水果官方给出的分类，即Category是不能添加属性的，但是我们是谁；我们有黑魔法啊，runtime 操练起来啊

从xcode6开始到xcode 7创建分类都要在
command+N，之后选中Objective -C File，然后选中你要给谁创建分类，给谁的分类添加属性。
我这里选择万象“NSObject”

.h

//  NSObject+IndieBandName.h
//  CategoryAddVar
//
//  Created by Vizard on 16/7/28.
//  Copyright © 2016年 Vizard. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSObject (IndieBandName)

@property (nonatomic, strong) NSString *indieBandName;

@end

.m

//  NSObject+IndieBandName.m
//  CategoryAddVar
//
//  Created by Vizard on 16/7/28.
//  Copyright © 2016年 Vizard. All rights reserved.
//

#import "NSObject+IndieBandName.h"
#import <objc/runtime.h>

static const void *IndieBanNameKey = &IndieBanNameKey;

@implementation NSObject (IndieBandName)

@dynamic indieBandName;

- (NSString *)indieBandName
{
    return objc_getAssociatedObject(self, IndieBanNameKey);
}

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

@end

然后在VC中

就可以使用了。
相信看官看到这里已经看透了，黑魔法的关键点了

 1.#import <objc/runtime.h>
2.@dynamic indieBandName;
3. objc_getAssociatedObject
4. objc_setAssociatedObject