Associated.swift

import Foundation

private class Associated<Type>: NSObject {
    let value: Type
    init(_ value: Type) {
        self.value = value
    }
}

protocol Associable {}

extension Associable where Self: AnyObject {
    
    func getAssociatedObject<T>(_ key: UnsafeRawPointer) -> T? {
        return (objc_getAssociatedObject(self, key) as? Associated<T>).map { $0.value }
    }
    
    func setAssociatedObject<T>(_ key: UnsafeRawPointer, _ value: T?) {
        objc_setAssociatedObject(self, key, value.map { Associated<T>($0) }, .OBJC_ASSOCIATION_RETAIN)
    }
    
}

extension NSObject: Associable {}