docking?

I am writing an application which extends the fucntionality of another and I would like it to open underneath the 'parent' application. I am guessing I have to read the size and topleft pos of the original and open my app so it 'attaches' to the bottom..

I have some idea on how to do this, but need some help in figuring this out. I would also like to follow the original app if it resizes. Can I detect a change in size of another app or do I just check it at regular intervals..

any help appreciated..

anyone?