Is this the correct place to use weak self in closure?

Question

I have a viewController that has a collectionView outlet. In the cellForRowAt method I call this code:

guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "photoCell", for: indexPath) as? MediaPhotoCell else {fatalError("Could not initialize cell")}
       cell.configureForMedia(obj, completion: { [weak self](image:UIImage?) in
            if let loadedImage = image {
                    let photo = self?.loadedPhotos[indexPath.row]
                    photo?.image = loadedImage
                    self?.loadedPhotos[indexPath.row] = photo
                    self?.photosViewController?.updateImage(for: photo)
                }
            })
return cell

I've been spending time on learning how to resolve/prevent memory leaks through retain cycles. I added a capture list [weak self] before capturing the value in the closure (image: UIIimage?). My logic is that since viewController owns the collectionview, and that collectionview owns the cell, which has a closure method in it's custom implementation (custom cell), and closure references self, i'm creating a retain cycle if i do not declare self as weak.

Is this necessary? Or is this not necessary if the outlet (the collectionView) is declared as weak?

@DanielLyon is absolutely spot on with that. You have avoided a capturing self properly — Erik
Thank you so much! Do either of you follow anyone that has a good guide to finding memory leaks/improving app performance? — leahyjwilliam
@leahyjwilliam I don't have a specific guide but one thing I can recommend is to watch the memory usage graph in Xcode while using the app. If it seems like the memory keeps increasing and never decreases when it should (e.g. when a view controller is dismissed or images are scrolled away in a table view), you might have a memory leak. — Paolo
It is not clear to me why this is needed. I am unfamiliar with configureForMedia, but usually completion blocks are not retained indefinitely and are intended to be released after the method's action is taken. In this case, it may be more beneficial to keep a strong reference to self. There is no retain cycle here. — Ian MacDonald

Daniel Lyon Daniel Lyon · Accepted Answer · 2018-08-16T20:33:56

Your example works to create a weak reference to self for use in a closure.

As to @IanMacDonald comment about it not being required for your example, he is also correct. So you would only need to do this if self could be nil in the closure.

A good example case for using [weak self] would be for an asynchronous network request, in a view controller where that request is used to populate the view. If the user backs out, we no longer need to populate the view, nor do we need a reference to the view controller.

I really liked this medium article about reference counting in closures.

Is this the correct place to use weak self in closure?

1 Answers