给category增加属性

//分类的头文件
@interface ClassName (CategoryName)
@property (nonatomic, strong) NSString *str;
@end
//实现文件
#import "ClassName + CategoryName.h"
#import <objc/runtime.h>

static void *strKey = &strKey;

@implementation ClassName (CategoryName)
-(void)setStr:(NSString *)str 
{ 
    objc_setAssociatedObject(self, & strKey, str, OBJC_ASSOCIATION_COPY); 
} 

-(NSString *)str 
{ 
    return objc_getAssociatedObject(self, &strKey); 
}
@end





这里要注意objc_setAssociatedObject里传的参数，枚举值等等有的时候是copy有的时候是strong或者retain