Swift shared data singleton with data persistence and simple callback support

SharedDataSingleton.swift

/**
Usage: var mySharedData = SharedData.instance
*/
class SharedData {
	static let instance: SharedData = {
		let instance = SharedData()
		
		// Update instance.defaultsName if desired
		
		if let defaults = NSUserDefaults.standardUserDefaults().objectForKey(instance.defaultsName) as? [String:AnyObject] {
			instance.data = defaults
		}
		
		return instance
	}()
	
	typealias Callback = (Any) -> Void
	
	internal var defaultsName = "SharedDataObject"
	private var watches       = [String:[Callback]]()
	private var data          = [String: Any]()
	
	private init() {
		// Init
	}
	
	subscript(key: String) -> Any? {
		get {
			return self.data[key] ?? nil
		}
		set(newValue) {
			if let value = newValue {
				self.data[key] = value
				NSUserDefaults.standardUserDefaults().setObject(self.data as? AnyObject, forKey: self.defaultsName)
				self.trigger(key, value: value)
			}
		}
		
	}
	
	func watch(key: String, callback: Callback) -> Void{
		if var callbacks = self.watches[key] {
			callbacks.append(callback)
		} else {
			self.watches[key] = [callback]
		}
	}
	
	func trigger(key: String, value: Any) -> Void{
		guard let callbacks = self.watches[key] else {
			return
		}
		for callback: Callback in callbacks {
			callback(value)
		}
	}
	
}