"self" can not detect its properties in Xcode indeed it even doesn't know which class itself is

Hi everyone I have a really strange problem. I create a new subclass of NSObject, but I just cannot use "self.someproperty" in the implementation. Normally when I type the word "self", Xcode will guess what I'm typing and give me the property name after dot. But in this case, it doesn't and give me an red error. I have check my code for a night and give up now. So, wish some one give me a advise please let me know this might be a small problem somewhere?

Here's my code:

#import <Foundation/Foundation.h>
 
 @interface FlickrPhotoCache : NSObject
 
 +(BOOL)isInCache:(NSDictionary *)photo;
 
 @end
 
 
 
 #import "FlickrPhotoCache.h"
 
 @interface FlickrPhotoCache()
 
 @property (nonatomic, strong) NSMutableArray *photos;
 
 @end
 
 @implementation FlickrPhotoCache
 @synthesize photos = _photos;
 
 +(BOOL)isInCache:(NSDictionary *)photo
 {
     self.photos // here I get error
 }
 
 @end
 

Thanks in advance! WHT

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