How to get the return state from a sendAction method in a component

I have a component and click the button will invoke an external event handler defined inside the controller. I would like to know the return state from a sendAction method. See image below for details.

1. enter text in textarea
2. click "BTN SAVE"
3. handle action in index controller
4. perform ajax request in btnPressed actions obj
5. MY QUESTION How to notify the component the return state from the step (4)

Once I know the request state I can perform the correspond action(s) in my component.

Thanks