"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.

Academic

They can't really. SCNNode is probably an Obj-C extension and this is how the Swift interface got generated. It's not valid Swift really. – Gradation

It can be a computed property, they look the same as stored properties in the generated interface. – Equalitarian

Answer 1

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

Beriosova

Answer 2

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)
        }
    }
}

Kolyma

Answer 3

Funny I have the same feeling as the OP. Check this blog post by Apple itself: https://developer.apple.com/swift/blog/?id=37

You will notice they clearly break their only rules, and they explain how to use JSON with a code that isn't compilable.

extension Restaurant {
    private let urlComponents: URLComponents // base URL components of the web service
    private let session: URLSession // shared session for interacting with the web service

    static func restaurants(matching query: String, completion: ([Restaurant]) -> Void) {
        var searchURLComponents = urlComponents
        searchURLComponents.path = "/search"
        searchURLComponents.queryItems = [URLQueryItem(name: "q", value: query)]
        let searchURL = searchURLComponents.url!

        session.dataTask(url: searchURL, completion: { (_, _, data, _)
            var restaurants: [Restaurant] = []

            if let data = data,
                let json = try? JSONSerialization.jsonObject(with: data, options: []) as? [String: Any] {
                for case let result in json["results"] {
                    if let restaurant = Restaurant(json: result) {
                        restaurants.append(restaurant)
                    }
                }
            }

            completion(restaurants)
        }).resume()
    }
}

That entire article is about how to handle JSON in Swift, so "Restaurants" is not defined in any Objective-C code.

Towhee

Answer 4

in Swift 5, Xcode 13 I was able to add the property directly to the extension using static (no need to create an inner struct etc..)

For example:

extension String {
   static var onboarded = "onboarded"
   static var locationAllowed = "locationAllowed"
}

Usage:

@AppStorage(.locationAllowed) var locationAllowed = false
@AppStorage(.onboarded) var a = false

@AppStorage need a "String" as a parameter, so I just used .onboarded from the Extension

Owlish

Answer 5

You may try this code, I'm using Xcode 8.3.3 and Swift 3.1:

import SceneKit
import GameplayKit

extension SCNNode {

    @available(OSX 10.13, *)
    public var entity: GKEntity? {
        return self.entity
    }
}

Lust

get only properties are allowed – Demmy