Does Objective-C allow circular dependencies?

问题

I'm rewriting a Java library in Objective-C and I've come across a strange situation. I've got two classes that import each other. It's a circular dependency. Does Objective-C support such a situation? If not, how do you recommend I rewrite it?

回答1:

Importing a class is not inheritance. Objective-C doesn't allow circular inheritance, but it does allow circular dependencies. What you would do is declare the classes in each other's headers with the @class directive, and have each class's implementation file import the other one's header. To wit:

ClassA.h

@class ClassB;

@interface ClassA : NSObject {
    ClassB *foo;
}
@end

ClassA.m

#import "ClassB.h"

@implementation ClassA
    // Whatever goes here
@end

ClassB.h

@class ClassA;

@interface ClassB : NSObject {
    ClassA *foo;
}

@end

ClassB.m

#import "ClassA.h"

@implementation ClassB
    // Whatever goes here
@end

来源：https://stackoverflow.com/questions/5425465/does-objective-c-allow-circular-dependencies