Problem Statement

Demonstrate how to dynamically add variables to an object (a class instance) at runtime. This is useful when the methods/variables of an instance are based on a data file that isn't available until runtime. Hal Fulton gives an example of creating an OO CSV parser at An Exercise in Metaprogramming with Ruby. This is referred to as "monkeypatching" by Pythonistas and some others.

Let's start with the solution:

import Foundation
 
let fooKey = UnsafeMutablePointer<UInt8>.alloc(1)
 
class MyClass { }
let e = MyClass()
 
// set
objc_setAssociatedObject(e, fooKey, 1, .OBJC_ASSOCIATION_RETAIN)
 
// get
if let associatedObject = objc_getAssociatedObject(e, fooKey) {
  print("associated object: \(associatedObject)")
} else {
  print("no associated object")
}