Safer use of responder chain in Swift

gistfile1.swift

import UIKit

protocol FollowResponder: NSObjectProtocol {
	func followUser( completion:(()->())? )
	func unfollowUser( completion:(()->())? )
}

extension UIResponder {
	
	enum FollowAction: Selector {
		case FollowUser = "followUser:"
		case UnfollowUser = "unfollowUser:"
	}
	
	func targetForAction( followAction: FollowAction, withSender sender: AnyObject?) -> AnyObject? {
		return self.targetForAction( followAction.rawValue, withSender: sender )
	}
}

extension AppDelegate : FollowResponder {
	
	func followUser( completion:(()->())? ) {
		print( "Following user..." )
		completion?()
	}
	
	func unfollowUser( completion:(()->())? ) {
		print( "Unfollowing user..." )
		completion?()
	}
}

class FollowViewController: UIViewController {
	
	@IBAction func onFollowButtonPressed( sender: AnyObject! ) {
		guard let responder = self.targetForAction( .FollowUser, withSender: self ) as? FollowResponder else {
			fatalError( "Unable to find FollowResponder in chain." )
		}
		
		responder.followUser {
			print( "User has been followed." )
		}
	}
}