Swift thread-safe dependency injection

thread-safe-dependency-injection.swift

import Foundation

protocol HatType { }

class Hat: HatType {
    init() {
        print("real hat 1")
    }
}

class Hat2: HatType {
    init() {
        print("real hat 2")
    }
}

struct HatInjector {
    private static let queue = DispatchQueue(label: "HatInjector.queue")
    
    private static var _hat: HatType?
    static var hat: HatType {
        get {
            guard _hat == nil else { return _hat! }
            queue.sync {
                guard _hat == nil else { return }
                _hat = Hat2()
            }
            return _hat!
        }
        set { queue.sync { _hat = newValue } }
    }
}

DispatchQueue.global().async {
    DispatchQueue.concurrentPerform(iterations: 1000) { _ in
        HatInjector.hat
    }
}

DispatchQueue.global().async {
    DispatchQueue.concurrentPerform(iterations: 1000, execute: { _ in
        HatInjector.hat = Hat()
    })
}