Observable.swift

//
//  Observable.swift
//  Created by Josh Wright on 9/1/17.
//

import UIKit

class Observable<T>: NSObject {

    public typealias ValueChanged = (T) -> Void
    
    public var value: T {
        didSet {
            for obs in _observations {
                obs.response(value)
            }
        }
    }
    
    private var _observations: [Observation] = []
    
    init(value: T) {
        self.value = value
        super.init()
    }
    
    public func observe(_ valueChanged: @escaping ValueChanged) -> Observation {
        return Observation(observable: self, response: valueChanged)
    }
    
    fileprivate func remove(observation: Observation) {
        if let index = _observations.index(of: observation) {
            _observations.remove(at: index)
        }
    }
    
    class Observation: NSObject {
        
        weak var observable: Observable<T>?
        
        var response: Observable.ValueChanged
        
        init(observable: Observable<T>, response: @escaping Observable.ValueChanged) {
            self.observable = observable
            self.response = response
            super.init()
        }
        
        func cancel() {
            observable?.remove(observation: self)
        }
        
    }
}