让Category支持添加属性与成员变量

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;
@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;

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

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

@end