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Better Codable through Property Wrappers
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README.md

Better Codable through Property Wrappers

Level up your Codable structs through property wrappers. The goal of these property wrappers is to avoid implementing a custom init(from decoder: Decoder) throws and suffer through boilerplate.

@LossyArray

@LossyArray decodes Arrays and filters invalid values if the Decoder is unable to decode the value. This is useful when the Array contains non-optional types and your API serves elements that are either null or fail to decode within the container.

Usage

Easily filter nulls from primitive containers

struct Response: Codable {
    @LossyArray var values: [Int]
}

let json = #"{ "values": [1, 2, null, 4, 5, null] }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

print(result) // [1, 2, 4, 5]

Or silently exclude failable entities

struct Failable: Codable {
    let value: String
}

struct Response: Codable {
    @LossyArray var values: [Failable]
}

let json = #"{ "values": [{"value": 4}, {"value": "fish"}] }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

print(result) // [Failable(value: "fish")]

@LossyDictionary

@LossyDictionary decodes Dictionaries and filters invalid key-value pairs if the Decoder is unable to decode the value. This is useful if the Dictionary is intended to contain non-optional values and your API serves values that are either null or fail to decode within the container.

Usage

Easily filter nulls from primitive containers

struct Response: Codable {
    @LossyDictionary var values: [String: String]
}

let json = #"{ "values": {"a": "A", "b": "B", "c": null } }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

print(result) // ["a": "A", "b": "B"]

Or silently exclude failable entities

struct Failable: Codable {
    let value: String
}

struct Response: Codable {
    @LossyDictionary var values: [String: Failable]
}

let json = #"{ "values": {"a": {"value": "A"}, "b": {"value": 2}} }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

print(result) // ["a": "A"]

@DefaultCodable

@DefaultCodable provides a generic property wrapper that allows for default values using a custom DefaultCodableStrategy. This allows one to implement their own default behavior for missing data and get the property wrapper behavior for free. Below are a few common default strategies, but they also serve as a template to implement a custom property wrapper to suit your specific use case.

While not provided in the source code, it's a sinch to create your own default strategy for your custom data flow.

struct RefreshDaily: DefaultCodableStrategy {
    static var defaultValue: CacheInterval { return CacheInterval.daily }
}

struct Cache: Codable {
    @DefaultCodable<RefreshDaily> var refreshInterval: CacheInterval
}

let json = #"{ "refreshInterval": null }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Cache.self, from: json)

print(result) // Cache(refreshInterval: .daily)

@DefaultFalse

Optional Bools are weird. A type that once meant true or false, now has three possible states: .some(true), .some(false), or .none. And the .none condition could indicate truthiness if BadDecisions™ were made.

@DefaultFalse mitigates the confusion by defaulting decoded Bools to false if the Decoder is unable to decode the value, either when null is encountered or some unexpected type.

Usage

struct UserPrivilege: Codable {
    @DefaultFalse var isAdmin: Bool
}

let json = #"{ "isAdmin": null }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

print(result) // UserPrivilege(isAdmin: false)

@DefaultEmptyArray

The weirdness of Optional Booleans extends to other types, such as Arrays. Soroush has a great blog post explaining why you may want to avoid Optional Arrays. Unfortunately, this idea doesn't come for free in Swift out of the box. Being forced to implement a custom initializer in order to nil coalesce nil arrays to empty arrays is no fun.

@DefaultEmptyArray decodes Arrays and returns an empty array instead of nil if the Decoder is unable to decode the container.

Usage

struct Response: Codable {
    @DefaultEmptyArray var favorites: [Favorite]
}

let json = #"{ "favorites": null }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

print(result) // Response(favorites: [])

@DefaultEmptyDictionary

As mentioned previously, Optional Dictionaries are yet another container where nil and emptiness collide.

@DefaultEmptyDictionary decodes Dictionaries and returns an empty dictionary instead of nil if the Decoder is unable to decode the container.

Usage

struct Response: Codable {
    @DefaultEmptyDictionary var scores: [String: Int]
}

let json = #"{ "scores": null }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

print(result) // Response(values: [:])

@LosslessValue

All credit for this goes to Ian Keen.

Somtimes APIs can be unpredictable. They may treat some form of Identifiers or SKUs as Ints for one response and Strings for another. Or you might find yourself encountering "true" when you expect a boolean. This is where @LosslessValue comes into play.

@LosslessValue will attempt to decode a value into the type that you expect, preserving the data that would otherwise throw an exception or be lost altogether.

Usage

struct Response: Codable {
    @LosslessValue var sku: String
    @LosslessValue var isAvailable: Bool
}

let json = #"{ "sku": 12345, "isAvailable": "true" }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

print(result) // Response(sku: "12355", isAvailable: true)

Date Wrappers

One common frustration with Codable is decoding entities that have mixed date formats. JSONDecoder comes built in with a handy dateDecodingStrategy property, but that uses the same date format for all dates that it will decode. And often, JSONDecoder lives elsewhere from the entity forcing tight coupling with the entities if you choose to use its date decoding strategy.

Property wrappers are a nice solution to the aforementioned issues. It allows tight binding of the date formatting strategy directly with the property of the entity, and allows the JSONDecoder to remain decoupled from the entities it decodes. The @DateValue wrapper is generic across a custom DateValueCodableStrategy. This allows anyone to implement their own date decoding strategy and get the property wrapper behavior for free. Below are a few common Date strategies, but they also serve as a template to implement a custom property wrapper to suit your specific date format needs.

The following property wrappers are heavily inspired by Ian Keen.

ISO8601Strategy

ISO8601Strategy relies on an ISO8601DateFormatter in order to decode String values into Dates. Encoding the date will encode the value into the original string value.

Usage

struct Response: Codable {
    @DateValue<ISO8601Strategy> var date: Date
}

let json = #"{ "date": "1996-12-19T16:39:57-08:00" }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

// This produces a valid `Date` representing 39 minutes and 57 seconds after the 16th hour of December 19th, 1996 with an offset of -08:00 from UTC (Pacific Standard Time).

RFC3339Strategy

RFC3339Strategy decodes RFC 3339 date strings into Dates. Encoding the date will encode the value back into the original string value.

Usage

struct Response: Codable {
    @DateValue<RFC3339Strategy> var date: Date
}

let json = #"{ "date": "1996-12-19T16:39:57-08:00" }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

// This produces a valid `Date` representing 39 minutes and 57 seconds after the 16th hour of December 19th, 1996 with an offset of -08:00 from UTC (Pacific Standard Time).

TimestampStrategy

TimestampStrategy decodes Doubles of a unix epoch into Dates. Encoding the date will encode the value into the original TimeInterval value.

Usage

struct Response: Codable {
    @DateValue<TimestampStrategy> var date: Date
}

let json = #"{ "date": 978307200.0 }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

// This produces a valid `Date` representing January 1st, 2001.

YearMonthDayStrategy

@DateValue<YearMonthDayStrategy> decodes string values into Dates using the date format y-MM-dd. Encoding the date will encode the value back into the original string format.

Usage

struct Response: Codable {
    @DateValue<YearMonthDayStrategy> var date: Date
}

let json = #"{ "date": "2001-01-01" }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

// This produces a valid `Date` representing January 1st, 2001.

Or lastly, you can mix and match date wrappers as needed where the benefits truly shine

struct Response: Codable {
    @DateValue<ISO8601Strategy> var updatedAt: Date
    @DateValue<YearMonthDayStrategy> var birthday: Date
}

let json = #"{ "updatedAt": "2019-10-19T16:14:32-05:00", "birthday": "1984-01-22" }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Response.self, from: json)

// This produces two valid `Date` values, `updatedAt` representing October 19, 2019 and `birthday` January 22nd, 1984.

Installation

Swift Package Manager

Attribution

This project is licensed under MIT. If you find these useful, please tell your boss where you found them.

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