self 何时保留闭包？

Question

我一直在阅读内存管理一段时间，我知道当闭包保留 self 和 self 保留闭包时，我们应该放 [weak self] 来打破循环，我的问题是闭包什么时候保留 self 什么时候 self 保留一个关闭？

Answer 1

It's usually pretty obvious, though not always.

• The closure retains thing if it mentions thing or any property of thing .

• thing retains the closure if it has a property that is or contains a function and that function is the closure.

The artificial example I use in my book is:

class FunctionHolder {
    var function : (() -> ())?
}
func testFunctionHolder() {
    let fh = FunctionHolder()
    fh.function = {
        print(fh)
    }
}

Here, thing is fh , a FunctionHolder instance. We assign to the function property of fh a closure that mentions fh . That's a retain cycle.

The problem in real life is that this happens without your realizing it. You know you're retaining something , but you might not realize that the something is or contains a closure. The classic example is when you call

let ob = NotificationCenter.default.addObserver(forName:...

That call returns an observer object, which you retain so that you will get notifications. But you also hand that call a closure, and if it mentions you, you've got a retain cycle, because you are retaining the observer which contains the closure which mentions you.

And there are a number of other common pitfall situations of that sort.

Answer 2

Here is an example:

class ViewController: UIViewController {

    var data = Data() {
        didSet {
            // do something
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        Service.loadData { data in
            self.data = data
        }
        perform(#selector(tryToDismiss), with: self, afterDelay: 0.5)
    }

    @objc func tryToDismiss() {
        dismiss(animated: true)
    }

    deinit {
        print("ViewController got deinit")
    }
}

class Service {

    static func loadData(completion: @escaping (Data) -> Void) {
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            completion(Data())
        }
    }
}

You can see that even though the view controller is getting dismissed after 0.5 seconds of getting loaded it's not getting deinitialized because of the closure that completes the load data after 1 second.