Relay offers a Swift, type safe way to find a conforming target along the Responder chain and invoke any protocol method on it.

Relay.swift

//
//  Relay.swift
//  XjSwiftLab
//
//  Created by Jason Jobe on 5/11/15.
//  Copyright (c) 2015 Jason Jobe. All rights reserved.
//  https://gist.github.com/wildthink/19d43871964821c4f293
//
import Foundation

/* Examle usage

    protocol SomeClassOrProtocol.self) {
        func someClassOrProtocolFunction (some:Int, args:[String])
    }

    relay (SomeClassOrProtocol.self) {
       $0.someClassOrProtocolFunction(some, args)
    }

    relay { (target: SomeClassOrProtocol) in
        target.someClassOrProtocolFunction(some, args)
    }

*/


protocol Relay
{
    func relay <T> (type: T.Type, from: Any?, @noescape call: (T) -> Void) -> Void
    
    func nextRelay() -> Relay?
    func willPerformRelay <T> (type: T.Type, sender: Any) -> Bool
}

extension Relay {

    func relay <T> (type: T.Type, from: Any? = nil, @noescape call: (T) -> Void) -> Void
    {
        if willPerformRelay(type, sender: from) {
            if let target = self as? T {
                call (target)
            }
        }
        else {
            nextRelay()?.relay(type, from: from ?? self, call: call)
        }
    }
    
    func nextRelay() -> Relay? {
        return nil
    }
    func willPerformRelay <T> (type: T.Type, sender: Any) -> Bool {
        return self is T
    }
}

#if os(iOS)
    import UIKit.UIResponder
    public typealias Responder = UIResponder
    public typealias Application = UIApplication
#else
    import AppKit.NSResponder
    public typealias Responder = NSResponder
    public typealias Application = NSApplication
#endif

extension Responder : Relay
{
    func  nextRelay() -> Relay? {
        let next = nextResponder() ?? Application.sharedApplication()
        return next
    }
}