Thread-Safe.swift

class OAO {
    let concurrent = DispatchQueue(label: "OAO", attributes: .concurrent)
    let arrayQueue = DispatchQueue(label: "OAO.array", attributes: .concurrent)

    // Thread-Safe
    private var _array: [Int] = []

    private var array: [Int] {
        get {
            arrayQueue.sync {
                _array
            }
        }
        set {
            arrayQueue.async(flags: .barrier) {
                self._array = newValue
            }
        }
    }

    // Not-Thread-Safe
//    private var array: [Int] = []

    func test() {
        concurrent.async {
            print(self.array)
        }

        concurrent.async {
            self.array = [1, 2, 3]
        }

        concurrent.async {
            print(self.array)
        }

        concurrent.async {
            self.array = [100, 10, 1]
        }

        concurrent.async {
            print(self.array)
        }
    }
}