在类目中添加属性

.h文件

//分类的头文件
@interface ClassName (CategoryName)
@property (nonatomic, strong) NSString *str;//此为新增的属性

//此为新增的属性

@end

.m文件

//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