Decoding a heterogenous JSON array

Example.swift

// This is intended to be dropped in a Playground.

import Foundation

let json =
"""
    {
      "name": "Casey's Corner",
      "menu": [
        {
          "itemType": "drink",
          "drinkName": "Dry Vodka Martini"
        },
        {
          "itemType": "drink",
          "drinkName": "Jack-and-Diet"
        },
        {
          "itemType": "appetizer",
          "appName": "Nachos"
        },
        {
          "itemType": "entree",
          "entreeName": "Steak",
          "temperature": "Medium Rare"
        },
        {
          "itemType": "entree",
          "entreeName": "Caesar Salad"
        },
        {
          "itemType": "entree",
          "entreeName": "Grilled Salmon"
        }
      ]
    }
"""

struct Drink: Decodable {
    let drinkName: String
}

struct Appetizer: Decodable {
    let appName: String
}

struct Entree: Decodable {
    let entreeName: String
    let temperature: String?
}

struct Restaurant: Decodable {
    let name: String
    let menu: [Any]
    
    // The normal, expected CodingKey definition for this type
    enum RestaurantKeys: CodingKey {
        case name
        case menu
    }
    
    // The key we use to decode each menu item's type
    enum MenuItemTypeKey: CodingKey {
        case itemType
    }
    
    // The enumeration that actually matches menu item types;
    // note this is **not** a CodingKey
    enum MenuItemType: String, Decodable {
        case drink
        case appetizer
        case entree
    }
    
    init(from decoder: Decoder) throws {
        // Get the decoder for the top-level object
        let container = try decoder.container(keyedBy: RestaurantKeys.self)
        
        // Decode the easy stuff: the restaurant's name
        self.name = try container.decode(String.self, forKey: .name)
        
        // Create a place to store our menu
        var inProgressMenu: [Any] = []
        // Get a copy of the array for the purposes of reading the type
        var arrayForType = try container.nestedUnkeyedContainer(forKey: .menu)
        // Make a copy of this for reading the actual menu items.
        var array = arrayForType
        
        // Start reading the menu array
        while !arrayForType.isAtEnd {
            // Get the object that represents this menu item
            let menuItem = try arrayForType.nestedContainer(keyedBy: MenuItemTypeKey.self)
            // Get the type from this menu item
            let type = try menuItem.decode(MenuItemType.self, forKey: .itemType)
            
            // Based on the type, create the appropriate menu item
            
            // Note we're switching to using `array` rather than `arrayForType`
            // because we need our place in the JSON to be back before we started
            // reading this menu item.
            switch type {
            case .drink:
                let drink = try array.decode(Drink.self)
                inProgressMenu.append(drink)
            case .appetizer:
                let appetizer = try array.decode(Appetizer.self)
                inProgressMenu.append(appetizer)
            case .entree:
                let entree = try array.decode(Entree.self)
                inProgressMenu.append(entree)
            }
        }
        
        // Set our menu
        self.menu = inProgressMenu
    }
}

let data = json.data(using: .utf8)!
let restaurant = try! JSONDecoder().decode(Restaurant.self, from: data)

print("\(restaurant.name)")
for item in restaurant.menu {
    if let d = item as? Drink {
        print("  +-- Drink: \(d.drinkName)")
    } else if let a = item as? Appetizer {
        print("  +-- Appetizer: \(a.appName)")
    } else if let e = item as? Entree {
        print("  +-- Entree: \(e.entreeName)")
        if let temp = e.temperature {
            print("      Temperature: \(temp)")
        }
    }
}

/* Expected output:
* Casey's Corner
*   +-- Drink: Dry Vodka Martini
*   +-- Drink: Jack-and-Diet
*   +-- Appetizer: Nachos
*   +-- Entree: Steak
*       Temperature: Medium Rare
*   +-- Entree: Caesar Salad
*   +-- Entree: Grilled Salmon
*/

@cliss Love this, thanks for sharing! A change I'd recommend for increased type-safety here would be to narrow the menu items based on a protocol. eg:

protocol MenuItem: Decodable {}

struct Drink: MenuItem {
    let drinkName: String
}

struct Appetizer: MenuItem {
    let appName: String
}

struct Entree: MenuItem {
    let entreeName: String
    let temperature: String?
}

struct Restaurant: Decodable {
    let name: String
    let menu: [MenuItem]

    // The normal, expected CodingKey definition for this type
    enum RestaurantKeys: CodingKey {
        case name
        case menu
    }

    // The key we use to decode each menu item's type
    enum MenuItemTypeKey: CodingKey {
        case itemType
    }

    // The enumeration that actually matches menu item types;
    // note this is **not** a CodingKey
    enum MenuItemType: String, Decodable {
        case drink
        case appetizer
        case entree

        var associatedItemType: MenuItem.Type {
            switch self {
            case .drink:
                return Drink.self
            case .appetizer:
                return Appetizer.self
            case .entree:
                return Entree.self
            }
        }
    }

    init(from decoder: Decoder) throws {
        // Get the decoder for the top-level object
        let container = try decoder.container(keyedBy: RestaurantKeys.self)

        // Decode the easy stuff: the restaurant's name
        self.name = try container.decode(String.self, forKey: .name)

        // Create a place to store our menu
        var inProgressMenu: [MenuItem] = []
        // Get a copy of the array for the purposes of reading the type
        var arrayForType = try container.nestedUnkeyedContainer(forKey: .menu)
        // Make a copy of this for reading the actual menu items.
        var array = arrayForType

        // Start reading the menu array
        while !arrayForType.isAtEnd {
            // Get the object that represents this menu item
            let menuItem = try arrayForType.nestedContainer(keyedBy: MenuItemTypeKey.self)
            // Get the type from this menu item
            let type = try menuItem.decode(MenuItemType.self, forKey: .itemType)

            // Based on the type, create the appropriate menu item

            // Note we're switching to using `array` rather than `arrayForType`
            // because we need our place in the JSON to be back before we started
            // reading this menu item.
            inProgressMenu.append(try array.decode(type.associatedItemType))
        }

        // Set our menu
        self.menu = inProgressMenu
    }
}

This way you can be confident that only types conforming to MenuItem might show up in your menu, and an exception will be thrown if an unrecognized type is found.

@cliss Love this, thanks for sharing! A change I'd recommend for increased type-safety here would be to narrow the menu items based on a protocol. eg:
[...]
This way you can be confident that only types conforming to MenuItem might show up in your menu, and an exception will be thrown if an unrecognized type is found.

We 100% agree, but I wanted to show an example of completely disparate types, which are possible in JSON.

You could also use an enum I guess to preserve the actual types.
Something like

enum MyEnum {
    case drink(Drink)
    case appetizer(Appetizer)
    ....
}

and then use that like
let menu: [MyEnum]

What do you think about that?

@dehlen Coincidentally, in the app I'm working on, that's precisely what I did. But that's not the point of the blog post nor this code snippet. :)

@cliss Great idea. Never thought to do this.

Another issue I run into often with foreign APIs is handling unexpected values in enums like MenuItemType. If an itemType value is returned that is something other than "drink", "appetizer", or "entree" then the entire Restaurant object fails to decode. That's typically not what I want to happen. So, I write a custom init to handle unknown values.

https://gist.github.com/michaelbiggs/e1c09cbf6d78a0e9f7b32277fa8cda07

@michaelbiggs For sure.

Another issue I run into often with foreign APIs is handling unexpected values in enums like MenuItemType. If an itemType value is returned that is something other than "drink", "appetizer", or "entree" then the entire Restaurant object fails to decode. That's typically not what I want to happen. So, I write a custom init to handle unknown values.

If you run into this or similar issues caused by unstable or partially broken APIs fairly often, I can recommend the BetterCodable or EvenBetterCodable packages. With those you can say how to handle unknown or incorrect values, e.g. by returning nil for the specific value or by dropping it from the array.