Encode/Decode Array of Types conforming to protocol with JSONEncoder
I'm trying to find the best way to Encode/Decode an array of structs conforming to a swift protocol using the new JSONDecoder/Encoder in Swift 4.
I made up a little example to illustrate the problem:
First we have a protocol Tag and some Types that conform to this protocol.
protocol Tag: Codable {
var type: String { get }
var value: String { get }
}
struct AuthorTag: Tag {
let type = "author"
let value: String
}
struct GenreTag: Tag {
let type = "genre"
let value: String
}
Then we have a Type Article which has an Array of Tags.
struct Article: Codable {
let tags: [Tag]
let title: String
}
Finally we encode or decode the Article
let article = Article(tags: [AuthorTag(value: "Author Tag Value"), GenreTag(value:"Genre Tag Value")], title: "Article Title")
let jsonEncoder = JSONEncoder()
let jsonData = try jsonEncoder.encode(article)
let jsonString = String(data: jsonData, encoding: .utf8)
And this is the JSON structure that I like to have.
{
"title": "Article Title",
"tags": [
{
"type": "author",
"value": "Author Tag Value"
},
{
"type": "genre",
"value": "Genre Tag Value"
}
]
}
The problem is that at some point I have to switch on the type property to decode the Array but to Decode the Array I have to know its type.
EDIT:
It's clear to me why Decodable can not work out of the box but at least Encodable should work. The following modified Article struct compiles but crashes with the following error message.
fatal error: Array<Tag> does not conform to Encodable because Tag does not conform to Encodable.: file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-900.0.43/src/swift/stdlib/public/core/Codable.swift, line 3280
struct Article: Encodable {
let tags: [Tag]
let title: String
enum CodingKeys: String, CodingKey {
case tags
case title
}
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(tags, forKey: .tags)
try container.encode(title, forKey: .title)
}
}
let article = Article(tags: [AuthorTag(value: "Author Tag"), GenreTag(value:"A Genre Tag")], title: "A Title")
let jsonEncoder = JSONEncoder()
let jsonData = try jsonEncoder.encode(article)
let jsonString = String(data: jsonData, encoding: .utf8)
And this is the relevant part from Codeable.swift
guard Element.self is Encodable.Type else {
preconditionFailure("\(type(of: self)) does not conform to Encodable because \(Element.self) does not conform to Encodable.")
}
Source: https://github.com/apple/swift/blob/master/stdlib/public/core/Codable.swift
Answer
The reason why your first example doesn't compile (and your second crashes) is because protocols don't conform to themselves – Tag is not a type that conforms to Codable, therefore neither is [Tag]. Therefore Article doesn't get an auto-generated Codable conformance, as not all of its properties conform to Codable.
Encoding and decoding only the properties listed in the protocol
If you just want to encode and decode the properties listed in the protocol, one solution would be to simply use an AnyTag type-eraser that just holds those properties, and can then provide the Codable conformance.
You can then have Article hold an array of this type-erased wrapper, rather than of Tag:
struct AnyTag : Tag, Codable {
let type: String
let value: String
init(_ base: Tag) {
self.type = base.type
self.value = base.value
}
}
struct Article: Codable {
let tags: [AnyTag]
let title: String
}
let tags: [Tag] = [
AuthorTag(value: "Author Tag Value"),
GenreTag(value:"Genre Tag Value")
]
let article = Article(tags: tags.map(AnyTag.init), title: "Article Title")
let jsonEncoder = JSONEncoder()
jsonEncoder.outputFormatting = .prettyPrinted
let jsonData = try jsonEncoder.encode(article)
if let jsonString = String(data: jsonData, encoding: .utf8) {
print(jsonString)
}
Which outputs the following JSON string:
{
"title" : "Article Title",
"tags" : [
{
"type" : "author",
"value" : "Author Tag Value"
},
{
"type" : "genre",
"value" : "Genre Tag Value"
}
]
}
and can be decoded like so:
let decoded = try JSONDecoder().decode(Article.self, from: jsonData)
print(decoded)
// Article(tags: [
// AnyTag(type: "author", value: "Author Tag Value"),
// AnyTag(type: "genre", value: "Genre Tag Value")
// ], title: "Article Title")
Encoding and decoding all properties of the conforming type
If however you need to encode and decoded every property of the given Tag conforming type, you'll likely want to store the type information in the JSON somehow.
I would use an enum in order to do this:
enum TagType : String, Codable {
// be careful not to rename these – the encoding/decoding relies on the string
// values of the cases. If you want the decoding to be reliant on case
// position rather than name, then you can change to enum TagType : Int.
// (the advantage of the String rawValue is that the JSON is more readable)
case author, genre
var metatype: Tag.Type {
switch self {
case .author:
return AuthorTag.self
case .genre:
return GenreTag.self
}
}
}
Which is better than just using plain strings to represent the types, as the compiler can check that we've provided a metatype for each case.
Then you just have to change the Tag protocol such that it requires conforming types to implement a static property that describes their type:
protocol Tag : Codable {
static var type: TagType { get }
var value: String { get }
}
struct AuthorTag : Tag {
static var type = TagType.author
let value: String
var foo: Float
}
struct GenreTag : Tag {
static var type = TagType.genre
let value: String
var baz: String
}
Then we need to adapt the implementation of the type-erased wrapper in order to encode and decode the TagType along with the base Tag:
struct AnyTag : Codable {
var base: Tag
init(_ base: Tag) {
self.base = base
}
private enum CodingKeys : CodingKey {
case type, base
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let type = try container.decode(TagType.self, forKey: .type)
self.base = try type.metatype.init(from: container.superDecoder(forKey: .base))
}
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(type(of: base).type, forKey: .type)
try base.encode(to: container.superEncoder(forKey: .base))
}
}
We're using a super encoder/decoder in order to ensure that the property keys for the given conforming type don't conflict with the key used to encode the type. For example, the encoded JSON will look like this:
{
"type" : "author",
"base" : {
"value" : "Author Tag Value",
"foo" : 56.7
}
}
If however you know there won't be a conflict, and want the properties to be encoded/decoded at the same level as the "type" key, such that the JSON looks like this:
{
"type" : "author",
"value" : "Author Tag Value",
"foo" : 56.7
}
You can pass decoder instead of container.superDecoder(forKey: .base) & encoder instead of container.superEncoder(forKey: .base) in the above code.
As an optional step, we could then customise the Codable implementation of Article such that rather than relying on an auto-generated conformance with the tags property being of type [AnyTag], we can provide our own implementation that boxes up a [Tag] into an [AnyTag] before encoding, and then unbox for decoding:
struct Article {
let tags: [Tag]
let title: String
init(tags: [Tag], title: String) {
self.tags = tags
self.title = title
}
}
extension Article : Codable {
private enum CodingKeys : CodingKey {
case tags, title
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
self.tags = try container.decode([AnyTag].self, forKey: .tags).map { $0.base }
self.title = try container.decode(String.self, forKey: .title)
}
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(tags.map(AnyTag.init), forKey: .tags)
try container.encode(title, forKey: .title)
}
}
This then allows us to have the tags property be of type [Tag], rather than [AnyTag].
Now we can encode and decode any Tag conforming type that's listed in our TagType enum:
let tags: [Tag] = [
AuthorTag(value: "Author Tag Value", foo: 56.7),
GenreTag(value:"Genre Tag Value", baz: "hello world")
]
let article = Article(tags: tags, title: "Article Title")
let jsonEncoder = JSONEncoder()
jsonEncoder.outputFormatting = .prettyPrinted
let jsonData = try jsonEncoder.encode(article)
if let jsonString = String(data: jsonData, encoding: .utf8) {
print(jsonString)
}
Which outputs the JSON string:
{
"title" : "Article Title",
"tags" : [
{
"type" : "author",
"base" : {
"value" : "Author Tag Value",
"foo" : 56.7
}
},
{
"type" : "genre",
"base" : {
"value" : "Genre Tag Value",
"baz" : "hello world"
}
}
]
}
and can then be decoded like so:
let decoded = try JSONDecoder().decode(Article.self, from: jsonData)
print(decoded)
// Article(tags: [
// AuthorTag(value: "Author Tag Value", foo: 56.7000008),
// GenreTag(value: "Genre Tag Value", baz: "hello world")
// ],
// title: "Article Title")