"Extensions may not contain stored properties" unless your are Apple? What am I missing?

Question

How come Apple can do this:

import CoreGraphics
import GameplayKit
import simd

/**
 @header


 SceneKit framework category additions related to GameplayKit integration.


 @copyright 2017 Apple, Inc. All rights reserve.

 */

extension SCNNode {


    /**
     * The GKEntity associated with the node via a GKSCNNodeComponent.
     *
     * @see GKEntity
     */
    @available(OSX 10.13, *)
    weak open var entity: GKEntity?
}

/**
 * Adds conformance to GKSceneRootNodeType for usage as rootNode of GKScene 
 */
extension SCNScene : GKSceneRootNodeType {
}

... and I cannot do this:

extension SCNNode {
    weak open var ntity: GKEntity?
}

and get two errors:

• 'weak' may only be applied to class and class-bound protocol types, not '<<error type>>'
• Extensions may not contain stored properties

What I would like to actually do is to provide an entity property on OSX versions before 10.13, so additional suggestions for that are also welcome.

Answer 1

Swift 4 / iOS 11 / Xcode 9.2

The answers here are technically right, but here's a solution anyway.

In your extension, define a private struct with the fields you're going to want. Make everything static, like this:

private struct theAnswer {
    static var name: String = ""
}

I know, I know, static means it's a class-wide variable/property, not an instance one. But we are not actually going to store anything in this struct.

In the code below, obj_getAssociatedObject and objc_setAssociatedObject both require an UnsafeRawPointer as a key. We use these functions to store a key/value pair which is associated with this unique instance.

Here's the complete example with a String:

import Foundation

class ExtensionPropertyExample {

    // whatever
}


extension ExtensionPropertyExample {
    private struct theAnswer {
        static var name: String = ""
    }

    var name: String {
        get {
            guard let theName = objc_getAssociatedObject(self, &theAnswer.name) as? String else {
                return ""
            }
            return theName
        }
        set {
            objc_setAssociatedObject(self, &theAnswer.name, newValue, .OBJC_ASSOCIATION_RETAIN)
        }
    }
}

Answer 2

This is currently not possible in Swift. As noted by Sulthan this is an Objective-C category for which you see the Swift version, which is generated by Xcode.

Now, Objective-C does not easily support adding properties in categories (extensions are called categories in Objective-C), but you can use associated objects to get what you want.

Mattt Thompson has a great article about associated objects on his NSHipster blog: Associated Objects - NSHipster