JASS - Unit leave Unit range?

Is there a function that is triggered when a certain unit leaves the range of another one? I'm trying to make a Stealth Generator that turn invisible everything in a ceratain range, they are invisibles until they leave this one.

I don't believe the event you're describing exists. The closest thing is TriggerRegisterLeaveRegion( trigger whichTrigger, region whichRegion, boolexpr filter ).

The big problem with a region its because it isnt round and I would need to make one each time a Stealth Generator enter the map and leave the map. And to make it "round" I would need 3 Regions...

You are sure there isn't any other ways?

Actually there are functions available at The JASS Vault for creating differently shaped regions.

Edit: I never said there aren't other ways, there are several other ways. Simulating an aura that cloaks units would work and just look at how many methods people have developed to create auras.