How to parse JSON using custom decoder init with incrementing keys in Swift

Question

I have some meal json which I want to convert into a MealData struct

let randomMealJson = """
{
    "meals": [{
        "idMeal": "52812",
        "strMeal": "Beef Brisket Pot Roast",
        "strDrinkAlternate": null,
        "strCategory": "Beef",
        "strArea": "American",
        "strInstructions": "Meal instructions",
        "strMealThumb": "https://www.themealdb.com/images/media/meals/ursuup1487348423.jpg",
        "strTags": "Meat",
        "strYoutube": "https://www.youtube.com/watch?v=gh48wM6bPWQ",
        "strIngredient1": "Beef Brisket",
        "strIngredient2": "Salt",
        "strIngredient3": "Onion",
        "strIngredient4": "Garlic",
        "strIngredient5": "Thyme",
        "strIngredient6": "Rosemary",
        "strIngredient7": "Bay Leaves",
        "strIngredient8": "beef stock",
        "strIngredient9": "Carrots",
        "strIngredient10": "Mustard",
        "strIngredient11": "Potatoes",
        "strIngredient12": null,
        "strIngredient13": null,
        "strIngredient14": null,
        "strIngredient15": null,
        "strIngredient16": null,
        "strIngredient17": null,
        "strIngredient18": null,
        "strIngredient19": null,
        "strIngredient20": null,
        "strMeasure1": "4-5 pound",
        "strMeasure2": "Dash",
        "strMeasure3": "3",
        "strMeasure4": "5 cloves",
        "strMeasure5": "1 Sprig",
        "strMeasure6": "1 sprig ",
        "strMeasure7": "4",
        "strMeasure8": "2 cups",
        "strMeasure9": "3 Large",
        "strMeasure10": "1 Tbsp",
        "strMeasure11": "4 Mashed",
        "strMeasure12": "",
        "strMeasure13": "",
        "strMeasure14": "",
        "strMeasure15": "",
        "strMeasure16": "",
        "strMeasure17": "",
        "strMeasure18": "",
        "strMeasure19": "",
        "strMeasure20": "",
        "strSource": "http://www.simplyrecipes.com/recipes/beef_brisket_pot_roast/",
        "dateModified": null
    }]
}
"""

Here is the structure I want to make...

struct MealData: Decodable {
    let meals: [Meal]
}

struct Meal: Decodable {
    let items: [Item]
}

extension Meal {
    struct Item: Decodable {
        let ingredient: String
        let measure: String
    }
}

extension Meal {
    init(from decoder: Decoder) throws {
//        var items: [Item] = (0...20).map { num in      
//           what to do here?
//        }
    }
}

I'd like to go through the ingredients and measures and create a Meal.Item from them but I can't work out how to achieve this. It is possible in the way I have indicated in the code?

Answer 1

Following @joakim-danielson's answer. You can do the same thing inside init(from:) initializer, like your original intention.

Just decode the ingredients and measures of each Meal as [String: String?] and compose your custom Items:

extension Meal {
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        
        let mealDict = try container.decode([String: String?].self)
        var index = 1
        var items = [Item]()
        while
            let ingredient = mealDict["strIngredient\(index)"] as? String,
            let measure = mealDict["strMeasure\(index)"] as? String,
            !measure.isEmpty
        {
            items.append(Item(ingredient: ingredient, measure: measure))
            index += 1
        }
        self.items = items
    }
}

Answer 2

I would do this in 2 steps. First decode to a dictionary

struct MealData: Decodable {
    let meals: [[String: String?]]
}

then I would convert to another type in a separate step

struct Recipie {
    var ingredients: [Ingredient]
}

struct Ingredient {
    let ingredient: String
    let measure: String
}

Here is the full code

do {
    let result = try JSONDecoder().decode(MealData.self, from: randomMealJson)

    var recipies = [Recipie]()
    for meal in result.meals {
        var recipie = Recipie(ingredients: [])
        var index = 1
        while true {
            guard let ingredientValue = meal["strIngredient\(index)"],
                  let measureValue = meal["strMeasure\(index)"],
                  let ingredient = ingredientValue,
                  let measure = measureValue, !measure.isEmpty
            else { break }

            recipie.ingredients.append(Ingredient(ingredient: ingredient, measure: measure))
            recipies.append(recipie)
            index += 1
        }
    }
} catch {
    print(error)
}