ThreadSafe Singleton

ThreadSafeSingleton.swift

final class FeaturedTopics {
    private let concurrentQueue = DispatchQueue(label: "concurrentQueue", attributes: .concurrent)
    private var topics: [String: Any] = [:]
    
    static let shared = FeaturedTopics()
    
    private init() {}
    
    subscript(key: String) -> Any? {
        get {
            var topic: Any?
            // conocurrent queue allows executing read operations in parallel
            concurrentQueue.sync {
                topic = self.topics[key]
            }
            return topic
        }
        set {
            // barrier turns concurrent queue into serial queue temporarily
            // As a result no other thread can modify internal dictionary while this block is executing
            concurrentQueue.async(flags: .barrier) {
                self.topics[key] = newValue
            }
        }
    }
    
    func reset() {
        topics.removeAll()
    }
}