FriendProtocol.swift

import UIKit

/*
  Protocol Definition: A protocol defines a blueprint of methods, properties, and other requirements that suit a particular 
  task or piece of functionality.
  (https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html)
*/

/*
  This protocol, GreetFriendDelegate, requires any conforming type to have an instance method called greetFriend, which 
  returns a String value whenever it’s called
*/
protocol GreetFriendDelegate {
    func greetFriend() -> String
}

/*
  This is the initial view controller. I want to navigate to my FriendViewController and have Liam greet me!
*/
class MeViewController: UIViewController, GreetFriendDelegate {
    let myName = "Alec"
    
    // Im getting ready to navigate to my FriendViewController... //
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if let destination = segue.destination as? FriendViewController {
            print("Navigating to FriendViewController...")
            destination.delegate = self
            print("Set MeViewController as the delegate for FriendViewController.")
        }
    }
    
    /*
      You need to implement your function definition here to CONFORM to the protocol. When my friend Liam is ready, 
      he will use a button to greet me! Liam needs to know my name, though. So I'll remind him by sending it to 
      him when he asks for it.
    */
    func greetFriend() -> String {
        print("I am a protocol method getting called from FriendViewController.")
        return self.myName
    }
}

/*
  This is the secondary view controller that is reached by navigating from the MeViewController.
*/
class FriendViewController: UIViewController {
    let myName = "Liam"
    /* 
      We set our delegate property here to have reference to the MeViewController so we can ask it for the name to greet 
      when Liam needs it. 
    */
    var delegate: GreetFriendDelegate?
    
    // Liam needs his friends name here! Hurry, ask MeViewController! //
    @IBAction func greetFriendAction(_ sender: Any) {
        // Liam is asking MeViewController who he should greet //
        let myFriend = self.delegate?.greetFriend()
        print("Hello, \(myFriend)! My name is \(self.myName)")
    }
}