基于oc的ios开发，如何给一个系统类添加一个属性？

基于oc的ios开发，如何给一个系统类添加一个属性？ 目前我知道可以用继承或类别实现。但不知道哪种方式好

用类别：
.h文件
#import

@interface UIButton (TLButton)
@property(nonatomic,strong,readwrite)NSNumber* cityId;
@property(nonatomic,strong,readwrite)NSNumber* typeId;
@property(nonatomic,readwrite)NSNumber* pross;
@end
.m文件
#import "UIButton+TLButton.h"
@implementation UIButton (TLButton)
-(NSNumber *)cityId
{
return objc_getAssociatedObject(self, @selector(cityId));
}

• (void)setCityId:(NSNumber *)cityId
  {

  [self willChangeValueForKey:@"cityId"];
  objc_setAssociatedObject(self, @selector(cityId), cityId, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
  [self didChangeValueForKey:@"cityId"];
  }
  -(NSNumber )typeId
  {
  return objc_getAssociatedObject(self, @selector(typeId));
  }
  -(void)setTypeId:(NSNumber *)typeId
  {
  [self willChangeValueForKey:@"typeId"];
  objc_setAssociatedObject(self, @selector(typeId), typeId, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
  [self didChangeValueForKey:@"typeId"];
  }
  -(NSNumber)pross
  {
  return objc_getAssociatedObject(self, @selector(pross));
  }
  -(void)setPross:(NSNumber*)pross
  {
  [self willChangeValueForKey:@"pross"];
  objc_setAssociatedObject(self, @selector(pross), pross, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
  [self didChangeValueForKey:@"pross"];
  }
  @end

http://zhidao.baidu.com/link?url=pQeR_z63jG8rHX8ZZE1Ql6L2qDqa-BnfLbjYqxhaPinP2GWEFJ2fAjxwYmzJrjx_zaBi7SLQp_56U12tGBJq3a

楼上正解，重写getter setter 方法。