Volatile Resource Wrapper inspired by Siesta

Resource.swift

//
//  Resource.swift
//
//  Copyright (c) 2019 Jason Jobe. All rights reserved.
//  https://gist.github.com/wildthink/567608a37253da877eb6a2f69d7e99fc
//  Inspired by https://bustoutsolutions.github.io/siesta/
//  Created by Jason Jobe on 5/11/19.
//
import Foundation

public class Resource<C: Codable> {
    public enum State { case ready, requested, failed, received }
    
    public let name: String
    public let url: URL
    public private(set) var value: C?
    public private(set) var timestamp: Date?
    public private(set) var last_error: Error?
    public private(set) var state: State = .ready
    
    public init (_ name: String, url: URL) {
        self.name = name
        self.url = url
    }
    
    public func decode(response: URLResponse?, data: Data) throws -> C? {
        // FIXME: Confirm its really JSON
        return try JSONDecoder().decode(C.self, from: data)
    }
    
    public func fetch (complete: ((Resource, URLResponse?) -> Void)? = nil) {
        guard state != .requested else { return }
        
        let task = URLSession.shared.dataTask(with: url) { [weak self]
            (data, response, error) in
            guard let self = self else { return }
            if let data = data {
                do {
                    self.value = try self.decode(response: response, data: data)
                }
                catch {
                    self.last_error = error
                }
            } else {
                self.last_error = error
            }
            self.state = (self.last_error == nil ? .received : .failed)
            self.timestamp = Date()
            complete?(self, response)
        }
        state = .requested
        timestamp = Date()
        task.resume()
    }
}