Atomic as a property wrapper

Atomic.swift

import Dispatch
import PlaygroundSupport

@propertyDelegate
struct Atomic<A> {
  private var _value: A
  private let queue = DispatchQueue(label: "property wrapper")

  init(initialValue: A) {
    _value = initialValue
  }

  var value: A {
    queue.sync { _value }
  }

  mutating func mutate(_ transform: (inout A) -> Void) {
    queue.sync {
      transform(&_value)
    }
  }
}

class C {
  @Atomic var counter: Int = 0

  func hammer() {
    DispatchQueue.concurrentPerform(iterations: 1000) { _ in
      $counter.mutate { $0 += 1 }
    }
  }
}

PlaygroundPage.current.needsIndefiniteExecution = true

let c = C()
c.hammer()
c.$counter
print(c.counter)