Is it necessary to use [unowned self] in closures

问题:

    UIView.animateWithDuration(1,
        animations: { [unowned self] in
            self.box.center = self.boxTopRightPosition
        },
        completion: { [unowned self] completed in
            self.box.hidden = true
    })

Is it necessary to avoid memory leak?

回答1:

No, it is not needed in this case. animations and completion are not retained by self so there is no risk of strong retain cycle.

回答2:

Well, "necessary" isn't the same as "recommended". If your question is if it's necessary then @Kirsteins' response is fine, however imagine the situation where you want to animate something in your view controller after some work, but your view controller has been released (because it is not in the view hierarchy anymore or any other reason). In this case, if you don't use [weak self], your view controller won't get released until finishing the animation because you are retaining it in the animation block, but does it make sense to keep it retained until animating something which is not in the view anymore?

So, in few words, you don need to use a weak reference to self when animating UIKit, however, you don't need to keep your view retained if it's released, because an animation with no view doesn't make sense, so using weak is a good option.

回答3:

@Plabo, as @Kirsteins said, animations and completion are not retained by self, so even if you start a animation and for any reason your view controller has been released, it will deallocated instantaneously. So, you don't need captured 'self'. Consider the silly example bellow:

class ViewController: UIViewController {

    @IBOutlet weak var button : UIButton!

    override func viewDidLoad() {
        super.viewDidLoad()

        print("viewDidLoad ViewController")
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        UIView.animate(withDuration: 20, animations: {
            self.button.frame = CGRect(x: 0, y: 300, width: 30, height: 30)
        }) { finished in
            self.button.frame = CGRect(x: 0, y: 100, width: 30, height: 30)
        }
    }

    deinit {
        print("deinit ViewController")
    }

}

As soon as it be deallocated, the deinitializer will be called and the completion will never be executed.

回答4:

No it's not needed. As Kirsteins says:

No, it is not needed in this case. animations and completion are not retained by self so there is no risk of strong retain cycle.

But lhmgrassi says:

As soon as it be deallocated, the deinitializer will be called and the completion will never be executed.

I don't think this is true. The completion block will always be called. And if you use a strong self your object won't be deallocated until the completion block is executed.

However, if you use a [weak self], your object is not (temporary) retained by the completion block and might be deallocated before the completion block is fired. The completion block is still fired but self is already nil.

If you use a [unowned self] in your completion handler, you object might also be deallocated before the completion handler is called, which could result in a crash!

I've made an example illustrating this.

Full source can be found on Github

回答5:

Just the opposite. You want self to continue to exist long enough for the completion block to be called. Therefore having self be strong and retained through the escaping completion handler is a good thing.

The worry that usually leads people to use weak self is a retain cycle. But this isn't that. A retain cycle is when self retains the closure which retains self, causing a leak because now self can never be released. But this is not that situation at all. the closure, and therefore self, is being retained, but not by self! So there is some retaining going on, temporarily, but it is good, not bad.