How can I get specific instance when I use static method - ios

Question

I hope to get a specific instance when I use my static method. For example:
class Food: NSObject {
var name: String
class func initFruit() -> Food? {
let fruitName = NSStringFromClass(self).components(separatedBy: ".").last! as String
if "Apple" == fruitName {
return Apple(name: fruitName)
} else if "Orange" == fruitName {
return Orange(name: fruitName)
}
return nil
}
init(name: String) {
self.name = name
}
}
class Apple: Food {
}
class Orange: Food {
}
When I create an Apple instance with the method:
let apple = Apple.initFruit() as? Apple
How can I get the specific instance apple rather than use as? Apple?. I wonder how to modify the method:
static func initFruit() -> Food?

Answer 1

There are a couple of problems with your design, let me try and enumerate them:
base classes should not be aware of their subclasses, it's bad practice and it's not scalable, as adding a new subclass would require maintaining the base class method
the static method is not needed at all, at least in the shape it's written in the question, you could simply directly call the initializers for the subclasses
Leaving all those aside, you can use Self as return type for the static method, this will allow dynamic results.
class Food: NSObject {
var name: String
class func initFruit() -> Self {
let fruitName = NSStringFromClass(self).components(separatedBy: ".").last! as String
return self.init(name: fruitName)
}
required init(name: String) {
self.name = name
}
}
class Apple: Food {
}
class Orange: Food {
}
let apple = Apple.initFruit() // is an Apple, no cast needed

Answer 2

i think it's not good idea because Food is parent class and Apple inherits Food. Apple may know it's parent class cause is extends Food but Food does not.
So, if you want to create instance by some string or some variable. I would like to recommend you to adopt "Factory pattern"
reference here:
https://medium.com/swift-programming/design-patterns-creational-patterns-factory-pattern-in-swift-d049af54235b

Answer 3

Inspired by the question Generics in Swift - "Generic parameter 'T' could not be inferred I find another way to resolve this question. I add a method to infer the specific type.
func ascertainFruitType<T>() -> T {
return self as! T // as! is dangerous
}
Then the method initFruit is changed on below:
class func initFruit() -> Self {
let fruitName = NSStringFromClass(self).components(separatedBy: ".").last! as String
if "Apple" == fruitName {
return Apple(name: fruitName).ascertainFruitType()
} else {
return Orange(name: fruitName).ascertainFruitType()
}
}