Extension that adds a tag property to UIViewController

UIViewController+Tag.swift

/*
*
*   This class provides a UIViewController extension which adds a tag property to all UIViewController subclasses.
*
*/

import Foundation

private var tagAssociationKey: UInt8 = 0

extension UIViewController {
    public var tag: String! {
        get {
            return objc_getAssociatedObject(self, &tagAssociationKey) as? String
        }
        set(newValue) {
            objc_setAssociatedObject(self, &tagAssociationKey, newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN))
        }
    }
}

Objective-C?

update for Swift 5

    public var tag: String {
        get {
            return objc_getAssociatedObject(self, &tagAssociationKey) as? String ?? "❤️"
        }
        set(newValue) {
            objc_setAssociatedObject(self, &tagAssociationKey, newValue, .OBJC_ASSOCIATION_RETAIN)
        }
    }