SwiftでProtocolの配列をプロパティに持つオブジェクトをJSONにencodeする

概要

タイトルの通り、以下のようなJSONをencodeしようとしてハマったため、こちらの記事に簡単にメモしておきます。

{
    "value": 5,
    "list": [
        {
            "className": "A"
        },
        {
            "className": "B"
        }
    ]
}

実装例。containerをネストさせます。

import Foundation

protocol P : Encodable {}

class A : P {
    var className = "A"
}

class B : P {
    var className = "B"
}

class Data : Encodable {

    var value: Int = 5
    
    var list: Array<P> = [A(), B()]

    public func encode(to encoder: Encoder) throws {
        var rootContainer = encoder.container(keyedBy: CodingKeys.self)
        try rootContainer.encode(value, forKey: .value)

        var listContainer = rootContainer.nestedUnkeyedContainer(forKey: .list)
        for p in list {
            try listContainer.encode(p)
        }
    }

    enum CodingKeys: String, CodingKey {
        case value
        case list
    }
}


let data = try JSONEncoder().encode(Data())
print(String(data: data, encoding: .utf8)!)
// -> {"value":5,"list":[{"className":"A"},{"className":"B"}]}

検索用 エラーの例

error: type 'any P' cannot conform to 'Encodable'
        try rootContainer.encode(list, forKey: .list)

note: only concrete types such as structs, enums and classes can conform to protocols
        try rootContainer.encode(list, forKey: .list)

note: requirement from conditional conformance of 'Array<any P>' to 'Encodable'
        try rootContainer.encode(list, forKey: .list)