[?]Class A imports B, B needs to import A too...

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[?]Class A imports B, B needs to import A too...

(7 posts) (5 voices)

Started 3 months ago by Duckwit
Latest reply from Karl

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Duckwit
Member

This is one of those stupid technical problem that I should be able to solve by now, but being bogged down by work and struggling to find programming time I can't seem to figure this out.

Class A is a singleton, it receives input from the user interface using Interface Builder.
Class B is a CCLayer subclass (HelloWorldLayer)

B needs to know about a to send it some data via a method.
A needs to know about B to call some methods in it when it receives certain file data from the NSOpenPanel.

B #imports A
But if A imports B there are problems - (circular importing)

How should I properly reference each class?

Posted 3 months ago #
Martin
Member

I don't know whats the most proper solution, but you could use a protocol that one object conforms to, to define the methods the other object will use. Then both objects will import the file containing the protocol definition(s).
And/Or you could import the header file of the other object only in the implementation file of your object. Afaik importing in the implementation does not generate cyclic dependencies.
If your singleton stores a reference to the object you could use @class to "promise" the definition of the class will follow "later" (importing its header in the .m file). If I remember correctly that's how I usually do it.

p.s.: aaaand read this :)
http://stackoverflow.com/questions/322597/class-vs-import

Posted 3 months ago #
frilla
Member
As @Martin stated, you can use a forward declaration @class classNameHere on your header files so tell the compiler that you'll be refering to classes with this name in the header file but you won't be able to use them in the header file (for example call one of their methods).

In the implementation file, you'll have to import the header files in order to actually use the objects.

classA.h:
```
@class classB;

classB *_pointerToClassB;
```
classB.h:
```
@class classA

classA *_pointerToClassA;
```
classA.m:
```
#import "classB.h"

//you can now use classB
```
classB.m:
```
#import "classA.h
//you can now use classA
```
Posted 3 months ago #
Duckwit
Member

Thanks @Martin and @frilla - I implemented this just after @Martin posted and it is working beautifully. :)

Posted 3 months ago #
Birkemose
Moderator

While forward declarations work, they are no good.
It is a common problem that you "suddenly" needs to cross-include two classes. Imo it should always be fixed by rethinking your class lay-out, because it will in the end result in a stronger OO-hierachy.

Posted 3 months ago #
Duckwit
Member

@Birkemose - Very wise advice, and I would love to take the time to implement it - but this here is just for a quick level-editor I made for my personal use only, so the code doesn't really need to work 'well' it just needs to work. (Which it does, and very smoothly + extremely fast as well).

Posted 3 months ago #
Karl
Member

When you find that classes require two-way communication, it can commonly be a sign of bad OO design, as birkemose said. However, there are cases where it's completely warranted. In such a case, you have three choices:

1. Make both aware of each other using the @class forward declaration, as mentioned earlier (this is usually the wrong approach).
2. Use the delegate pattern, which is a commonly used pattern in UIKit (UIApplicationDelegate being used in EVERY iOS app).
3. Register callbacks for the object that doesn't need to know about the other, except for notifications. This can be done with target/selector, blocks, or even monitoring NSNotifications or KVO (which is useful when multiple classes may be interested in events a class could generate).

Posted 3 months ago #

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