Observable.swift

import Foundation

let nc = NotificationCenter()

extension Notification.Name {
    public static let ObservableDidChange = NSNotification.Name("ObservableDidChange")
}

let ObservableNewValueUserInfoKey = "new"
let ObservableOldValueUserInfoKey = "old"

public class Observable<T> {
    public init(_ value: T) {
        self.value = value
    }

    public var value: T {
        didSet {
            let userInfo = [
                ObservableOldValueUserInfoKey: oldValue,
                ObservableNewValueUserInfoKey: value
            ]
            nc.post(name: .ObservableDidChange, object: self, userInfo: userInfo)
        }
    }

    public func observe(_ closure: @escaping (_ old: T, _ new: T) -> Void) -> Observer<T> {
        return Observer(observable: self, closure: closure)
    }

}

public class Observer<T> {
    let observable: Observable<T>
    let nco: NSObjectProtocol

    deinit {
        detach()
    }

    init (observable: Observable<T>, closure: @escaping (_ old: T, _ new: T) -> Void) {
        self.observable = observable

        nco = nc.addObserver(forName: .ObservableDidChange, object: observable, queue: nil) { note in
            guard let userInfo = note.userInfo,
                let oldValue = userInfo[ObservableOldValueUserInfoKey] as? T,
                let newValue = userInfo[ObservableNewValueUserInfoKey] as? T
                else {
                    return
            }

            closure(oldValue, newValue)
        }
    }

    func detach() {
        nc.removeObserver(nco)
    }
}