Advanced Swift: Values and References, Episode 7: Challenge: COW | Kodeco, the new raywenderlich.com

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1

In this challenge you will try to implement a copy-on-write that reduces reference counting traffic.

This is a companion discussion topic for the original entry at https://www.kodeco.com/1940309-advanced-swift-values-and-references/lessons/7
2

Hi Ray and team,

Why do we make the copy inside of the getter rather than the setter? Wouldn’t that make a copy on read rather than on write?

Thank you!

3

Sorry, I get it now. We only use _mustatingStorage computed property inside of the setters. I guess the alternative would be something like this:

struct InventoryItem {
  private var storage: Storage

  private final class Storage {
    //...
  }

  init(name: String, cost: String, barcode: String, color: UIColor, images: [UIImage], comment: String) {
    self.storage = Storage(name: name, cost: cost, barcode: barcode, color: color, images: images, comment: comment)
  }

  var name: String {
    get { storage.name }
    set {
      if !isKnownUniquelyReferenced(&storage) {
        storage = Storage(name: newValue, cost: storage.cost, barcode: storage.barcode, color: storage.color, images: storage.images, comment: storage.comment)
        return
      }
      storage.name = newValue
    }
  }

// ...
}

Then we only need a single private property, but it requires a lot more code as the isKnownUniquelyReferenced logic needs to be repeated for each property.

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