Auto-implement class/struct as envelope for top level arrays in JSON by implementing this simple protocol

TopLevelCollection.swift

protocol TopLevelCollection: Codable {
    associatedtype ElementType: Codable
    var elements: [ElementType] { get set }
    init(elements: [ElementType])
}

extension TopLevelCollection {
    public init(from decoder: Decoder) throws {
        var container = try decoder.unkeyedContainer()
        var elements: [ElementType] = []
        while !container.isAtEnd {
            let subdecoder = try container.superDecoder()
            let subcontainer = try subdecoder.singleValueContainer()
            let element = try subcontainer.decode(ElementType.self)
            elements.append(element)
        }
        self.init(elements: elements)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        try container.encode(contentsOf: elements)
    }
}


/// USAGE:
struct Foo: TopLevelCollection {
    var elements: [String] 
}
// ["foo", "bar", "blah"] JSON will now be mapped to 'elements' and will be encoded back into the same format

struct Bar: TopLevelCollection {
    var elements: [BarCodable]
}

struct BarCodable: Codable {
    let name: String
}
/// [ { "name": "Jerry" }, { "name": "Bob" } ] will be mapped to 'Bar''s elements and vice-versa