class ViewController: UIViewController {
    struct Test {
        let value: Int
    }

    private let syncQueue = DispatchQueue(label: "sync queue")
    private var test = [String: Test](minimumCapacity: 4) {
        didSet {
            self.syncQueue.async {
                print("setting from \(oldValue)")
                let values = Array(self.test.values.map { $0.value })
                print(values)
            }
        }
    }

    var lastKey = ""
    private var timer1: Timer!

    override func viewDidLoad() {
        super.viewDidLoad()

        timer1 = Timer(timeInterval: 1, repeats: true) { (timer) -> Void in
            self.syncQueue.async {
                self.test[self.lastKey] = nil
                print("active: \(self.test.keys)")// crash on this line
            }
        }
        RunLoop.current.add(timer1, forMode: RunLoopMode.defaultRunLoopMode)

        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 3) {
            self.syncQueue.async {
                self.test["one"] = Test(value: 0)
            }
        }
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 4) {
            self.syncQueue.async {
                self.test["two"] = Test(value: 0)
            }
        }
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 5) {
            self.lastKey = "two"
        }
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 6) {
            self.lastKey = "one"
        }
    }
}

print("active: \(self.test.keys.map { $0 })")