Way to observe focus change?

Hi all

I have two components in SG. Component A calls an async service and if that returns an error Component B displays that error.

Component B handles the focusing between the two components by storing the previous Component that had focus and returning focus to that Component when a button is selected.

The issue I'm trying to solve is is there a way for Component A to know when it loses focus and when focus is returned? - So it can change the appearance of an element within Component A.

Thanks

Charlie