Subclass issue in iOS

Question

In my case, UIViewController B is a subclass of UIViewController A. B can surely access all the methods and variables from A, since B is subclassing from A (i.e. A is the parent of B).

However, A needs a variable from B. Is there possible to do that ?

Thanks.

Are you talking about accessing actual variable values at runtime? If so there are a few options, e.g. notifications, saving data to nsuserdefaults, etc. — ader, Jan 27 '12 at 09:54
I want A (parent) can access the instance variables which set by B (child) in runtime. How to design and code that? — user403015, Jan 27 '12 at 09:57
The superclass should not know anything about the subclass. Can you please detail the scenario you're dealing with? In particular why the superclass would need to access the subclass' variable? — John Mead, Jan 27 '12 at 10:01

score 0 · Answer 1 · answered Jan 27 '12 at 09:53

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Add that variable as a default in subclass A. Then it's also available in subclass B?

If that's something you don't want, then I suppose there is something wrong with your design?

answered Jan 27 '12 at 09:53

Glenn

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What I want is that A (parent) needs to access the instance variables which set by B (child) in runtime. – user403015 Jan 27 '12 at 09:58

score 0 · Answer 2 · answered Jan 27 '12 at 10:21

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If the instance variable declared in the subclass has a getter method, any method in the superclass could always ask an object of its class if it respondsToSelector: for that getter method, and if so, call it to get the value of the instance variable.

answered Jan 27 '12 at 10:21

hotpaw2

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Thanks. Could you please show me some sample code of your idea? It would be a great help to me. – user403015 Jan 27 '12 at 10:25

SVGreg · Accepted Answer · 2012-01-27T10:48:29.227

Actually, if you need this type of relations - your design is wrong. I mean - you do not need inheritance relationship in you case, but something like aggregation or composition. For example your type of relations breaks The Liskov Substitution Principle.

BUT. Objective C accepts reverse relationships. You can use delegates (@protocol) to describe interfaces which can retrieve some data from unknown objects who accepts this @protocol.

So, in your case class B should conform a protocol which provide access to some properties of B. And A should be able to work with this protocol, i.e. to know getters which A needs.

Subclass issue in iOS

3 Answers3