    private let concurrentQueue = DispatchQueue(label: "UserSingletonConcurrentQueue", qos: .default, attributes: .concurrent)
    
    // https://developer.apple.com/videos/play/wwdc2016/720/
    public var count: Int {
        get {
            return serialQueue.sync {
                _count
            }
        }
        set (newCount) {
            // serial queue -> safe writes!
            serialQueue.async {
                self._count = newCount
            }
        }
    }
    
    public var likes: Int {
        get {
            // Read concurrently
            return concurrentQueue.sync {
                _likes
            }
        }
        set (newLikes) {
            // A dispatch barrier ensures that a piece of code will be executed, and while it’s being executed, no other piece of code will be executed
            // Writes are executed exclusively
            concurrentQueue.async(flags: .barrier) {
                self._likes = newLikes
            }
        }
    }
    
}