Extensions giving Swift's Codable API type inference super powers
Welcome to Codextended — a suite of extensions that aims to make Swift’s Codable API easier to use by giving it type inference-powered capabilities and conveniences. It’s not a wrapper, nor is it a brand new framework, instead it augments Codable directly in a very lightweight way.
Codable is awesome!
No third-party serialization framework can beat the convenience of Codable. Since it’s built in, it can both leverage the compiler to automatically synthesize all serialization code needed in many situations, and it can also be used as a common bridge between multiple different modules — without having to introduce any shared dependencies.
However, once some form of customization is needed — for example to transform parts of the decoded data, or to provide default values for certain keys — the standard Codable API starts to become really verbose. It also doesn’t take advantage of Swift’s robust type inference capabilities, which produces a lot of unnecessary boilerplate.
That’s what Codextended aims to fix.
Examples
Here are a few examples that demonstrate the difference between using “vanilla” Codable and the APIs that Codextended adds to it. The goal is to turn all common serialization operations into one-liners, rather than having to set up a ton of boilerplate.
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Top-level API
Codextended makes a few slight tweaks to the top-level API used to encode and decode values, making it possible to leverage type inference and use methods on the actual values that are being encoded or decoded.
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With vanilla Codable:
// Encoding
let encoder = JSONEncoder()
let data = try encoder.encode(value)
// Decoding
let decoder = JSONDecoder()
let article = try decoder.decode(Article.self, from: data)
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With Codextended:
// Encoding
let data = try value.encoded()
// Decoding
let article = try data.decoded() as Article
// Decoding when the type can be inferred
try saveArticle(data.decoded())
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Overriding the behavior for a single key
While Codable is amazing as long as the serialized data’s format exactly matches the format of the Swift types that’ll use it — as soon as we need to make just a small tweak, things quickly go from really convenient to very verbose.
As an example, let’s just say that we want to provide a default value for one single property (without having to make it an optional, which would make it harder to handle in the rest of our code base). To do that, we need to completely manually implement our type’s decoding — like below for the tags property of an Article type.
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With vanilla Codable:
struct Article: Codable {
enum CodingKeys: CodingKey {
case title
case body
case footnotes
case tags
}
var title: String
var body: String
var footnotes: String?
var tags: [String]
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
title = try container.decode(String.self, forKey: .title)
body = try container.decode(String.self, forKey: .body)
footnotes = try container.decodeIfPresent(String.self, forKey: .footnotes)
tags = (try? container.decode([String].self, forKey: .tags)) ?? []
}
}
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With Codextended:
struct Article: Codable {
var title: String
var body: String
var footnotes: String?
var tags: [String]
init(from decoder: Decoder) throws {
title = try decoder.decode("title")
body = try decoder.decode("body")
footnotes = try decoder.decodeIfPresent("footnotes")
tags = (try? decoder.decode("tags")) ?? []
}
}
Codextended includes decoding overloads both for CodingKey-based values and for string literals, so that we can pick the approach that’s the most appropriate/convenient for each given situation.
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Using date formatters
Codable already comes with support for custom date formats through assigning a DateFormatter to either a JSONEncoder or JSONDecoder. However, requiring each call site to be aware of the specific date formats used for each type isn’t always great — so with Codextended, it’s easy for a type itself to pick what date format it needs to use.
That’s really convenient when working with third-party data, and we only want to customize the date format for some of our types, or when we want to produce more readable date strings when encoding a value.
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With vanilla Codable:
struct Bookmark: Codable {
enum CodingKeys: CodingKey {
case url
case date
}
struct DateCodingError: Error {}
static let dateFormatter = makeDateFormatter()
var url: URL
var date: Date
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
url = try container.decode(URL.self, forKey: .url)
let dateString = try container.decode(String.self, forKey: .date)
guard let date = Bookmark.dateFormatter.date(from: dateString) else {
throw DateCodingError()
}
self.date = date
}
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(url, forKey: .url)
let dateString = Bookmark.dateFormatter.string(from: date)
try container.encode(dateString, forKey: .date)
}
}
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With Codextended:
struct Bookmark: Codable {
static let dateFormatter = makeDateFormatter()
var url: URL
var date: Date
init(from decoder: Decoder) throws {
url = try decoder.decode("url")
date = try decoder.decode("date", using: Bookmark.dateFormatter)
}
func encode(to encoder: Encoder) throws {
try encoder.encode(url, for: "url")
try encoder.encode(date, for: "date", using: Bookmark.dateFormatter)
}
}
Again, we could’ve chosen to use a CodingKeys enum above to represent our keys, rather than using inline strings.
Mix and match
Since Codextended is 100% implemented through extensions, you can easily mix and match it with “vanilla” Codable code within the same project. It also doesn’t change what makes Codable so great — the fact that it often doesn’t require any manual code at all, and that it can be used as a bridge between frameworks.
All it does is give Codable a helping hand when some form of customization is needed.
Installation
Since Codextended is implemented within a single file, the easiest way to use it is to simply drag and drop it into your Xcode project.
But if you wish to use a dependency manager, you can either use the Swift Package Manager by declaring Codextended as a dependency in your Package.swift file:
.package(url: "https://github.com/JohnSundell/Codextended", from: "0.1.0")
For more information, see the Swift Package Manager documentation.
You can also use CocoaPods by adding the following line to your Podfile:
pod "Codextended"
Contributions & support
Codextended is developed completely in the open, and your contributions are more than welcome.
Before you start using Codextended in any of your projects, it’s highly recommended that you spend a few minutes familiarizing yourself with its documentation and internal implementation (it all fits in a single file!), so that you’ll be ready to tackle any issues or edge cases that you might encounter.
To learn more about the principles used to implement Codextended, check out “Type inference-powered serialization in Swift” on Swift by Sundell.
This project does not come with GitHub Issues-based support, and users are instead encouraged to become active participants in its continued development — by fixing any bugs that they encounter, or improving the documentation wherever it’s found to be lacking.
If you wish to make a change, open a Pull Request — even if it just contains a draft of the changes you’re planning, or a test that reproduces an issue — and we can discuss it further from there.
Hope you’ll enjoy using Codextended!
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