Implementing the Observable class

We'll implement Observable as a generic class, as it should be able to wrap any kind of object:

class Observable<Type> {
    typealias Observer = (Type) -> ()
    typealias Token = NSObjectProtocol
    
    private var observers = [(Token, Observer)]()
    
    var value: Type {
        didSet {
            notify()
        }
    }
    
    init(_ value: Type) {
        self.value = value
    }

    @discardableResult
    func bind(_ observer: @escaping Observer) -> Token {
        defer { observer(value) }
        let obj = NSObject()
        observers.append((obj, observer))
        return obj
    }

    func unbind(_ token: Token) {
        observers.removeAll { $0.0.isEqual(token) }
    }
    
    private func notify() {
        observers.forEach { (_, observer) in
            observer(value)
        }
    }
}

We can demonstrate the capabilities of this Observer<Type> object through the following snippet:

let observable = Observable("")
let token0 = observable.bind {
    print("Changed 0: \($0)")
}
observable.value = "OK"
let token1 = observable.bind {
    print("Changed 1: \($0)")
}
observable.unbind(token1)
observable.value = "NOK"

// Output:
Changed 0:             // initial binding
Changed 0: OK          // the value is set to OK, notifying
Changed 1: OK          // initial binding of second observer
Changed 0: NOK         // The value is set to NOK, notifying

With the Observable<Type> class, we are able to subscribe to changes occurring at the model layer. The next step is to bind the observable to UIKit elements, which can have their values change.