FeedItem Decoder

FeedItem.swift

import Foundation

enum FeedItem {
    case article(Article)
    case ad(Ad)
}

struct Article: Decodable {
    let id: UInt
    let title: String
}

struct Ad: Decodable {
    let id: UInt
    let unitId: String
}

//FeedItem+Decodable.swift

extension FeedItem: Decodable {
    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let type = try container.decode(ItemType.self, forKey: .type)
        
        switch type {
        case .article:  self = .article(try container.decode(Article.self, forKey: .data))
        case .ad:       self = .ad(try container.decode(Ad.self, forKey: .data))
        }
    }
    
    enum CodingKeys: CodingKey {
        case type
        case data
    }
    
    enum ItemType: String, Codable {
        case article = "article"
        case ad = "ad_unit"
    }
}