Default initialiser in protocol requires unwanted mutable properties - ios

Question

In Swift 3: Imagine you want your models to be value types (struct) through out your app. But you would also like persistence of said models using Core Data/Realm, which requires you to create classes. Then you can convert your structs to classes and vice verse using JSON (which would require structs and classes to both support JSON deserialization and serialization).
Wouldn't it be neat if you don't have to write JSON deserialization (and analogously for serialization, but I'm focusing on deserialization here) in two places, but use put deserialization in a protocol, that both your struct and class uses.
Using structs we want our JSON model to have immutable fields, thus all properites being let constants. But using a protocol implementation of the deserialization does not allow for this AFAIK.
The code example below works, but it is ugly, because of all unwanted requirements (UR) marked in comments in the code.
struct JSON {
let json: [String: Any]
func value<Value>(_ key: String) throws -> Value {
guard let value = json[key] as? Value else { throw NSError() }
return value
}
}
protocol JSONDeserializable {
init(json: JSON) throws
}
protocol UserModel: JSONDeserializable {
var username: String { get set } // Unwanted requirement (UR) #1: property needs "set" so that it can be initialized within protocol
init() // UR2: needs empty init, because of default implementation of `init(json: JSON)` in `extension UserModel`
}
extension UserModel {
init(json: JSON) throws {
self.init() // UR3: Needs to call this otherwise compilation error: `'self' used before chaining to another self.init requirement`
username = try json.value("username")
}
}
struct UserStruct: UserModel {
// UR4: property cannot be `let`, beause of `set` in protocol.
var username: String = "" // UR5: Property have to have default value because of it being a empty init
init() {}
}
final class UserClass: NSObject, UserModel {
// UR6: analogue with UR4
var username: String = "" // UR7: analogue with UR5
}
let json: JSON = JSON(json: ["username": "Sajjon"])
let u1 = try UserStruct(json: json)
let u2 = try UserClass(json: json)
print(u1.username) // prints "Sajjon"
print(u2.username) // prints "Sajjon"
Is there another way of achieving this, with a lower amount of unwanted requirements? Or an optimal solution with zero UR? 🙄

Answer 1

Thanks to what #hamish pointed out, the best solution would be (where struct JSON and protocol JSONDeserializable remains the same as in the question). This is not a perfect solution since you have to implement the initializer of the class. The neat part is that you don't have to implement any initializer for the struct, since it has one implicitly.
protocol UserModel: JSONDeserializable {
var username: String { get }
var firstname: String { get }
var country: String { get }
init(
username: String,
firstname: String,
country: String
)
}
extension UserModel {
init(json: JSON) throws {
self.init(
username: try json.value("username"),
firstname: try json.value("firstname"),
country: try json.value("country")
)
}
}
struct UserStruct: UserModel {
let username: String
let firstname: String
let country: String
// struct has default initializer
}
final class UserClass: UserModel {
let username: String
let firstname: String
let country: String
init(
username: String,
firstname: String,
country: String
) {
self.username = username
self.firstname = firstname
self.country = country
}
}
let json: JSON = JSON(json: [
"username": "Sajjon",
"firstname": "Alexander",
"country": "Sweden"
])
let u1 = try UserStruct(json: json)
let u2 = try UserClass(json: json)
print(u1.username) // prints "Sajjon"
print(u2.username) // prints "Sajjon"