Kodable

Platforms

Kodable is an extension of the Codable functionality through property wrappers. The main goal is to remove boilerplate while also adding useful functionality.

Features:

No need to write your own init(from decoder: Decoder) or CodingKeys
Provide a custom key for decoding
Access nested values using the . notation
Add a default value in case the value is missing
Overriding the values decoded (i.e. trimming a string)
Validation of the values decoded
Automatically tries to decode String and Bool from other types as a fallback
Transformer protocol to implement your own additional functionality on top of the existing ones
Special error handling and better readability of Swift’s DecodingError

Kodable
Table of contents

Installation

Swift Package Manager

If you’re working directly in a Package, add Kodable to your Package.swift file

dependencies: [
    .package(url: "https://github.com/JARMourato/Kodable.git", .upToNextMajor(from: "1.1.0")),
]

If working in an Xcode project select File->Swift Packages->Add Package Dependency... and search for the package name: Kodable or the git url:

https://github.com/JARMourato/Kodable.git

Usage

Provided Wrappers

Coding

Just make your type conform to Kodable and you’ll have access to all of the features Coding brings. You can mix and match Codable values with Coding properties.

Declare your model:

struct User: Kodable {
    var identifier: String = ""
    var social: String?
    @Coding("first_name") var firstName: String
    @Coding(default: "+1 123456789") var phone: String
    @Coding("address.zipCode") var zipCode: Int
}

// Instead of

struct CodableUser: Codable {

    enum Keys: String, CodingKey {
        case identifier, social, firstName = "first_name", phone, address
    }

    enum NestedKeys: String, CodingKey {
        case zipCode
    }

    var identifier: String = ""
    var social: String?
    var firstName: String
    var phone: String
    var zipCode: Int

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: Keys.self)
        identifier = try container.decode(String.self, forKey: .identifier)
        social = try container.decodeIfPresent(String.self, forKey: .social)
        firstName = try container.decode(String.self, forKey: .firstName)
        phone = try container.decodeIfPresent(String.self, forKey: .phone) ?? "+1 123456789"
        let addressContainer = try container.nestedContainer(keyedBy: NestedKeys.self, forKey: .address)
        zipCode = try addressContainer.decode(Int.self, forKey: .zip)
    }
}

Then

let json = """
{
    identifier: "1",
    "social": 987654321,
    "first_name": John,
    "address": {
        "zipCode": 94040,
    }
}
""".data(using: .utf8)!

let result = try JSONDecoder().decode(User.self, from: json)

// or using the provided syntactic sugar

let user = try User.decode(from: json)

CodableDate

This wrapper allows decoding dates on per-property strategy basis. By default, CodableDate uses the iso8601 strategy. The built-in strategies are: iso8601, iso8601WithMillisecondPrecision, rfc2822, rfc3339, and timestamp. There is also the option of using a custom format by providing a valid string format to the option .format().

struct Dates: Kodable {
    @CodableDate var iso8601: Date
    @CodableDate(.format("y-MM-dd"), .key("simple_date")) var simpleDate: Date
    @CodableDate(.rfc2822, .key("rfc2822")) var rfc2822Date: Date
    @CodableDate(.rfc3339, .key("rfc3339")) var rfc3339Date: Date
    @CodableDate(.timestamp, .key("timestamp")) var timestamp: Date
}

let json = """
{
    "iso8601": "1996-12-19T16:39:57-08:00",
    "simple_date": "2001-01-01",
    "rfc2822": "Thu, 19 Dec 1996 16:39:57 GMT",
    "rfc3339": "1996-12-19T16:39:57-08:00",
    "timestamp": 978307200.0,
}
""".data(using: .utf8)!

let dates = Dates.decode(from: json)
print(dates.iso8601.description) // Prints "1996-12-20 00:39:57 +0000"
print(dates.simpleDate.description) // Prints "2001-01-01 00:00:00 +0000"
print(dates.rfc2822Date.description) // Prints "1996-12-19 16:39:57 +0000"
print(dates.rfc3339Date.description) // Prints "1996-12-20 00:39:57 +0000"
print(dates.timestamp.description) // Prints "2001-01-01 00:00:00 +0000"

Note that there’s no built-in support for ISO8601 dates with precision greater than millisecond (e.g. microsecond or nanosecond), because Apple doesn’t officially supports such precision natively, yet. Should you feel the necessity to have those, or any other custom date formatter, you can implement your own DateConvertible and use .custom(dateConvertible) DateCodingStrategy. If you think your use case should make its way into the official library, PRs are always welcome!

Advanced Usage

Lossy Type Decoding

For the types Array, Bool, and String, some lossy decoding was introduced. More types can be added later on, but for now these sufficed my personal usage. To disable this behavior for a specific property, in case you want decoding to fail when the type is not correct, just provide the enforceTypeDecoding option to the Coding property wrapper.

Array

The lossy decoding on Array is done by trying to decode each element from a Array.Element type in a non-lossy way (even if they are Bool or String) and ignores values that fail decoding.

struct LossyArray: Kodable {
    @Coding("failable_array", .lossy) var array: [String]
}

let json = """
{
    "failable_array": [ "1", 1.5, "2", true, "3", null, 4 ]
}
""".data(using: .utf8)!

let lossy = try LossyArray.decode(from: json)
print(lossy.array) // Prints [ "1", "2", "3" ]

Bool

Tries to decode a Bool from Int or String if Bool fails

struct Fail: Kodable {
    @Coding("string_bool", .enforceTypeDecoding) var notBool: Bool
}

struct Success: Kodable {
    @Coding("string_bool") var stringBool: Bool
    @Coding("int_bool") var intBool: Bool
}

let json = """
{
    "string_bool": "false",
    "int_bool": 1,
}
""".data(using: .utf8)!

let success = try Success.decode(from: json)
print(success.stringBool) // prints false
print(success.intBool) // prints true

let fail = try Fail.decode(from: json) // Throws KodableError.invalidValueForPropertyWithKey("string_bool")

String

Tries to decode a String from Double or Int if String fails

struct Amounts: Kodable {
    @Coding("double") var double: String
    @Coding("int") var integer: String
    @Coding var string: String
}

let json = """
{
    "double": 629.9,
    "int": 1563,
    "string": "999.9"
}
""".data(using: .utf8)!

let amounts = try Amounts.decode(from: json)
print(amounts.double) // prints "629.9"
print(amounts.integer) // prints "1563"
print(amounts.string) // prints "999.9"

Overriding Values

You can provide a KodableModifier.custom modifier with an overriding closure so that you can modify the decoded value before assigning it to the property.

struct Project: Kodable {
    @Coding(.modifier(Project.trimmed)) var title: String
    
    static var trimmed: KodableModifier<String> { 
        KodableModifier { $0.trimmingCharacters(in: .whitespacesAndNewlines) } 
    }
}

let json = #"{ "title": "  A New Project    " }"#.data(using: .utf8)!

let project = try Project.decode(from: json)
print(project.title) // Prints "A New Project"

There are a few built in modifiers provided already:

String

trimmed : Applies trimmingCharacters(in: .whitespacesAndNewlines) to the value decoded

String?

trimmed : Applies trimmingCharacters(in: .whitespacesAndNewlines) to the value decoded
trimmedNifIfEmpty : Applies trimmingCharacters(in: .whitespacesAndNewlines) to the value decoded, returns nif if empty

Sorting When the type conforms to the Comparable protocol:

ascending or descending: Sorts the elements of an array in ascending (or descending) order, using the type’s underlying comparison function.

When the type doesn’t conform to the Comparable protocol, but one of its properties does:

ascending(by: KeyPath) or descending(by: KeyPath): Sorts the elements of an array in ascending (or descending) order, based on the KeyPath property passed.

If there’s no conformance to Comparable at all, you can resort to basic sorting functionality:

sorted(using: Comparator): Sorts the elements of an array using a Comparator closure that determines whether the items are in increasing order. In Swifty words: func < (lhs: Value.Element, rhs: Value.Element) -> Bool

Comparable

clamping(to:) : Clamps the value in a range.
range() : Constrains the value inside a provided range.
max() : Constrains the value to a maximum value.
min() : Constrains the value to a minimum value.

Validating Values

You can provide a KodableModifier.validation modifier with a validation closure, where you can verify if the value is valid.

struct Image: Kodable {
    @Coding(.validation({ $0 > 500 })) var width: Int
}

let json = #{ "width": 400 }#.data(using: .utf8)!

let image = try Image.decode(from: json)
// Throws KodableError.validationFailed(property: "width", parsedValue: 400)

Custom Wrapper

Kodable was built based on a protocol called KodableTransform

public protocol KodableTransform {
    associatedtype From: Codable
    associatedtype To
    func transformFromJSON(value: From) throws -> To
    func transformToJSON(value: To) throws -> From
    init()
}

If you want to add your own custom behavior, you can create a type that conforms to the KodableTransform protocol.:

struct URLTransformer: KodableTransform {
    
    enum Error: Swift.Error {
        case failedToCreateURL
    }

    func transformFromJSON(value: String) throws -> URL {
        guard let url = URL(string: value) else { throw Error.failedToCreateURL }
        return url
    }
    
    func transformToJSON(value: URL) throws -> String {
        value.absoluteString
    }
}

Then use the KodableTrasformable property wrapper, upon which all other wrappers are based:

typealias CodingURL = KodableTransformable<URLTransformer>

And voilà

struct Test: Kodable {
    @CodingURL("html_url") var url: URL
}

Encode Null Values

By default optional values won’t be encoded so:

struct User: Kodable {
    @Coding var firstName: String
    @Coding var lastName: String?
}

let user = User()
user.firstName = "João"

When encoded will output:

{
    "firstName": "João"
}

However, if you want to explicitly encode null values, then you can add the encodeAsNullIfNil option:

struct User: Kodable {
    @Coding var firstName: String
    @Coding(.encodeAsNullIfNil) var lastName: String?
}

let user = User()
user.firstName = "João"

Which will then output:

{
    "firstName": "João",
    "lastName": null
}

Debugging

While developing it might be useful to know what JSON is being received, so that we can be sure that the options chosen lead to correct decoding. There are several ways to do this, however, for simplicity sake, Kodable provides a simple way to print the JSON value received.

Let’s take for example the following JSON and Kodable models:

{
    "identifier": "1",
    "social": 987654321,
    "first_name": "John",
    "address": {
        "zipCode": 94040,
        "state": "CA"
    },
    "aliases": [ "Jay", "Doe" ]
}

struct Address: Codable {
    let zipCode: Int
    let state: String 
}

struct User: Kodable {
    var identifier: String = ""
    var social: String?
    @Coding("first_name") var firstName: String
    @Coding(default: "+1 123456789") var phone: String
    @Coding var address: Address
}

Kodable provides 2 ways to debug the JSON that will be used to decode the User model. The first is to check the whole JSON value for the model. To achieve that, conform the model to the DebugJSON protocol:

struct User: Kodable, DebugJSON {
    /.../
}

Whenever an instance of the User model is decoded you’ll get the following message in the console

Decoded JSON for type User:
{
    "identifier": "1",
    "social": 987654321,
    "first_name": "John",
    "address": {
        "zipCode": 94040,
        "state": "CA"
    },
    "aliases": [ "Jay", "Doe" ]
}

However, sometimes the model can be quite extensive and you’re only interested in a specific nested model. In that case, there is a second option which is to mark only the property you want with the option .debugJSON:

struct User: Kodable {
    var identifier: String = ""
    var social: String?
    @Coding("first_name") var firstName: String
    @Coding(default: "+1 123456789") var phone: String
    @Coding(.debugJSON) var address: Address
}

In which case, for every instance of the User model that is decoded, you’ll get the following message in the console:

Decoded JSON for the address property of type User:
{
    "zipCode": 94040,
    "state": "CA"
}

Contributions

If you feel like something is missing or you want to add any new functionality, please open an issue requesting it and/or submit a pull request with passing tests 🙌

License

MIT

Special thanks to

Better Decoding Error Messages - via @nunogoncalves

Contact

João (@_JARMourato)

21 improve messages from errors