Resolved: Binding didSet variable to update computed var

In this post, we will see how to resolve Binding didSet variable to update computed var

Question:

I am creating a nutrition tracker where I need the nutrients to update when the amount of the food is changed. This is dictated by the nutrition in the food and the amount eaten (obviously). I am trying to have my computed var (nutrients) update when the amount changes but what I have so far just reinitializes the object’s amount back to 1 instead of what is set in the Binding textfield.
There is also a measurement selector that needs to update and set the nutrient values but for the sake of simplicity I have just included the amount field. Can someone tell me where my thinking is wrong?
Example Struct

struct Intake: Identifiable, Equatable, Hashable {
    var id : String = UUID().uuidString
    var amount: Double = 1 {
        didSet {
            updateNutrients()
    }

    var nutrients: Nutrients = Nutrients()

    init(food: Food) {
        self.food = food
        print("reinitalizing food")
        updateNutrients()
    }

    private mutating func updateNutrients() {
        nutrients = Nutrients(
            carb: self.food.consumableProps.nutrients.carb ?? 0 * self.amount, 
            fat: self.food.consumableProps.nutrients.fat ?? 0 * self.amount,
            protein: self.food.consumableProps.nutrients.protein ?? 0 * self.amount,
            transFat: self.food.consumableProps.nutrients.transFat ?? 0 * self.amount,
            saturatedFat: self.food.consumableProps.nutrients.saturatedFat ?? 0 * self.amount,
            sodium: self.food.consumableProps.nutrients.sodium ?? 0 * self.amount,
            fiber: self.food.consumableProps.nutrients.fiber ?? 0 * self.amount,
            sugars: self.food.consumableProps.nutrients.sugars ?? 0 * self.amount,
            cholesterol: self.food.consumableProps.nutrients.cholesterol ?? 0 * self.amount,
            calcium: self.food.consumableProps.nutrients.calcium ?? 0 * self.amount,
            iron: self.food.consumableProps.nutrients.iron ?? 0 * self.amount,
            potassium: self.food.consumableProps.nutrients.potassium ?? 0 * self.amount,
            magnesium: self.food.consumableProps.nutrients.magnesium ?? 0 * self.amount,
            calories: self.food.consumableProps.nutrients.calories ?? 0 * self.amount
        )
    }

Example binding in View:

struct IntakeRow:View{
   @Binding var intake:Intake

   var body: some View{
      VStack{
         TextField("Amount", value: $intake.amount, formatter: NumberFormatter())
         HStack(spacing: 10) {
                nutrientText(macro: .carb, nutrients.carb ?? 0, format: "%.0f")
                    .frame(width: g.size.width/5)
                nutrientText(macro: .fat, nutrients.fat ?? 0, format: "%.0f")
                    .frame(width: g.size.width/5)
                nutrientText(macro: .protein, nutrients.protein ?? 0, format: "%.0f")
                    .frame(width: g.size.width/5)
            }.frame(height:g.size.height)
        }
    }
    
    private func nutrientText(macro: Macro, _ value: Double, format: String) -> some View {
        return Text(String(format: format, value))
            .font(.system(size: 14, weight: .semibold))
            .foregroundColor(macro.color)
            .minimumScaleFactor(0.8)
            .lineLimit(1)
    }

Nutrients struct:

struct Nutrients: Codable, Sequence {
    var carb: Double? = 0
    var fat: Double? = 0
    var protein: Double? = 0
    var transFat: Double? = nil
    var saturatedFat: Double? = nil
    var sodium: Double? = nil
    var fiber: Double? = nil
    var sugars: Double? = nil
    var cholesterol: Double? = nil
    var calcium: Double? = nil
    var iron: Double? = nil
    var potassium: Double? = nil
    var magnesium: Double? = nil
    var calories: Double? = nil
    
    
    static func +(lhs: Nutrients, rhs: Nutrients) -> Nutrients {
        return Nutrients(
            carb: (lhs.carb ?? 0) + (rhs.carb ?? 0),
            fat: (lhs.fat ?? 0) + (rhs.fat ?? 0),
            protein: (lhs.protein ?? 0) + (rhs.protein ?? 0),
            transFat: (lhs.transFat ?? 0) + (rhs.transFat ?? 0),
            saturatedFat: (lhs.saturatedFat ?? 0) + (rhs.saturatedFat ?? 0),
            sodium: (lhs.sodium ?? 0) + (rhs.sodium ?? 0),
            fiber: (lhs.fiber ?? 0) + (rhs.fiber ?? 0),
            sugars: (lhs.sugars ?? 0) + (rhs.sugars ?? 0),
            cholesterol: (lhs.cholesterol ?? 0) + (rhs.cholesterol ?? 0),
            calcium: (lhs.calcium ?? 0) + (rhs.calcium ?? 0),
            iron: (lhs.iron ?? 0) + (rhs.iron ?? 0),
            potassium: (lhs.potassium ?? 0) + (rhs.potassium ?? 0),
            magnesium: (lhs.magnesium ?? 0) + (rhs.magnesium ?? 0),
            calories: (lhs.calories ?? 0) + (rhs.calories ?? 0)
        )
    }
    
    func makeIterator() -> AnyIterator<Double?> {
        var index = 0
        let values: [Double?] = [carb, fat, protein, transFat, saturatedFat, sodium, fiber, sugars, cholesterol, calcium, iron, potassium, magnesium, calories]
        return AnyIterator {
            defer { index += 1 }
            return index < values.count ? values[index] : nil
        }
    }
}

Food Struct:

enum Food: Codable {
    case foodItem(FoodItem)
    case recipe(Recipe)
    case restaurantMeal(RestaurantMeal)
    
    private enum CodingKeys: String, CodingKey {
        case type, properties
    }
    
    private enum FoodType: String, Codable {
        case foodItem = "FoodItem"
        case recipe = "Recipe"
        case restaurantMeal = "RestaurantMeal"
    }
    
    func getFood() -> any Consumable{
        switch self {
        case .foodItem(let foodItem):
            return foodItem
        case .recipe(let recipe):
            return recipe
        case .restaurantMeal(let restaurantMeal):
            return restaurantMeal
        }
    }
    
    subscript<T>(keyPath: WritableKeyPath<any Consumable, T>) -> T {
        get {
            switch self {
            case let .foodItem(foodItem): return foodItem[keyPath: keyPath]
            case let .recipe(recipe): return recipe[keyPath: keyPath]
            case let .restaurantMeal(restaurantMeal): return restaurantMeal[keyPath: keyPath]
            }
        }
    }
    
    
    
    var consumableProps: any Consumable {
        get {
            switch self {
            case let .foodItem(foodItem): return foodItem
            case let .recipe(recipe): return recipe
            case let .restaurantMeal(restaurantMeal): return restaurantMeal
            }
        }
        set{
            switch self {
            case .foodItem(var foodItem):
                foodItem = newValue as! FoodItem
                self = .foodItem(foodItem)
            case .recipe(var recipe):
                recipe = newValue as! Recipe
                self = .recipe(recipe)
            case .restaurantMeal(var restaurantMeal):
                restaurantMeal = newValue as! RestaurantMeal
                self = .restaurantMeal(restaurantMeal)
            }
        }
        
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let type = try container.decode(FoodType.self, forKey: .type)
        switch type {
        case .foodItem:
            let foodItem = try container.decode(FoodItem.self, forKey: .properties)
            self = .foodItem(foodItem)
        case .recipe:
            let recipe = try container.decode(Recipe.self, forKey: .properties)
            self = .recipe(recipe)
        case .restaurantMeal:
            let restaurantMeal = try container.decode(RestaurantMeal.self, forKey: .properties)
            self = .restaurantMeal(restaurantMeal)
        }
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        switch self {
        case .foodItem(let foodItem):
            try container.encode(FoodType.foodItem, forKey: .type)
            try container.encode(foodItem, forKey: .properties)
        case .recipe(let recipe):
            try container.encode(FoodType.recipe, forKey: .type)
            try container.encode(recipe, forKey: .properties)
        case .restaurantMeal(let restaurantMeal):
            try container.encode(FoodType.restaurantMeal, forKey: .type)
            try container.encode(restaurantMeal, forKey: .properties)
        }
    }
}

Best Answer:

Your syntax is slightly off when you pass in the new nutrient values:

self.food.consumableProps.nutrients.magnesium ?? 0 * self.amount

is evaluated as the current food.consumableProp.nutrients.magnesium value or 0 * amount (which of course is always 0). Adding parentheses should fix your issue:

(self.food.consumableProps.nutrients.magnesium ?? 0) * self.amount

This change makes it work on my end, but I did still have to change a few things to get it to compile so let me know if it doesn’t work for you.

If you have better answer, please add a comment about this, thank you!

Source: Stackoverflow.com

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