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