How do I restrict one sprites movement to a boundary?

Hey,

Could any kind person explain to me the simplest way to restrict one sprites movement to be within the confines of another sprite? i.e. like in a virtual thumbstick?

I'm trying to get the top sprite to be able to be dragged within the boundary created by the larger sprite below and snap back to its original location when you let go again.

Thanks

How are both sprites shaped ?

They are circular?

Hehe, worked on this, came back and found a more elegant solution then mine.

Thank you both for looking at this, I sorted it by ammending features in the capx posted above.