Swift retain cycle explanation

Question

Here is my custom view:

class CustomVIew: UIView {

    deinit {
        print("custom view deinit")
    }

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

and the 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() {

    }
}

When the controller is popped from the navigation stack, everything is fine:

B deinit
custom view deinit

But when I replace this code:

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

with this:

 customView.onTapViewHandler = didTapBlue

then, nothing is printed on the console. The CustomView and ViewController are not released, why?

Why does customView.onTapViewHandler = didTapBlue capture a reference to self?

Answer 1

Swift function is a type of closure. So like closure(Block in objective c) does functions can capture references.

when customView.onTapViewHandler = didTapBlue gets executes a reference to self ie ViewControllerB reference in this case will be captured by the function call.

Same time ViewControllerB's view holds strong reference to CustomVIew so it makes retain cycle.

About using unowned, Apple document says:

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.

That means no circular reference and retain cycle.