extension UIViewController: HasAssociatedObjects {

    // new stored property
    var storedString: String? {
        get {
            return associatedObjects.value as? String
        }
        set {
            associatedObjects.value = newValue
        }
    }

}

extension UIViewController: HasAssociatedObjects {

    var storedString: String? {
        get {
            return associatedObjects["STRING"] as? String
        }
        set {
            associatedObjects["STRING"] = newValue ?? ""
        }
    }

    var storedInt: Int {
        get {
            guard let value = associatedObjects["INT"] as? Int else {
                return 0 //< default value
            }
            return value
        }
        set {
            associatedObjects["INT"] = newValue
        }
    }

}

extension Subject: HasAssociatedObjects {
}

subject.associatedObjects.value = 10

let storedValue = subject.associatedObjects.value as? Int
XCTAssertEqual(10, storedValue)

// subject is struct
struct AnySubject {
    let identifier: Int
}

// a appropriate property that can be cleaned
var propertyOfSomeone: [Int: AssociatedObjects] = [:]

// You can customize `associatedObjects`
extension AnySubject: HasAssociatedObjects {
    var associatedObjects: AssociatedObjects {
        guard let associatedObjects = propertyOfSomeone[hashValue] else {
            let associatedObjects = AssociatedObjects()
            propertyOfSomeone[hashValue] = associatedObjects
            return associatedObjects
        }
        return associatedObjects
    }
}

extension AnySubject: Hashable {
    var hashValue: Int {
        return identifier
    }

    static func == (lhs: AnySubject, rhs: AnySubject) -> Bool {
        return lhs.hashValue == rhs.hashValue
    }
}