Swifty NSMapTable that can deal with weak referenced object - subclassing edition

ZMapTable.swift

//  ZMapTable.swift
//
//  Copyright (c) 2016 Kaz Yoshikawa.  Released under MIT License.


class ZMapTable<K: AnyObject, V: AnyObject>: CustomStringConvertible {
	let mapTable: NSMapTable
	private init(mapTable: NSMapTable) {
		self.mapTable = mapTable
	}
	subscript(key: K) -> V? {
		get { return self.mapTable.objectForKey(key) as? V }
		set { self.mapTable.setObject(newValue, forKey: key) }
	}
	var description: String {
		return mapTable.description
	}
}

class ZStrongToWeakMapTable<K: AnyObject, V: AnyObject>: ZMapTable<K, V> {
	init() {
		super.init(mapTable: NSMapTable.strongToWeakObjectsMapTable())
	}
}

class ZWeakToStrongMapTable<K: AnyObject, V: AnyObject>: ZMapTable<K, V> {
	init() {
		super.init(mapTable: NSMapTable.weakToStrongObjectsMapTable())
	}
}

class ZWeakToWeakMapTable<K: AnyObject, V: AnyObject>: ZMapTable<K, V> {
	init() {
		super.init(mapTable: NSMapTable.weakToWeakObjectsMapTable())
	}
}

class ZStrongToStrongMapTable<K: AnyObject, V: AnyObject>: ZMapTable<K, V> {
	init() {
		super.init(mapTable: NSMapTable.strongToStrongObjectsMapTable())
	}
}

Sample (note: String is not AnyObject use NSString)

var mapTable = ZWeakToStrongMapTable<NSString, NSString>()
var alice: NSString? = "Alice"
var bob: NSString? = "Bob"
mapTable[alice!] = toronto
mapTable[bob!] = newyork
print(mapTable.description) // [Bob -> New York, Alice -> Toronto]

alice = nil
print(mapTable.description)  // Alice is till in the map table

dispatch_async(dispatch_get_main_queue()) {
    print(self.mapTable.description) // sometime later alice is gone
}