Codable variable inside Codable struct causes error

Question

Please take a look at my code:

No errors

struct Person: Codable {
    var name: String
    var age: Double
    var birthday: Date
    
    var selectedItem: Car
}

struct Car: Codable {
    var companyName: String
    var creationDate: Date
}

struct iPhone: Codable {
    var model: String
    var creationDate: Date
    var isScreenBroken: Bool = true
}

Build error

struct Person: Codable { // "Type 'Person' does not conform to protocol 'Decodable'", "Type 'Person' does not conform to protocol 'Encodable'"
    var name: String
    var age: Double
    var birthday: Date
    
    var selectedItem: Codable // I've changed this line
}

struct Car: Codable {
    var companyName: String
    var creationDate: Date
}

struct iPhone: Codable {
    var model: String
    var creationDate: Date
    var isScreenBroken: Bool = true
}

Type 'Person' does not conform to protocol 'Decodable'

Type 'Person' does not conform to protocol 'Encodable'

I don't understand why is this happening. It knows that selectedItem is conforming to Encodable & Decodable:

var selectedItem: Codable

I'm new to protocols in Swift so please when answering try to explain what's happening here.

Thanks!

how it knows selectedItem is conforming to Codable ? Obviously selected item is not Car type anymore. — zeytin, Jan 01 '21 at 08:42
It would have to decode `selectedItem` to a specific type, which isn’t declared. Just stating that it conforms to `Codable` does not help with decoding if it does not have a type. — Chris, Jan 01 '21 at 08:48
You could make a new type for `selectedItem` that conforms to Codable, or set it to a concrete type like `Int` or `String`. — Chris, Jan 01 '21 at 08:54
Can you please provide an answer and give an example? @Chris — אורי orihpt, Jan 01 '21 at 08:55

Joakim Danielson · Accepted Answer · 2021-01-01T09:06:29.463

The problem for the compiler here is that normally when a type is defined to conform to Codable then the compiler synthesises code for you to make Person in this case conform to the protocol. It does this by creating an implementation of init(from decoder: Decoder) throws and one of func encode(to encoder: Encoder) throws for you.

But when you change selectedItem to be of type Codable then the compiler can no longer synthesise these methods since it needs to know exactly what properties the type of selectedItem has to correctly generate the code.

What you need to do here is to use generics

struct Person<T: Codable>: Codable {
    var name: String
    var age: Double
    var birthday: Date

    var selectedItem: T
}

struct Car: Codable {
    var companyName: String
    var creationDate: Date
}

struct iPhone: Codable {
    var model: String
    var creationDate: Date
    var isScreenBroken: Bool = true
}

Then the compiler is happy again and you can use it like

let person = Person(name: "Joe", age: 40, birthday: date, selectedItem: Car(companyName: "Ford", creationDate: Date()))

Codable variable inside Codable struct causes error

No errors

Build error

1 Answers1