class ViewController: UIViewController { class Process { let processingQueue = DispatchQueue(label: "com.kishikawakatsumi.test") weak var parent: ViewController? init(parent: ViewController) { self.parent = parent } func run() { while let parent = parent { } } } lazy var process = Process(parent: self) override func viewDidLoad() { super.viewDidLoad() process.processingQueue.async { [weak self] in self?.process.run() } } }