PersistentReusableContainer.swift

final class Container<Key: Hashable & RawRepresentable> {
    private var storage = [Key: [ContainerItem]]() {
        didSet {
            didChange()
        }
    }
    
    init(content: [Key: [ContainerItem]] = [:]) {
        storage = content
    }
    
    func add(_ item: ContainerItem, key: Key) {
        if var current = storage[key] {
            current.append(item)
            storage[key] = current
        }
        else {
            storage[key] = [item]
        }
    }
    
    func items<T: ContainerItem>(forKey key: Key) -> [T] {
        guard let all = storage[key] else { return [] }
        return all as! [T]
    }
    
    var didChange: () -> Void = {}
}

protocol ContainerItem {
    init?(jsonData: Data)
    var jsonDataRepresentation: Data { get }
}

extension ContainerItem where Self: Codable {
    init?(jsonData: Data) {
        guard let object = try? JSONDecoder().decode(Self.self, from: jsonData) else { return nil }
        self = object
    }
    
    var jsonDataRepresentation: Data {
        return try! JSONEncoder().encode(self)
    }
}