一定要在 类别中添加实例变量怎么办

那我偏偏想要在类别中添加实例变量该怎么办呢？这时候就要用到runtime了，不要忘记了Objective-C是动态语言。一种常见的办法是通过runtime.h中objc_getAssociatedObject / objc_setAssociatedObject来访问和生成关联对象。这两个方法可以让一个对象和另一个对象关联，就是说一个对象可以保持对另一个对象的引用，并获取那个对象。

//NSObject+IndieBandName.h

@interface NSObject (IndieBandName)

@property (nonatomic, strong) NSString *indieBandName;

@end

// NSObject+IndieBandName.m

#import "NSObject+Extension.h"

#import <objc/runtime.h> 

static const void *IndieBandNameKey = &IndieBandNameKey; 

@implementation NSObject (IndieBandName) 
@dynamic indieBandName;
 
- (NSString *)indieBandName { 
　　return objc_getAssociatedObject(self, IndieBandNameKey); 
} 

- (void)setIndieBandName:(NSString *)indieBandName { 
　　objc_setAssociatedObject(self, IndieBandNameKey, indieBandName, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
} 

@end

参考：

https://blog.csdn.net/intheair100/article/details/72718625

http://www.cnblogs.com/crazypebble/p/3439261.html
http://quotation.github.io/objc/2015/05/21/objc-runtime-ivar-access.html
http://www.cnblogs.com/wupher/archive/2013/01/05/2845338.html