Observable.swift

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

import Cocoa

class Observation<T>: Hashable {
    var hashValue: Int {
        return ObjectIdentifier(self).hashValue
    }
    
    static func ==(lhs: Observation<T>, rhs: Observation<T>) -> Bool {
        return lhs === rhs
    }
    
    let observer: (T) -> ()
    
    init(observer: @escaping (T) -> ()) {
        self.observer = observer
    }
}

class Observable<T> {
    var value: T {
        didSet {
            for observation in observations {
                observation.observer(value)
            }
        }
    }
    private var observations: Set<Observation<T>> = []
    
    init(value: T) {
        self.value = value
    }
    
    func observe<U>(_ keyPath: KeyPath<T, U>,
                    callback: @escaping  (U) -> ()) -> (() -> ()) {
        observations.insert(Observation<T> {
            callback($0[keyPath: keyPath])
        })
        
        return {}
    }
}

struct User {
    let name: String
    let age: Int
}

let observableUser = Observable(value: User(name: "Jonh", age: 35))

let nameObservation = observableUser.observe(\.name) { name in
    print("New name:", name)
}

let ageObservation = observableUser.observe(\.age) { age in
    print("New age:", age)
}

observableUser.value = User(name: "Dow", age: 40)