Swift extensions *can* add stored properties

Ok, the title is a complete lie: Swift extensions can only add computed properties. But you can got something just as good, if you’re willing to use Objective-C associated objects. There’s a lot of boilerplate, though, and if you do it the obvious way you sacrifice type-safety. Can we do better?

Yes we can. Here are a couple of little functions that, by the magic of Swift’s type-inference, let you write type-safe non-Optional computed properties that act just like stored properties, in a class or protocol extension.

import Foundation
func associatedObject<ValueType: AnyObject>(
        base: AnyObject, 
        key: UnsafePointer<UInt8>, 
        initialiser: () -> ValueType) 
        -> ValueType {
    if let associated = objc_getAssociatedObject(base, key) 
        as? ValueType { return associated }
    let associated = initialiser()
    objc_setAssociatedObject(base, key, associated, 
                             .OBJC_ASSOCIATION_RETAIN)
    return associated
}
func associateObject<ValueType: AnyObject>(
        base: AnyObject, 
        key: UnsafePointer<UInt8>, 
        value: ValueType) {
    objc_setAssociatedObject(base, key, value, 
                             .OBJC_ASSOCIATION_RETAIN)
}

That’s all. You use them like so:

class Miller {} // Here's the class we will extend
class Cat { // Every Miller should have a Cat
    var name = “Puss”
}
private var catKey: UInt8 = 0 // We still need this boilerplate
extension Miller {
    var cat: Cat { // cat is *effectively* a stored property
        get { 
            return associatedObject(self, key: &catKey) 
                { return Cat() } // Set the initial value of the var
        }
        set { associateObject(self, key: &catKey, value: newValue) }
    }
}

If you’re following along at home, you can drop the following lines in a playground to confirm that it works as expected.

let grumpy = Miller()
grumpy.cat.name // shows "Puss"
grumpy.cat.name = “Hephaestos”
grumpy.cat.name // shows "Hephaestos"

Easy as that!

Credit where credit is due: at least half of this implementation comes from Eric-Paul Lecluse. He used associated objects in an extension, I extracted the pattern into a reusable class, then together we rendered the class down into the two little functions you see above. The writeup, and any errors it contains, is my own.