Protocol with Encodable protocol

Question

I have struct that has parameter with protocol type

protocol CheckoutOrder {}

struct CheckoutOrderEntity: Encodable {
    private enum CodingKeys: String, CodingKey {
        case amount
        case payment
    }
    
    let amount: Int
    let payment: CheckoutOrder
}

Also have 2 structs which implements protocol CheckoutOrder

struct ApplePayOrderEntity: Encodable {
    private enum CodingKeys: String, CodingKey {
        case token
    }
    
    let token: ApplePayTokenEntity
}

extension ApplePayOrderEntity: CheckoutOrder { }

struct CheckoutPaymentEntity: Encodable {
    private enum CodingKeys: String, CodingKey {
        case terminalName
        case shouldUseBalance
    }
    
    let shouldUseBalance: Bool
    let terminalName: String
}

extension CheckoutPaymentEntity: CheckoutOrder { }

I have an error: Type 'CheckoutOrderEntity' does not conform to protocol 'Encodable' and CheckoutOrder protocol requires function 'encode(to:)'.... So, the main reason of using protocol is that payment can be type of ApplePayOrderEntity or CheckoutPaymentEntity. How to solve problem or what is best practice in this case?

The linked question is for arrays, but you can convert it to just handle a single element by using `try container.encode(payment, forKey: .payment)`. It just won't write an automatic conformance for you. — Rob Napier, Aug 09 '22 at 13:58
(You also need to add `: Encodable` to the protocol definition as Joakim Danielson notes. But the deeper issue is that you won't get automatic conformance because the compiler just won't do that today.) — Rob Napier, Aug 09 '22 at 14:06

score 2 · Accepted Answer · answered Aug 09 '22 at 11:53

First you need to make the protocol extend Encodable to tell the compiler that anything that conforms to CheckoutOrder also conforms to Encodable

protocol CheckoutOrder: Encodable {}

Then I would use generics for this and change CheckoutOrderEntity to

struct CheckoutOrderEntity<Payment: CheckoutOrder>: Encodable {
    private enum CodingKeys: String, CodingKey {
        case amount
        case payment
    }

    let amount: Int
    let payment: Payment
}

Example

let checkout = CheckoutOrderEntity(amount: 1000,
                                   payment: CheckoutPaymentEntity(shouldUseBalance: true,
                                                                  terminalName: "terminal"))

Protocol with Encodable protocol

1 Answers1