Category是Objective-C中常用的语法特性，通过它可以很方便的为已有的类来添加函数。但是Category不允许为已有的类添加新的属性或者成员变量。
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。通过这种方法来模拟生成属性。
//NSObject+IndieBandName.h
@interface NSObject (IndieBandName)
@property (nonatomic, strong) NSString *indieBandName;
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@end上面是头文件声明，下面的实现的.m文件：
// NSObject+IndieBandName.m
#import "NSObject+Extension.h"
#import <objc/runtime.h>
static const void *IndieBandNameKey = &IndieBandNameKey;
@implementation NSObject (IndieBandName)
@dynamic indieBandName;
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(NSString *)indieBandName { return objc_getAssociatedObject(self, IndieBandNameKey); }
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(void)setIndieBandName:(NSString *)indieBandName{ objc_setAssociatedObject(self, IndieBandNameKey, indieBandName, OBJC_ASSOCIATION_RETAIN_NONATOMIC); }
@end DLIntrospection 复制代码 这个和Category无关，但是也是runtime.h的一种应用。DLIntrospection，是 一个NSObject Category。它为NSObject提供了一系列扩展函数：
@interface NSObject (DLIntrospection)
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(NSArray *)classes;
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(NSArray *)properties;
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(NSArray *)instanceVariables;
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(NSArray *)classMethods;
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(NSArray *)instanceMethods;
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(NSArray *)protocols;
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(NSDictionary *)descriptionForProtocol:(Protocol *)proto;
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(NSString *)parentClassHierarchy; @end 复制代码
