Objective C Path drawing to link buttons

Does anyone have any idea on how to make a path drawing to "link" up two buttons like the ip开发者_开发问答hone game Airline Conqueror?

I haven't seen Airline Conqueror, but I suppose it might be like the lines that Interface Builder draws when you define button actions. That's done by drawing into a transparent window at a high level, probably the overlay level (kCGOverlayWindowLevelKey).