[swift-users] Decode a JSON object of unknown format into a Dictionary with Decodable in Swift 4

Randy Eckenrode kenada at me.com
Fri Jun 23 10:46:22 CDT 2017

> On Jun 17, 2017, at 10:07 PM, Chris Anderson via swift-users <swift-users at swift.org> wrote:
> Say I have a JSON object such as:
>   {
>     "id": "4yq6txdpfadhbaqnwp3",
>     "email": "john.doe at example.com <mailto:john.doe at example.com>",
>     "name":"John Doe",
>     "metadata": {
>       "link_id": "linked-id",
>       "buy_count": 4
>     }
>   }
> And with a struct of:
> struct User: Codable {
>   var id: String
>   var email: String
>   var name: String
> }
> How can I decode the `metadata` field into a Dictionary?
> I’ve tried doing things such as, in my struct,
> var metadata: Dictionary
> or
> var metadata: [String: Any]
> But I get the error 
> MyPlayground.playground:3:7: note: cannot automatically synthesize 'Encodable' because '<<error type>>' does not conform to 'Encodable'
>   var metadata: Dictionary 
> A meta or metadata field on many APIs (such as www.stripe.com <http://www.stripe.com/>) can contain whatever you want, and I still want to be able to process it on the Swift end. How can I store that meta data field into a Dictionary that I can parse apart manually after?

It’s possible, but you have to do most of the work yourself because you the compiler can’t create implementations for you. See below for a possible implementation. Note that I just ignore types I don’t handle. I also took a stab at doing something general in this gist (https://gist.github.com/kenada/069e121371eb8db41231edfcd4bd14a8 <https://gist.github.com/kenada/069e121371eb8db41231edfcd4bd14a8>), but it doesn’t implement very robust error handling or support encoding. It also doesn’t flatten down to Any/[Any]/[String: Any] (leaving it up to the user to destructure the enum).

import Foundation

struct User: Codable {
    var id: String
    var email: String
    var name: String
    var metadata: [String: Any]

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: StaticCodingKeys.self)
        self.id = try container.decode(String.self, forKey: .id)
        self.email = try container.decode(String.self, forKey: .email)
        self.name = try container.decode(String.self, forKey: .name)
        self.metadata = try User.decodeMetadata(from: container.superDecoder(forKey: .metadata))

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: StaticCodingKeys.self)
        try container.encode(self.id, forKey: .id)
        try container.encode(self.email, forKey: .email)
        try container.encode(self.name, forKey: .name)
        try encodeMetadata(to: container.superEncoder(forKey: .metadata))

   static func decodeMetadata(from decoder: Decoder) throws -> [String: Any] {
        let container = try decoder.container(keyedBy: DynamicCodingKeys.self)
        var result: [String: Any] = [:]
        for key in container.allKeys {
            if let double = try? container.decode(Double.self, forKey: key) {
                result[key.stringValue] = double
            } else if let string = try? container.decode(String.self, forKey: key) {
                result[key.stringValue] = string
        return result

    func encodeMetadata(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: DynamicCodingKeys.self)
        for (key, value) in metadata {
            switch value {
            case let double as Double:
                try container.encode(double, forKey: DynamicCodingKeys(stringValue: key)!)
            case let string as String:
                try container.encode(string, forKey: DynamicCodingKeys(stringValue: key)!)
                fatalError("unexpected type")

    private enum StaticCodingKeys: String, CodingKey {
        case id, email, name, metadata

    private struct DynamicCodingKeys: CodingKey {
        var stringValue: String

        init?(stringValue: String) {
            self.stringValue = stringValue

        var intValue: Int?

        init?(intValue: Int) {
            self.init(stringValue: "")
            self.intValue = intValue

let userJson = """
    "id": "4yq6txdpfadhbaqnwp3",
    "email": "john.doe at example.com",
    "name":"John Doe",
    "metadata": {
      "link_id": "linked-id",
      "buy_count": 4
""".data(using: .utf8)!

let decoder = JSONDecoder()
let user = try! decoder.decode(User.self, from: userJson)
// Prints: User(id: "4yq6txdpfadhbaqnwp3", email: "john.doe at example.com", name: "John Doe", metadata: ["buy_count": 4.0, "link_id": "linked-id"])

let encoder = JSONEncoder()
let data = try! encoder.encode(user)
print(String(data: data, encoding: .utf8)!)
// Prints: {"email":"john.doe at example.com","id":"4yq6txdpfadhbaqnwp3","metadata":{"link_id":"linked-id","buy_count":4},"name":"John Doe"}

Randy Eckenrode
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