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Swift JsonGen

JsonGen generates source code files with decoders and encoders for parsing JSON into immutable Swift structs.


  • Generates an extension with a decodeJson and encodeJson method for each struct
  • Works on individual .swift files or whole directories
  • Handles type aliases
  • Supports primitive types, nested types and custom generic types
  • Allow for custom encoders and decoders
  • Allow for part of the datastructure to remain untyped
  • Excelent error messages when JSON decoding fails: Improving error messages

Here's an example of a nice, detailed error message:

2 errors in Blog struct
 - subTitle: Value is not of expected type String?: `42`
  posts: 2 errors in array
     [1] 1 error in Post struct
       - title: Field missing
     [2] 3 errors in Post struct
       - title: Value is not of expected type String: `1`
       - body: Value is not of expected type String: `42`
        author: 2 errors in Author struct
          - name: Field missing
          - email: Field missing

See also the blog post: Swift + JSON with code generation

CocoaHeadsNL presentation

Tom Lokhorst presented at the January 2016 CocoaHeadsNL meetup. Comparing several Json decoding libraries for Swift and talking about code generation with JsonGen.


Install the latest release from NPM:

> npm install -g swift-json-gen

Include the Statham swift library in your own project. This library contains some encoders and decoders for default Swift and Foundation types.

With CocoaPods, this can be done with:

pod 'Statham'


Assuming you have a file example/Blog.swift containing one or more structs:

struct Blog {
  let id: Int
  let name: String
  let author: String?
  let needsPassword: Bool
  let url: URL

To generate Json decoders based a file of structs run:

> swift-json-gen example/Blog.swift

This will generate the file example/Blog+JsonGen.swift with the following (truncated) content:

extension Blog {
  static func decodeJson(json: Any) throws -> Blog {
    let decoder = try JsonDecoder(json: json)

    let _id = try decoder.decode("id", decoder: Int.decodeJson)
    let _name = try decoder.decode("name", decoder: String.decodeJson)
    let _author = try decoder.decode("author", decoder: Optional.decodeJson(String.decodeJson))
    let _needsPassword = try decoder.decode("needsPassword", decoder: Bool.decodeJson)
    let _url = try decoder.decode("url", decoder: URL.decodeJson)

      let id = _id,
      let name = _name,
      let author = _author,
      let needsPassword = _needsPassword,
      let url = _url
    else {
      throw JsonDecodeError.structErrors(type: "Blog", errors: decoder.errors)

    return Blog(id: id, name: name, author: author, needsPassword: needsPassword, url: url)

  func encodeJson() -> [String: Any] {
    var dict: [String: Any] = [:]

    dict["id"] = id.encodeJson()
    dict["name"] = name.encodeJson()
    dict["author"] = author.encodeJson({ $0.encodeJson() })
    dict["needsPassword"] = needsPassword.encodeJson()
    dict["url"] = url.encodeJson()

    return dict


Include the generated YourFile+JsonGen.swift file into your project. Make sure you've also included the Statham library.

The generated encoder and decoder can be used in conjunction with NSJSONSerialization like so:

let inputStr = "{ \"title\": \"Hello, World!\", \"published\": true, \"author\": { \"first\": \"Tom\", \"last\": \"Lokhorst\" } }"
let inputData = .utf8)!
let inputObj = try! JSONSerialization.jsonObject(with: inputData, options: [])

let blog = try! Blog.decodeJson(inputObj)

let outputObj = blog.encodeJson()
let outputData = try! outputObj, options: .prettyPrinted)
let outputStr = String(data: outputData, encoding: .utf8)!


If you want to differ from the default generated code you can provide your own decodeJson or encodeJson functions. If these already exist, no new function will be generated.

You also need to provide your own functions for kinds that are not supported, like enums and classes.

How it works

This program calls the Swift compiler and dumps the parsed AST. (Using the command xcrun swiftc -dump-ast SomeFile.swift)

This AST is traversed to look for struct definitions, for each struct decodeJson and encodeJson functions is generated:

extension SomeStruct {
  static func decodeJson(_ json: Any) throws -> SomeStruct {

  func encodeJson() -> Any {


This package is written in TypeScript. To make changes to the code of JsonGen, first install TypeScript:

> npm install -g typescript

Edit the .ts files and compile the code as follows:

> tsc


  • 1.2.0 - 2017-03-09 - Swift 3.1 support
  • 1.1.0 - 2017-03-07 - Add --accessLevel flag
  • 1.0.0 - 2016-09-30 - Swift 3 support
  • 0.8.0 - 2016-09-29 - Swift 2.3 support
  • 0.7.0 - 2016-04-07 - Generate missing init
  • 0.6.0 - 2016-03-03 - Move JsonGen.swift to separate library Statham
  • 0.5.0 - 2016-02-29 - Adds --output option for providing an output directory
  • 0.4.0 - 2016-02-21 - Generate code based on JsonDecodable class
  • 0.3.0 - 2015-11-19 - Decoders with throws, instead of returning an optional
  • 0.2.2 - 2015-09-22 - Bugfix, show correct error on missing field
  • 0.2.1 - 2015-09-14 - Bugfix, now works with released Xcode
  • 0.2.0 - 2015-09-11 - Update to Swift 2
  • 0.1.3 - 2015-07-22 - Show all Swift compiler errors
  • 0.1.2 - 2015-06-01 - Support for computed properties
  • 0.1.1 - 2015-05-28 - Don't generate empty files
  • 0.1.0 - 2015-05-25 - Initial public release
  • 0.0.0 - 2014-10-11 - Initial private version for project at Q42

Licence & Credits

JsonGen is written by Tom Lokhorst of Q42 and available under the MIT license, so feel free to use it in commercial and non-commercial projects.


Generate Json encoders and decoders based on Swift structs with good error handling







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