Why does defining CodingKeys for SOME properties require an initializer? - ios

Question

Compiles fine with no problems:
class User: Codable {
let name: String
let email: String
}
However, if we have a property not represented by a CodingKey, it demands an initializer:
class User: Codable { // Class 'User' has no initializers
let name: String
let email: String
private enum CodingKeys: String, CodingKey {
case name = "username"
}
}
Why does it decide that a lack of synthesized initializers are the problem, instead of having a warning or error requiring a CodingKey? Why does this break conformance to Decodable?
Edit: Since some people seem to be confused, I'm not asking how to resolve the error. That is obvious. I'm asking what is happening in the Codable protocol when you don't specify a CodingKey for a required property.

Answer 1

A class with at least one property that doesn't have a default value needs an initializer in order to initialize that property.
class User {
let name: String
let email: String
}
Class 'User' has no initializers
When you conform to Codable, or more specifically Decodable as you well noted, the class gets a synthesized initializer, namely User.init(from: Decoder), which fixes above issue. This synthesized initializer will use coding keys named after the properties.
class User: Decodable { ... }
As soon as you define an enum within User called CodingKeys (the exact name is important), the synthesis of the Decodable conformance will require that there is a key for each property. In other words, as soon as one key deviates from the property name, you have to define a coding key for each property.
The following does that and compiles:
class User: Decodable {
let name: String
let email: String
private enum CodingKeys: String, CodingKey {
case name = "username"
case email
}
}
If not specifying the email key, the compiler will not know how to set that property, only the developer would know how. Hence, the compiler will not synthesize it for you and you have to.
There might be a natural default as shown here:
class User: Decodable {
let name: String
let email: String
enum CodingKeys: String, CodingKey {
case name = "username"
}
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
name = try container.decode(String.self, forKey: .name)
// Default email:
email = "\(name)#example.com"
}
}

Answer 2

Xcode is throwing the first in a line of errors caused by the second example. If you added an initializer it would next probably tell you about something related to your lack of codingkey for the name property. However, when you don't define CodingKeys, it does is able to create CodingKeys as well as the default initializer for you.

Answer 3

Even though you're not going to need a CodingKey for email, you still have to declare its case, without any value as in:
private enum CodingKeys: String, CodingKey {
case name = "username"
case email
}