ObjectAssociable.swift

//
//  ObjectAssociable.swift
//  Created by Christopher Fuller for Southern California Public Radio
//  Original version at https://github.com/SCPR/swift-experiments/blob/master/source/ObjectAssociable.swift
//  Updated by Christopher Fuller for Swift 3 compatibility
//
//  Copyright (c) 2016 Southern California Public Radio
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in all
//  copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//  SOFTWARE.
//

protocol ObjectAssociable: AnyObject {}

extension ObjectAssociable {

    func associatedObject<T>(key: UnsafeRawPointer) -> T? {
        return objc_getAssociatedObject(self, key) as? T
    }

    func associateObject<T>(_ object: T?, key: UnsafeRawPointer) {
        objc_setAssociatedObject(self, key, object, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    }

    func dissociateObject(key: UnsafeRawPointer) {
        associateObject((nil as Any?), key: key)
    }

    func dissociateObjects() {
        objc_removeAssociatedObjects(self)
    }

}

Example Usage

class Example {}

extension Example: ObjectAssociable {

    private struct Key {
        static var dateA = 0
        static var dateB = 0
    }

    var dateA: Date? { // Date is just for example, can be any type.
        get {
            return associatedObject(key: &Key.dateA)
        }
        set {
            associateObject(newValue, key: &Key.dateA)
        }
    }

    var dateB: Date? { // Date is just for example, can be any type.
        get {
            return associatedObject(key: &Key.dateB)
        }
        set {
            associateObject(newValue, key: &Key.dateB)
        }
    }

    func removeDateA() {
        dissociateObject(key: &Key.dateA)
    }

    func removeDateB() {
        dissociateObject(key: &Key.dateB)
    }

    func removeDates() {
        dissociateObjects()
    }

}