Clash Royale CLAN TAG#URR8PPP

If I subclass a class, can I specify a certain subclass an instance variable should be?

I have an NSObject subclass, BaseClass. BaseClass is a placeholder class for a couple subclasses, SubClassA and SubClassB. There is an instance variable that I have present on both of the subclasses. They're the same name, and are both of a corresponding subclass of another object. They're often used in very similar ways, so I wanted to move some functionality from my SubClassA and SubClassB to the BaseClass. However, I need access to that variable.

BaseClass

SubClassA

SubClassB

If I move the variable into the BaseClass, I am unable to specify the proper subclass of it in SubClassA and SubClassB, saying I can't override it. If I use the common parent class of this instance variable in the BaseClass, I lose some access to things that aren't common between how SubClassA and SubClassB work.

BaseClass

SubClassA

SubClassB

BaseClass

SubClassA

SubClassB

This is a more primitive example, but the basics of what I'm trying to do. This example obviously does not work. Are my only options to choose having to define common functionality within SubClassA and SubClassB or is there a proper way to achieve my goal here?

SubClassA

SubClassB

class BaseClass: NSObject
var myObject: MyObject


class SubClassA: BaseClass
override var myObject: MyObjectA


class SubClassB: BaseClass
override var myObject: MyObjectB


class MyObject: NSObject

class MyObjectA: MyObject

class MyObjectB: MyObject


This gives me the error:

Property 'myObject' with type 'MyObjectA' cannot override a property with type 'MyObject'

3 Answers
3

How about using generic? For simplicity, I removed NSObject

class MyObject



class MyObjectA: MyObject



class MyObjectB: MyObject



class BaseClass<T> where T : MyObject
var myObject: T?


class SubClassA: BaseClass<MyObjectA>


class SubClassB: BaseClass<MyObjectB>



class BaseClass<T>: NSObject, DelegateProtocol where T:ServiceViewController {

Instead of putting the myObject related code into BaseClass, you could put it into a protocol extension. Consider this:

myObject

BaseClass

class BaseClass


class SubClassA: BaseClass, HasMyObject
var myObject: MyObjectA


class SubClassB: BaseClass
var myObject: MyObjectB


class MyObject

class MyObjectA: MyObject

class MyObjectB: MyObject

protocol HasMyObject
associatedtype MyObjectClass

var myObject: MyObjectClass get set



This is conceptually very similar to using generics, but would separate your myObject related code from the rest of the code in your class. Whether this is actually preferable over generics depends on your coding style and specific use-cases.

myObject

You can create a class function in your BaseClass that returns a class that inherits from a BaseObject (myObject) and override it for whichever class you need.

BaseClass

BaseObject

class BaseClassObject: NSObject



class BaseClass: NSObject
func generateClass() -> NSObject
return BaseClassObject()



class BranchedObject: BaseClassObject



class SubClassA: BaseClass

var myObject: NSObject?

override func generateClass() -> NSObject
return BranchedObject()


override init()
super.init()
self.myObject = self.generateClass()





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