How will SC2 handle interrupted building?

For example, what if I'm in the middle of building a Protoss mothership, and suddenly am attacked and need to revert to building some troopers or tanks or the like. What will happen to the progress of the Protoss mothership? Will I lose whatever progress was made, so that when I go back to building it, I'd have to start over from the beginning?

 

What exactly are you asking?

If you cancel the Mothership to build another air unit, then yes you have to start over. However, if you simply start building something else at a Gateway or another Stargate, you don't have to cancel the Mothership. Of course, if you need the Mothership resources for other units, you have to cancel the Mothership. Canceling a unit = losing all progress + gaining resources back.