/pure-swift-json

JSON encoding and decoding without the use of Foundation in pure Swift.

Primary LanguageSwiftApache License 2.0Apache-2.0

pure-swift-json

Swift 5.1 github-actions codecov

This package provides a json encoder and decoder in pure Swift (without the use of Foundation or any other dependency). The implementation is RFC8259 compliant. It offers a significant performance improvement compared to the Foundation implementation on Linux.

If you like the idea of using pure Swift without any dependencies, you might also like my reimplementation of Base64 in pure Swift: swift-base64-kit

‼️ NOTE: This is at a very early stage of development. Please take into account that the API might change while in pre-release.

Goals

  • does not use Foundation at all
  • does not use unsafe Swift syntax
  • no external dependencies other than the Swift STL
  • faster than Foundation implementation

Currently not supported

Alternatives

Usage

Add pure-swift-json as dependency to your Package.swift:

  dependencies: [
    .package(url: "https://github.com/fabianfett/pure-swift-json.git", .upToNextMajor(from: "0.1.0")),
  ],

Add PureSwiftJSONCoding to the target you want to use it in.

  targets: [
    .target(
      name: "MyFancyTarget",
      dependencies: ["PureSwiftJSONCoding"]),
  ]

Use it as you would use the Foundation encoder and decoder.

import PureSwiftJSONCoding

let bytesArray  = try JSONEncoder().encode(myEncodable)
let myDecodable = try JSONDecoder().decode(MyDecodable.self, from: bytes)

Performance

All tests have been run on a 2019 MacBook Pro (16" – 2,4 GHz 8-Core Intel Core i9). You can run the tests yourself by cloning this repo and

# change dir to perf tests
$ cd PerfTests

# compile in release mode - IMPORTANT ‼️
$ swift build -c release

# run tests
$ .build/release/PureSwiftJSONCodingPerfTests

Encoding

macOS Swift 5.1 macOS Swift 5.2 Linux Swift 5.1 Linux Swift 5.2
Foundation 2.61s 2.62s 13.03s 12.52s
PureSwiftJSON 1.23s 1.25s 1.13s 1.05s
Speedup ~2x ~2x ~10x ~10x

Decoding

macOS Swift 5.1 macOS Swift 5.2 Linux Swift 5.1 Linux Swift 5.2
Foundation 2.72s 3.04s 10.27s 10.65s
PureSwiftJSON 1.70s 1.72s 1.39s 1.16s
Speedup ~1.5x ~1.5x ~7x ~8x

Workarounds

What about Date and Data?

Date and Data are particular cases for encoding and decoding. They do have default implementations that are kind off special:

  • Date will be encoded as a float

    Example: 2020-03-17 16:36:58 +0000 will be encoded as 606155818.503831

  • Data will be encoded as a numeric array.

    Example: 0, 1, 2, 3, 255 will be encoded as: [0, 1, 2, 3, 255]

Yes, that is the default implementation. Only Apple knows why it is not ISO 8601 and Base64. 🙃 Since I don't want to link against Foundation, it is not possible to implement default encoding and decoding strategies for Date and Data like the Foundation implementation does. That's why, if you want to use another encoding/decoding strategy than the default, you need to overwrite encode(to: Encoder) and init(from: Decoder).

This could look like this:

struct MyEvent: Decodable {

  let eventTime: Date
  
  enum CodingKeys: String, CodingKey {
    case eventTime
  }

  init(from decoder: Decoder) {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    let dateString = try container.decode(String.self, forKey: .eventTime)
    guard let timestamp = MyEvent.dateFormatter.date(from: dateString) else {
      let dateFormat = String(describing: MyEvent.dateFormatter.dateFormat)
      throw DecodingError.dataCorruptedError(forKey: .eventTime, in: container, debugDescription:
        "Expected date to be in format `\(dateFormat)`, but `\(dateString) does not fulfill format`")
    }
    self.eventTime = timestamp
  }
  
  private static let dateFormatter: DateFormatter = MyEvent.createDateFormatter()
  private static func createDateFormatter() -> DateFormatter {
    let formatter = DateFormatter()
    formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"
    formatter.timeZone   = TimeZone(secondsFromGMT: 0)
    formatter.locale     = Locale(identifier: "en_US_POSIX")
    return formatter
  }
}

You can find more information about encoding and decoding custom types in Apple's documentation.

Of course you can use @propertyWrappers to make this more elegant:

import Foundation

@propertyWrapper
struct DateStringCoding: Decodable {
  var wrappedValue: Date
  
  init(wrappedValue: Date) {
    self.wrappedValue = wrappedValue
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    let dateString = try container.decode(String.self)
    guard let date = Self.dateFormatter.date(from: dateString) else {
      let dateFormat = String(describing: Self.dateFormatter.dateFormat)
      throw DecodingError.dataCorruptedError(in: container, debugDescription:
            "Expected date to be in format `\(dateFormat)`, but `\(dateString) does not fulfill format`")
    }
    self.wrappedValue = date
  }

  private static let dateFormatter: DateFormatter = Self.createDateFormatter()
  private static func createDateFormatter() -> DateFormatter {
    let formatter = DateFormatter()
    formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"
    formatter.timeZone   = TimeZone(secondsFromGMT: 0)
    formatter.locale     = Locale(identifier: "en_US_POSIX")
    return formatter
  }
}

struct MyEvent: Decodable {
  @DateStringCoding
  var eventTime: Date
}

Checkout a full example in the test file DateCodingTests.

UTF-16 and UTF-32

If your input is UTF-16 or UTF-32 encoded, you can easily convert it to UTF-8:

let utf16 = UInt16[]() // your utf-16 encoded data
let utf8  = Array(String(decoding: utf16, as: Unicode.UTF16.self).utf8)
let utf32 = UInt32[]() // your utf-32 encoded data
let utf8  = Array(String(decoding: utf32, as: Unicode.UTF32.self).utf8)

Contributing

Please feel welcome and encouraged to contribute to pure-swift-json. This is a very young endeavour and help is always welcome.

If you've found a bug, have a suggestion, or need help getting started, please open an Issue or a PR. If you use this package, I'd be grateful for sharing your experience.

Focus areas for the time being:

  • ensuring safe use of nested containers while encoding and decoding
  • supporting camelCase and snakeCase aka KeyEncodingStrategy

Credits