A simple Swift library for working with generic JSON structures

Generic JSON

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Generic JSON makes it easy to deal with freeform JSON strings without creating a separate, well-typed structure.

Codable and freeform JSON

Swift 4 introduced a new JSON encoding and decoding machinery represented by the Codable protocol. The feature is very nice and very type-safe, meaning it’s no longer possible to just willy-nilly decode a JSON string pulling random untyped data from it. Which is good™ most of the time – but what should you do when you do want to just willy-nilly encode or decode a JSON string without introducing a separate, well-typed structure for it? For example:

// error: heterogeneous collection literal could only be inferred to '[String : Any]';
// add explicit type annotation if this is intentional
let json = [
    "foo": "foo",
    "bar": 1,

// Okay then:
let json: [String:Any] = [
    "foo": "foo",
    "bar": 1,

// But: fatal error: Dictionary<String, Any> does not conform to Encodable because Any does not conform to Encodable.
let encoded = try JSONEncoder().encode(json)

So this doesn’t work very well. Also, the json value can’t be checked for equality with another, although arbitrary JSON values should support equality. Enter JSON.


Create a JSON structure

let json: JSON = [
    "foo": "foo",
    "bar": 1,

// "{"bar":1,"foo":"foo"}"
let str = try String(data: try JSONEncoder().encode(json), encoding: .utf8)!
let hopefullyTrue = (json == json) // true!

Convert Encodable objects into a generic JSON structure

struct Player: Codable {
    let name: String
    let swings: Bool

let val = try JSON(encodable: Player(name: "Miles", swings: true))
val == [
    "name": "Miles",
    "swings": true,
] // true

Query Values

Consider the following JSON structure:

let json: JSON = [
    "num": 1,
    "str": "baz",
    "bool": true,
    "obj": [
        "foo": "jar",
        "bar": 1,

Querying values can be done using optional property accessors, subscripting or dynamic member subscripting:

// Property accessors
if let str = json.objectValue?["str"]?.stringValue {  }
if let foo = json.objectValue?["obj"]?.objectValue?["foo"]?.stringValue {  }

// Subscripting
if let str = json["str"]?.stringValue {  }
if let foo = json["obj"]?["foo"]?.stringValue {  }

// Dynamic member subscripting
if let str = json.str?.stringValue {  }
if let foo = json.obj?.foo?.stringValue {  }

You may even drill through nested structures using a dot-separated key path:

let val = json[keyPath: ""] // "jar"


  • Swift Tools 5.0.0
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Last updated: Wed Apr 10 2024 12:52:26 GMT-0900 (Hawaii-Aleutian Daylight Time)