VIPER.swift

import UIKit

struct Person { // Entity (usually more complex e.g. NSManagedObject)
    let firstName: String
    let lastName: String
}

struct GreetingData { // Transport data structure (not Entity)
    let greeting: String
    let subject: String
}

protocol GreetingProvider {
    func provideGreetingData()
}

protocol GreetingOutput: class {
    func receiveGreetingData(greetingData: GreetingData)
}

class GreetingInteractor : GreetingProvider {
    weak var output: GreetingOutput!
    
    func provideGreetingData() {
        let person = Person(firstName: "David", lastName: "Blaine") // usually comes from data access layer
        let subject = person.firstName + " " + person.lastName
        let greeting = GreetingData(greeting: "Hello", subject: subject)
        self.output.receiveGreetingData(greeting)
    }
}

protocol GreetingViewEventHandler {
    func didTapShowGreetingButton()
}

protocol GreetingView: class {
    func setGreeting(greeting: String)
}

class GreetingPresenter : GreetingOutput, GreetingViewEventHandler {
    weak var view: GreetingView!
    var greetingProvider: GreetingProvider!
    
    func didTapShowGreetingButton() {
        self.greetingProvider.provideGreetingData()
    }
    
    func receiveGreetingData(greetingData: GreetingData) {
        let greeting = greetingData.greeting + " " + greetingData.subject
        self.view.setGreeting(greeting)
    }
}

class GreetingViewController : UIViewController, GreetingView {
    var eventHandler: GreetingViewEventHandler!
    let showGreetingButton = UIButton()
    let greetingLabel = UILabel()

    override func viewDidLoad() {
        super.viewDidLoad()
        self.showGreetingButton.addTarget(self, action: "didTapButton:", forControlEvents: .TouchUpInside)
    }
    
    func didTapButton(button: UIButton) {
        self.eventHandler.didTapShowGreetingButton()
    }
    
    func setGreeting(greeting: String) {
        self.greetingLabel.text = greeting
    }
    
    // layout code goes here
}
// Assembling of VIPER module, without Router
let view = GreetingViewController()
let presenter = GreetingPresenter()
let interactor = GreetingInteractor()
view.eventHandler = presenter
presenter.view = view
presenter.greetingProvider = interactor
interactor.output = presenter