Is it required to to use weak reference's within a singleton class?

Question

I came across a tutorial from raywenderlich were the author gave some good tips on handling threading issues in singleton. But when using closures from within the singleton class he is using 'weak' reference cycle. Is it really required so since the class is a singleton, it should have a single instance always right?

    final class PhotoManager {
      private init() {}
      static let shared = PhotoManager()

      private var unsafePhotos: [Photo] = []

    let concurrentPhotoQueue = DispatchQueue(label: "com.jeesson.googlypuff.photoQueue", attributes: .concurrent)
      var photos: [Photo] {
        var photoCopy:[Photo]!    
        concurrentPhotoQueue.sync {
            photoCopy = self.unsafePhotos
        }
        return photoCopy
      }

      func addPhoto(_ photo: Photo) {

// Do we need 'weak self here.. and why?
        concurrentPhotoQueue.async(flags: .barrier) {[weak self] in
            // 1
            guard let self = self else {
                return
            }
            self.unsafePhotos.append(photo)
            DispatchQueue.main.async { [weak self] in
                //self?.postContentAddedNotification()
            }
        }



      }

    }

The tutorial

Answer 1

In case of DispatchQueue closures don't add any capture list at all, nowhere.

DispatchQueue closures don't cause retain cycles because self doesn't own them.

Basically capture lists inside a singleton object are not needed as the singleton is never going to be deallocated.