在Objective-C中子类化时更改变量的类

Question

I have a simple class defined, called 'PostView', which subclasses UIView. Inside that class I have defined an 'item' in the implementation, which is an instance of 'Post'.

@interface PostView : UIView <UIScrollViewDelegate> {
  Post *item;
}

This works fine for me, but a problem occurs when I subclass it: 'PostPhotoView'. The 'item' variable now needs to be a 'PostPhoto' instance, but I'm not sure how I can go about doing this without defining 'item' in each subclass, which sound a bit repetitive and dirty..

Ideally I want to do this:

@interface PostPhotoView : PostView {
  PostPhoto *item;
}

But obviously I cannot redefine the variable. Maybe I am going about this in the wrong way?

Thanks.

Answer 1

There are two ways to deal with this:

• Typecast your access to the ivar/property: PostPhoto *theItem = (PostPhoto *)self.item
• Declare the ivar (or the return type of the accessor) as type id , so you can do PostPhoto *theItem = self.item . For completeness, if you're using a declared property of type id , you might also want to check the value in the setter: NSParameterAssert([theItem isKindOfClass:[Post class]]) .