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

Features

• 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 part of the datastructure to remain untyped

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

Installation

Install the latest release from NPM:

> npm install -g swift-json-gen

Also copy example/JsonGen.swift into your own project. This file contains some encoders and decoders for default Swift and Foundation types.

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:

extention SomeStruct {
  static func decodeJson(json: AnyObject) -> SomeStruct? {
    ...
  }
  
  func encodeJson() -> AnyObject {
    ...
  }
}

Customization

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.

Example

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: NSURL
}

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 content:

extension Blog {
  static func decodeJson(json: AnyObject) -> Blog? {
    let _dict = json as? [String : AnyObject]
    if _dict == nil { return nil }
    let dict = _dict!

    let id_field: AnyObject? = dict["id"]
    if id_field == nil { assertionFailure("field 'id' is missing"); return nil }
    let id_optional: Int? = Int.decodeJson(id_field!)
    if id_optional == nil { assertionFailure("field 'id' is not Int"); return nil }
    let id: Int = id_optional!

    let name_field: AnyObject? = dict["name"]
    if name_field == nil { assertionFailure("field 'name' is missing"); return nil }
    let name_optional: String? = String.decodeJson(name_field!)
    if name_optional == nil { assertionFailure("field 'name' is not String"); return nil }
    let name: String = name_optional!

    let author_field: AnyObject? = dict["author"]
    let author: String? = author_field == nil ? nil : Optional.decodeJson({ String.decodeJson($0) }, author_field!)

    let needsPassword_field: AnyObject? = dict["needsPassword"]
    if needsPassword_field == nil { assertionFailure("field 'needsPassword' is missing"); return nil }
    let needsPassword_optional: Bool? = Bool.decodeJson(needsPassword_field!)
    if needsPassword_optional == nil { assertionFailure("field 'needsPassword' is not Bool"); return nil }
    let needsPassword: Bool = needsPassword_optional!

    let url_field: AnyObject? = dict["url"]
    if url_field == nil { assertionFailure("field 'url' is missing"); return nil }
    let url_optional: NSURL? = NSURL.decodeJson(url_field!)
    if url_optional == nil { assertionFailure("field 'url' is not NSURL"); return nil }
    let url: NSURL = url_optional!

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

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

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

    return dict
  }
}

Compiling

This package is written in TypeScript. To make changes to the code of swift-json-gen, first install TypeScript:

> npm install -g typescript

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

> tsc lib/*.ts

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.