Determine UIView frame after being added to subview

I have custom UIView with autoResizeMask. Let's call it A view. I initWithFrame the A view and add it to subview B. After adding A view to B, B autoresizes A (the frame changes). To say it in other words -- B view fits in A view.

My question is, how can I determine the new frame within A view before showing it user? B开发者_运维问答ecause I want to make some changes to A according new frame.

(my A view is UIScrollView subclass, and I want to set correct contentSize).