Swift Variables in Extensions

I have recently started working on a project that uses Swift2.0! I have been using Obj-C for the past 3 years, and when Swift came I wasn't one of the early adopters. Problems with the language and with SourceKit always crashing in XCode 6 didn't make for a good dev environment. Now with the 2.0 released to the wild, the promise of open source and XCode 7, Swift is good to go!

One of the first problems I had is one that happens to many iOS developers that use extensions/categories in their projects: How do I add a variable (static or not) to an extension?

Since I seem to always be googling for this one, I'll log it here for future convenience.

Static Variables

For a static variable, there is actually a really neat way:

extension UIView {
  struct Static {
    static var defaultPadding : CGFloat = 16
  }
  func widthWithPadding() -> CGFloat {
    return self.bounds.width + Static.defaultPadding
  }
}

Since you can declare structs in an extension, you can use these value objects to hold your static variables and they function the same way as declaring a static variable on the class. If you don't like the extra Static. declaration, you can create wrappers using computed variables:

extension UIView {
  var defaultPadding : CGFloat { return Static.defaultPadding }
}

Instance Variables

For instance variables the solution is to use the obj-c runtime. You need to create computed variables that get and set an associated object using objc_getAssociatedObject() and objc_setAssociatedObject(). This is the same approach taken if using it with Obj-C.

import ObjectiveC

extension UIViewController {
  struct Static {
    static var MainTitle = "\_MainTitle"
  }
  var mainTitle: String? {
    get{
      return objc_getAssociatedObject(self,&Static.MainTitle) as? String
    }
    set(value){
      if newValue = newValue {
        objc_setAssociatedObject(self,&Static.GenericTitle,newValue as NSString?,.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
      }
    }
  }
}

That way you can extend to your heart desire! If you want to understand the difference between structs and classes check out the Apple docs. And to find out more about what kind of magic you can do with obj-c runtime check out NSHipster post on it.