Observable.swift

final class WeakBox<A: AnyObject> {
    weak var unbox: A?
    init(_ value: A) {
        unbox = value
    }
}

typealias Callback<T> = (T) -> ()

final class Observation<T> {

    let callback: Callback<T>
    
    typealias Deinitializer = (Observation) -> ()
    
    let deinitializer: Deinitializer?
    
    init(_ callback: @escaping Callback<T>, deinitializer: Deinitializer? = nil) {
        self.callback = callback
        self.deinitializer = deinitializer
    }
    
    deinit {
        self.deinitializer?(self)
    }
}

final class Observable<T> {
    private var observations: [WeakBox<Observation<T>>] = []
    
    func observe(callback: @escaping Callback<T>) -> Observation<T> {
        let observation = Observation(callback, deinitializer: self.remove(observation:))
        
        self.observations.append(WeakBox(observation))
        
        return observation
    }
    
    func update(with value: T) {
        for observation in observations {
            observation.unbox?.callback(value)
        }
    }
    
    func remove(observation: Observation<T>) {
        guard let index = self.observations.index(where: { return observation === $0 }) else {
            return
        }
        
        self.observations.remove(at: index)
    }
}