SortedDic.swift

//: A UIKit based Playground for presenting user interface
  
import UIKit
import Foundation

struct SortedDic: Sequence, IteratorProtocol {

    public init() { }

    public init(order: [String], dic: [String:Int]) {
        self.order = order
        self.dic = dic
    }

    private var index = 0
    private var dic: [String:Int] = [:]
    private var order: [String] = []

    func makeIterator() -> SortedDic {
        return SortedDic(order: order, dic: dic)
    }

    mutating func next() -> (String, Int)? {
        guard index < order.count else { return nil }
        let key = order[index]
        guard let value = dic[key] else { return nil }
        index += 1
        return (key, value)
    }

    subscript(key: String) -> Int? {
        get {
            return dic[key]
        }
        set(newValue) {
            dic[key] = newValue
            if newValue != nil {
                order.append(key)
            } else {
                order = order.filter({ $0 != key })
            }
        }
    }
}

var dic = SortedDic()
dic["A"] = 1
dic["B"] = 2
dic["C"] = 3
dic["B"] = nil

dic.forEach { (some) in
    print(some)
}