gistfile1.swift

//: Playground - noun: a place where people can play

import Foundation

protocol ObserverType {
	typealias Element
	
	func onNext(value: Self.Element)
	func onError()
	func onCompleted()
}

protocol ObservableType {
	typealias Element
	
	func subscribe<T : ObserverType where T.Element == Element>(observer: T)
}

class PrintingObserver<T> : ObserverType {
	typealias Element = T
	
	func onNext(value: T) {
		print("onNext \(value)")
	}
	
	func onError() {
		print("onError")
	}
	
	func onCompleted() {
		print("onCompleted")
	}
}

class NumbersObservable : ObservableType {
	typealias Element = Int
	
	func subscribe<T : ObserverType where T.Element == Int>(observer: T) {
		observer.onNext(1)
		observer.onNext(2)
		observer.onNext(3)
		observer.onCompleted()
	}
}

let numbers = NumbersObservable()
let observer = PrintingObserver<Int>()
numbers.subscribe(observer)