ios - Swift 保留周期解释

Question

这是我的自定义 View :

class CustomVIew: UIView {

    deinit {
        print("custom view deinit")
    }

    var onTapViewHandler: (()->Void)?
}

和 View Controller :

class ViewControllerB: UIViewController {

    var customView: CustomVIew!

    deinit {
        print("B deinit")
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        let customView = CustomVIew()
        customView.onTapViewHandler = { [unowned self] in
            self.didTapBlue()
        }
        customView.frame = CGRect(x: 50, y: 250, width: 200, height: 100)
        customView.backgroundColor = UIColor.blueColor()
        view.addSubview(customView)

        self.customView = customView

    }

    func didTapBlue() {

    }
}

当 Controller 从导航堆栈中弹出时，一切正常:

B deinit
custom view deinit

但是当我替换这段代码时:

customView.onTapViewHandler = { [unowned self] in
     self.didTapBlue()
}

用这个:

 customView.onTapViewHandler = didTapBlue

然后，控制台上不会打印任何内容。 CustomView 和 ViewController 没有发布，为什么？

为什么 customView.onTapViewHandler = didTapBlue 捕获对 self 的引用？

Answer 1

Swift 函数是一种闭包。所以像 closure(Block in objective c) does functions can capture references.

当 customView.onTapViewHandler = didTapBlue 执行对 self 的引用时，即 ViewControllerB 引用在这种情况下将被函数调用捕获。

同时 ViewControllerB 的 View 持有对 CustomVIew 的强引用，因此它形成了保留循环。

关于使用unowned，Apple document说:

Weak and unowned references enable one instance in a reference cycle to refer to the other instance without keeping a strong hold on it. The instances can then refer to each other without creating a strong reference cycle.

这意味着没有循环引用和保留周期。