EnvelopeDecodable.swift

import Foundation

/**
 A decodable type that expects its data to be enclosed in a
 keyed container.
 For example, using JSON, a `User` object might be delivered
 inside another object, with `"user"` as the key:
 ```
 { "user" : { "name" : "Alice", "rank" : "Colonel" } }
 ```
 */
protocol EnvelopeDecodable : Decodable {
    /** The key for this type in the enclosing container. */
    static var contentsKey: String { get }
}

/**
 Decodable representation for another decodable type
 that is delivered as a nested object in a keyed container.
 */
struct Envelope<T : EnvelopeDecodable> : Decodable {
    /** The wrapped value that we actually want to extract. */
	let contents: T

    private enum CodingKeys : CodingKey {
        case contents
        var stringValue: String { T.contentsKey }
    }
}

// Example:

struct User : EnvelopeDecodable {
    static let contentsKey = "user"

    let name: String
    let rank: String
    let favoriteCereal: String
}

let json = """
{
    "user" : {
        "name" : "Alice",
        "rank" : "Colonel",
        "favoriteCereal" : "Corn flakes"
    }
}
""".data(using: .utf8)!

let response = try! JSONDecoder().decode(Envelope<User>.self, from: json)
print(response.contents)    // User(name: "Alice", rank: "Colonel", favoriteCereal: "Corn flakes")