按我个人的理解，class与import的区分主要是为了解决引用死锁的问题。
stackOverFlow上有个人是这样解释这个问题的：
If you see this warning:
warning: receiver 'myCoolClass' is a forward class and corresponding @interface may not exist
you need to #import the file, but you can do that in your implementation file (.m), and use the @class declaration in your header file.
@class does not (usually) remove the need to #import files, it just moves the requirement down closer to where the information is useful.
For Example
If you say @class myCoolClass, the compiler knows that it may see something like:
myCoolClass *myObject;
It doesn't have to worry about anything other than myCoolClass is a valid class, and it should reserve room for a pointer to it (really, just a pointer). Thus, in your header, @class suffices 90% of the time.
However, if you ever need to create or access myObject's members, you'll need to let the compiler know what those methods are. At this point (presumably in your implementation file), you'll need to #import "myCoolClass.h", to tell the compiler additional information beyond just "this is a class".
先看看什么情况下应该用@class
1. file1.h
@class class2
@interface class1
{
class2* class2Object;
}
@end
@interface class2
{
class1* class1Object;
}
@end
2. class1.h中引入class2.h头文件
#import "class2.h"
@interface class1
{
class2* class2Object;
}
@end
|
class2.h中引入class1.h头文件
#import "class1.h"
@interface class2
{
class1* class1Object;
}
@end
|
这种情况会导致引用死锁，所以应该用forward declaration
class1.h, 在class1.m中添加 #import "class2.h"
@class class2;
@interface class1
{
class2* class2Object;
}
@end
|
class2.h，在class2.m中添加#import "class2.h"
@class class1;
@interface class2
{
class1* class1Object;
}
@end