How to pass generic property in swift? - swift

Question

Let's assume I have these two models:
struct Vegetarian: Menus {
var id: String? = UUID().uuidString
var vegiDishes: [String]?
var veganDishes: [String]?
}
struct Vegan: Menus {
var id: String? = UUID().uuidString
var veganDishes: [String]?
}
They are both conforms to this protocol:
protocol Menus: Identifiable, Codable, Hashable {
var id: String? {get set}
}
extension Menus {
var vegiDishes: [String]? {
get{ return [] } set{}
}
var veganDishes: [String]? {
get{ return [] } set{}
}
}
I have this object:
var myMenu: Vegetarian = Vegetarian()
If I want to add a dish, I'll do this:
addDish(menu: myMenu, dish: "Some Vegetarian Dish")
But since the two models have a different type of dishes properties, I'll have to have something like this:
func addDish<T: Menus> (menu: T, dish: String) {
switch menu {
case is Vegetarian:
var newMenu: Vegetarian = Vegetarian()
newMenu.veganDishes?.append(dish)
case is Vegan:
var newMenu: Vegan = Vegan()
newMenu.veganDishes?.append(dish)
default:
print("no such type")
}
}
Is there a way to simplify my method so I could specify a property in the call?
Let's say, something like this:
addDish(menu: myMenu, dish: "Some Vegetarian Dish", genericProperty: veganDishes)
Which will make my method look like this?
func addDishGenericProperty<T: Menus> (menu: T, dish: String, genericProperty?) {
menu.genericProperty.append(dish)
}

Answer 1

This seems perfect for a key-path! Or more specifically, a WritableKeyPath.
Since Vegetarian is a struct, it is a value type. This means we can't just set vegiDishes for example and expect that value to be reflected to our myMenu from outside the function. Instead you can use inout so myMenu is changed.
I had to also clear up the Menus and the types quite a bit, so they use non-optional arrays instead. If you need to check if the array is empty, use myMenu.vegiDishes.isEmpty.
Here is an example of how you can achieve all this:
protocol Menus: Identifiable, Codable, Hashable {
var id: String { get }
}
struct Vegetarian: Menus {
enum CodingKeys: CodingKey {
case vegiDishes
case veganDishes
}
let id = UUID().uuidString
var vegiDishes: [String]
var veganDishes: [String]
init(vegiDishes: [String] = [], veganDishes: [String] = []) {
self.vegiDishes = vegiDishes
self.veganDishes = veganDishes
}
}
struct Vegan: Menus {
enum CodingKeys: CodingKey {
case veganDishes
}
let id = UUID().uuidString
var veganDishes: [String]
init(veganDishes: [String] = []) {
self.veganDishes = veganDishes
}
}
Adding dishes:
func addDish<T: Menus>(menu: inout T, dish: String, kind: WritableKeyPath<T, [String]>) {
menu[keyPath: kind].append(dish)
}
var myMenu: Vegetarian = Vegetarian()
addDish(menu: &myMenu, dish: "Some Vegetarian Dish", kind: \.vegiDishes)
print(myMenu.vegiDishes)
Prints:
["Some Vegetarian Dish"]
Or alternatively, you could literally just do the following if possible in your case:
myMenu.vegiDishes.append("Some Vegetarian Dish")