JSON Converter

Perfect includes basic JSON encoding and decoding functionality. JSON encoding is provided through a series of extensions on many of the built-in Swift data types. Decoding is provided through an extension on the Swift String type.

It seems important to note that although Perfect provides this particular JSON encoding/decoding system, it is not required that your application uses it. Feel free to import your own favourite JSON-related functionality.

To use this system, first ensure that PerfectLib is imported:

import PerfectLib

Encoding To JSON Data

You can convert any of the following types directly into JSON string data:

  • String
  • Int
  • UInt
  • Double
  • Bool
  • Array
  • Dictionary
  • Optional
  • Custom classes which inherit from JSONConvertibleObject

Note that only Optionals which contain any of the above types are directly convertible. Optionals which are nil will output as a JSON "null".

To encode any of these values, call the jsonEncodedString() function which is provided as an extension on the objects. This function may potentially throw a JSONConversionError.notConvertible error.


let scoreArray: [String:Any] = ["1st Place": 300, "2nd Place": 230.45, "3rd Place": 150]
let encoded = try scoreArray.jsonEncodedString()

The result of the encoding would be the following String:

{"2nd Place":230.45,"1st Place":300,"3rd Place":150}

Decoding JSON Data

String objects which contain JSON string data can be decoded by using the jsonDecode() function. This function can throw a JSONConversionError.syntaxError error if the String does not contain valid JSON data.

let encoded = "{\"2nd Place\":230.45,\"1st Place\":300,\"3rd Place\":150}"
let decoded = try encoded.jsonDecode() as? [String:Any]

Decoding the String will produce the following dictionary:

["2nd Place": 230.44999999999999, "1st Place": 300, "3rd Place": 150]

Though decoding a JSON string can produce any of the permitted values, it is most common to deal with JSON objects (dictionaries) or arrays. You will need to cast the resulting value to the expected type.

Using the Decoded Data

Because decoded dictionaries or arrays are always of type [String:Any] or [Any], respectively, you will need to cast the contained values to usable types. For example:

var firstPlace = 0
var secondPlace = 0.0
var thirdPlace = 0

let encoded = "{\"2nd Place\":230.45,\"1st Place\":300,\"3rd Place\":150}"
guard let decoded = try encoded.jsonDecode() as? [String:Any] else {

for (key, value) in decoded {
    switch key {
    case "1st Place":
        firstPlace = value as! Int
    case "2nd Place":
        secondPlace = value as! Double
    case "3rd Place":
        thirdPlace = value as! Int

print("The top scores are: \r" + "First Place: " + "\(firstPlace)" + " Points\r" + "Second Place: " + "\(secondPlace)" + " Points\r" + "Third Place: " + "\(thirdPlace)" + " Points")

The output would be the following:

The top scores are: 
First Place: 300 Points
Second Place: 230.45 Points
Third Place: 150 Points

Decoding Empty Values from JSON Data

As JSON null values are untyped, the system will substitute a JSONConvertibleNull in place of all JSON nulls. Example:

let jsonString = "{\"1st Place\":300,\"4th place\":null,\"2nd Place\":230.45,\"3rd Place\":150}"

if let decoded = try jsonString.jsonDecode() as? [String:Any] {
    for (key, value) in decoded {
        if let value as? JSONConvertibleNull {
            print("The key \"\(key)\" had a null value")

The output would be:

The key "4th place" had a null value

JSON Convertible Object

Perfect's JSON system provides the facilities for encoding and decoding custom classes. Any eligible class must inherit from the JSONConvertibleObject base class which is defined as follows:

/// Base for a custom object which can be converted to and from JSON.
public class JSONConvertibleObject: JSONConvertible {
    /// Default initializer.
    public init() {}
    /// Get the JSON keys/value.
    public func setJSONValues(_ values:[String:Any]) {}
    /// Set the object properties based on the JSON keys/values.
    public func getJSONValues() -> [String:Any] { return [String:Any]() }
    /// Encode the object into JSON text
    public func jsonEncodedString() throws -> String {
        return try self.getJSONValues().jsonEncodedString()

Any object wishing to be JSON encoded/decoded must first register itself with the system. This registration should take place once when your application starts up. Call the JSONDecoding.registerJSONDecodable function to register your object. This function is defined as follows:

public class JSONDecoding {
    /// Function which returns a new instance of a custom object which will have its members set based on the JSON data.
    public typealias JSONConvertibleObjectCreator = () -> JSONConvertibleObject
    static public func registerJSONDecodable(name: String, creator: JSONConvertibleObjectCreator)

Registering an object requires a unique name which can be any string, provided it is unique. It also requires a "creator" function which returns a new instance of the object in question.

When the system encodes a JSONConvertibleObject it calls the object's getJSONValues function. This function should return a [String:Any] dictionary containing the names and values for any properties which should be encoded into the resulting JSON string. This dictionary must also contain a value identifying the object type. The value must match the name by which the object was originally registered. The dictionary key for the value is identified by the JSONDecoding.objectIdentifierKey property.

When the system decodes such an object, it will find the JSONDecoding.objectIdentifierKey value and look up the object creator which had been previously registered. It will create a new instance of the type by calling that function, and will then call the new object's setJSONValues(_ values:[String:Any]) function. It will pass in a dictionary containing all of the deconverted values. These values will match those previously returned by the getJSONValues function when the object was first converted. Within the setJSONValues function, the object should retrieve all properties which it wants to reinstate.

The following example defines a custom JSONConvertibleObject and converts it to a JSON string. It then decodes the object and compares it to the original. Note that this example object calls the convenience function getJSONValue, which will pull a named value from the dictionary and permits providing a default value which will be returned if the dictionary does not contain the indicated key.

This example is split up into several sections.

Define the class:

class User: JSONConvertibleObject {
    static let registerName = "user"
    var firstName = ""
    var lastName = ""
    var age = 0
    override func setJSONValues(_ values: [String : Any]) {
        self.firstName = getJSONValue(named: "firstName", from: values, defaultValue: "")
        self.lastName = getJSONValue(named: "lastName", from: values, defaultValue: "")
        self.age = getJSONValue(named: "age", from: values, defaultValue: 0)
    override func getJSONValues() -> [String : Any] {
        return [

Register the class:

// do this once
JSONDecoding.registerJSONDecodable(name: User.registerName, creator: { return User() })

Encode the object:

let user = User()
user.firstName = "Donnie"
user.lastName = "Darko"
user.age = 17

let encoded = try user.jsonEncodedString()

The value of "encoded" will look as follows:


Decode the object:

guard let user2 = try encoded.jsonDecode() as? User else {
    return // error

// check the values
XCTAssert(user.firstName == user2.firstName)
XCTAssert(user.lastName == user2.lastName)
XCTAssert(user.age == user2.age)

Note that the JSONDecoding.objectIdentifierKey (the value of which is "_jsonobjid") key/value pair that identifies the Swift object to be decoded must be in the incoming JSON string. If it is not present then the result of jsonDecode will be a regular Dictionary instead of the intended Swift object.

JSON Conversion Error

As an object is converted to or from a JSON string, the process may throw a JSONConversionError. This is defined as follows:

/// An error occurring during JSON conversion.
public enum JSONConversionError: ErrorProtocol {
    /// The object did not suppport JSON conversion.
    case notConvertible(Any)
    /// A provided key was not a String.
    case invalidKey(Any)
    /// The JSON text contained a syntax error.
    case syntaxError