问题
I have a hypothetical UIViewController class named "foo". foo inherits from class bar and class bar #import's "Class A", a class which foo uses extensively. The problem is, when I'm using an instance of class A in foo, I don't get any compiler errors, but I do get a warning for instance, that an instance of Class A does not respond to a particular method. Do I have to explicitly #import ClassA.h into class 'foo'? even though class foo extends extends bar, which already imports it?
Hope that's not too confusing. Let me know if I need to clear anything up.
回答1:
It sounds like you have a circular dependency issue. In order to resolve it, yes, each imlementation file (.m) needs to #import the proper header file. However, if you try to have the header files #import each other, you'll run into problems.
In order to use inheritance, you need to know the size of the superclass, which means you need to #import it. For other things, though, such as member variables which are pointers, or methods which take as a parameter or return the other type, you don't actually need the class definition, so you can use a forward reference to resolve the compiler errors.
// bar.h
@class A; // forward declaration of class A -- do not to #import it here
@interface bar : UIViewController
{
A *member; // ok
}
- (A) method:(A)parameter; // also ok
@end
// bar.m
#import "bar.h"
#import "A.h"
// can now use bar & A without any errors or warnings
来源：https://stackoverflow.com/questions/575160/objective-c-use-of-import-and-inheritance