Swift objc_getAssociatedObject always nil

Question

I am trying to associated a property to an Array Extension:

private var AssociatedObjectHandle: String = "BlaBLabla"
extension Array {
  var emptyIndex:Int {
    mutating get {
      if let object = objc_getAssociatedObject(self, &AssociatedObjectHandle) {
        return object as! Int
      }
      let index = self.searchEmptyIndex()
      self.emptyIndex = index

      return index
    }
    set {
      let new = (newValue as NSInteger)
      objc_setAssociatedObject(self, &AssociatedObjectHandle, new, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
    }
  }

  func searchEmptyIndex() -> Int {
      if let arr = self as? [Int] {
        return arr.index(of: -1)!
      }

      return -1
  }
}

The objc_getAssociatedObject call always returns nil!!

Anyone has any idea why?

I am banging my head for the last hour about this...

is `searchEmptyIndex` API your code too? Could you include that in your question? — Michael Dautermann, Aug 24 '17 at 10:43
I'm assuming `searchEmptyIndex` lives within your Array extension, right? In your code up there it's outside the extension. — Michael Dautermann, Aug 24 '17 at 10:58

Martin R · Accepted Answer · 2017-08-24T11:24:08.273

12

You cannot add associated objects to a Swift Array (or any Swift value type). objc_setAssociatedObject() and objc_getAssociatedObject() are from the Objective-C runtime, they expect an instance of NSObject as first argument.

Your code compiles and runs only because any Swift value is automatically bridged to an object if necessary.

When you call objc_setAssociatedObject(self, ...) then self is bridged to a (temporary) instance of NSArray, and the association is made on that object.
Later, when objc_getAssociatedObject(self, ...) is called, another (temporary) instance of NSArray is created, and that has no associated object.

That's why you get nil as the result.

edited Aug 24 '17 at 11:24

answered Aug 24 '17 at 11:13

Martin R

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So how can I add properties on Swift type extensions? – YanivH Aug 24 '17 at 11:24
3

@YanivH: You can't. – Martin R Aug 24 '17 at 11:24
And you cannot subclass an Array as well in Swift? – YanivH Aug 24 '17 at 11:47
2

@YanivH: No, `struct Array` is a value type, not a class. – Martin R Aug 24 '17 at 11:49

Swift objc_getAssociatedObject always nil

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