如何通过类别扩展添加属性

没有属性。category一般只使用扩展方法，可以使用全局变量，例如
@inferface MyClass (xxx)
{
int num;
}
- (void)myFunc;
但我曾用时出现莫名奇妙的数据错误，就不再用。

// Declaration

@interface MyObject (ExtendedProperties)
@property (nonatomic, strong, readwrite) id myCustomProperty;
@end

// Implementation

static void * MyObjectMyCustomPorpertyKey = (void *)@"MyObjectMyCustomPorpertyKey";

@implementation MyObject (ExtendedProperties)

- (id)myCustomProperty
{
return objc_getAssociatedObject(self, MyObjectMyCustomPorpertyKey);
}

- (void)setMyCustomProperty:(id)myCustomProperty
{
objc_setAssociatedObject(self, MyObjectMyCustomPorpertyKey, myCustomProperty, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end