Objective-C Associated Objects vs NSMapTable

Question

I recently discovered NSMapTable (doc, nshipster) and I have been wondering whether it can be used in place of associated objects.

Here's an example using standard associated objects:

var fooKey = "foo"
extension UIView {

  var foo: UIImage? {
    set {
      objc_setAssociatedObject(self, &fooKey, newValue, .OBJC_ASSOCIATION_RETAIN)
    }
    get {
      return objc_getAssociatedObject(self, &fooKey) as? UIImage
    }
  }
}

versus an implementation using NSMapTable:

let barTable = NSMapTable<UIView, UIImage>(keyOptions: [.weakMemory], valueOptions: [.strongMemory])

extension UIView {

  var bar: UIImage? {
      get {
        return barTable.object(forKey: self)
      }
      set {
        barTable.setObject(newValue, forKey: self)
      }
  }
}

I tried to google and understand the differences with no luck, as I don't even know how I can compare the two implementations.

How can I compare them? What are the differences?

Thanks

Answer 1

When an object is deallocated, all objects that are associated with it using OBJC_ASSOCIATION_RETAIN will be released (and sometimes deallocated if that was the last strong reference).

Your NSMapTable example won't clean up on dealloc.

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Beyond that, they are functionally the same, but have different implementation details. The most significant is the threading policy; NSMapTable is not thread safe whereas OBJC_ASSOCIATION_RETAIN is treated the same as an @property(atomic, strong).