Set new value in computed property - swift

Question

I declared a variable to be settable and gettable. In its getter, I have some logic, but not in setter. I decided to use computed property:
var product: Product? {
// setter
set(newProduct) {
// COMPILER WARNING: Attemping to modify 'product' within its own setter
product = newProduct
}
// getter
get {
let price = SOME_EQUATION
return Product(price)
}
}
The issue is in setter, compiler is warning me Attemping to modify 'product' within its own setter. But I do want to set the newProduct to the product instance variable. What is the correct way to do that?

Answer 1

I assume that SOME_EQUATION is using the value from the product, right? If not, then the setter is not needed at all.
Also, there's a side effect to the way you wrote the code: every time you get the value of the product property, you'll get a new instance.
But answering your question: for computed properties, there's no storage in the instance. So what you might want to do is to create a private property and store value into it instead:
private var _product: Product?
var product: Product? {
// setter
set(newProduct) {
// COMPILER WARNING: Attemping to modify 'product' within its own setter
_product = newProduct
}
// getter
get {
let price = SOME_EQUATION(_product)
return Product(price)
}
}

Answer 2

In your example you will always return Product(SOME_EQUATION). If this equation do not depend on the class containing the product property maybe consider moving this as a factory method in the Product.
Note that setting value in its' set is recursion.
To achieve real computed property:
var product: Product? {
return Product(SOME_EQUATION)
}
Note that let product = Product(SOME_EQUATION) is actually better than computed property if it is suitable in your case, because this equation will be executed only once.
Edit:
Computed properties are not used for what you are describing. Having no logic in the setter, and not using directly the value that you have set is wrong.
As I have described above, you need to create factory method in your Product class as:
static func create(for someEquation: YourType) -> Product {
return Product(someEquation)
}
And in your class you use it as:
Product.create(for: SOME_EQUATION)
More info on computed properties: you use them when you already have dependancies in your class, and you want to extract some point of your data for convenience.
If you depend on some parameter, and not dependancy, you would better make function, taking parameter!