How swift implement the default Decodable for struct? - swift

Question

struct Person: Decodable {
let firstName: String
}
var data = """
{"firstName": "Fai"}
""".data(using: .utf8)!
let decoder = JSONDecoder()
let parsed = try decoder.decode(Person.self, from: data)
JSONDecoder will decode the data, which is comfirmed to Decodable protocol.
So I want to know how swift implement this. But I can not get any idea in the source code: https://github.com/apple/swift/blob/main/stdlib/public/core/Codable.swift
The Decodable protocol only need to implement an init(from decoder: Decoder) function.
If I am going to do it, I will make an extension for struct:
extension struct: Decodable {
init(from decoder: Decoder) {...}
}
But when I delete the Decodable on my example, the compiler give errors:
Instance method 'decode(_:from:)' requires that 'Person' conform to 'Decodable'
So this is not the swift way to implement this. How's swift way? And where's the source code?

Answer 1

You can see what the compiler writes for you using -print-ast:
echo 'struct Person: Decodable {
let firstName: String
}' | swiftc -print-ast -
This will output most of the auto-generated code (it should include all of the Codable conformances, but there are a few other kinds of auto-generated code that won't include their implementation):
internal struct Person : Decodable {
internal let firstName: String
private enum CodingKeys : CodingKey {
case firstName
#_implements(Equatable, ==(_:_:)) fileprivate static func __derived_enum_equals(_ a: Person.CodingKeys, _ b: Person.CodingKeys) -> Bool {
private var index_a: Int
switch a {
case .firstName:
index_a = 0
}
private var index_b: Int
switch b {
case .firstName:
index_b = 0
}
return index_a == index_b
}
fileprivate func hash(into hasher: inout Hasher) {
private var discriminator: Int
switch self {
case .firstName:
discriminator = 0
}
hasher.combine(discriminator)
}
private init?(stringValue: String) {
switch stringValue {
case "firstName":
self = Person.CodingKeys.firstName
default:
return nil
}
}
private init?(intValue: Int) {
return nil
}
fileprivate var hashValue: Int {
get {
return _hashValue(for: self)
}
}
fileprivate var intValue: Int? {
get {
return nil
}
}
fileprivate var stringValue: String {
get {
switch self {
case .firstName:
return "firstName"
}
}
}
}
internal init(firstName: String)
internal init(from decoder: Decoder) throws {
#_hasInitialValue private let container: KeyedDecodingContainer<Person.CodingKeys> = try decoder.container(keyedBy: Person.CodingKeys.self)
self.firstName = try container.decode(String.self, forKey: Person.CodingKeys.firstName)
}
}
For the full implementation details, see DerivedConformanceCodable.cpp. Probably of most interest to your question is deriveBodyDecodable_init.