Threadsafe.swift

import Foundation

final class Threadsafe<Value> {
    init(_ value: Value) {
        self._value = value
    }
    
    var value: Value {
        return queue.sync { return _value }
    }
    
    private var _value: Value
    private let queue = DispatchQueue(label: "threadsafe")
    
    func mutate(_ work: @escaping (inout Value) -> ()) {
        queue.sync {
            work(&self._value)
        }
    }
}

var threadsafeValue = Threadsafe(3)

threadsafeValue.mutate { value in
    value += 1
}

print(threadsafeValue.value)