This is something that happens some times but not all the time:
header of a class
#import "DetailViewController.h"
@interface MasterViewController : UIViewController
@property (nonatomic, weak) DetailViewController *detailViewController;
error on the property line: unknown type name DetailViewController
then I add this line after the import and the error vanishes.
@class DetailViewController;
This is something annoying that only happens once in a while. I have a lot of classes just like that, without the class line that work perfectly. Some require the class. Ah, and sometimes it happens on a class that was working fine for days, but from a point on, Xcode decide it doesn't know the class anymore and decides to annoy you.
why?
You never need to do both @class and import the header file, you should only do one or the other. @class tells the compiler that the class exists somewhere in your project. In header files you should really only ever be using @class to reduce circular dependencies and compile faster. Then once you've written @class in your header, you can import the actual .h file inside your implementation.
In this case it looks like you do in fact have a circular dependency which is the reason for the error.
Bottom line, in .h files use @class in .m import the full header.
Note: If you're subclassing X then @class won't work and you'll have to import the full header in your .h
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