• (void)viewDidLoad {
  [super viewDidLoad];
  //定义一个 Person 类, 继承自 NSObject
  Class Person = objc_allocateClassPair([NSObject class], "Person", 0);
  //添加属性
  objc_property_attribute_t type = { "T", "@"NSString"" };
  objc_property_attribute_t ownership = { "C", "" }; // C = copy
  objc_property_attribute_t backingivar = { "V", "_privateName" };
  objc_property_attribute_t attrs[] = { type, ownership, backingivar };
  class_addProperty(Person, "name", attrs, 3);
  //添加方法
  class_addMethod(Person, @selector(name), (IMP)nameGetter, "@@:");
  class_addMethod(Person, @selector(setName:), (IMP)nameSetter, "v@:@");
  //注册该类
  objc_registerClassPair(Person);

  //获取实例
  id instance = [[Person alloc] init];
  NSLog(@"%@", instance);
  [instance setName:@"hxn"];

  NSLog(@"%@", [instance name]);

}
//get方法
NSString *nameGetter(id self, SEL _cmd) {
Ivar ivar = class_getInstanceVariable([self class], "_privateName");
return object_getIvar(self, ivar);
}
//set方法
void nameSetter(id self, SEL _cmd, NSString *newName) {
Ivar ivar = class_getInstanceVariable([self class], "_privateName");
id oldName = object_getIvar(self, ivar);
if (oldName != newName) object_setIvar(self, ivar, [newName copy]);
}