XPLM_API void       XPLMRegisterCommandHandler(
                         XPLMCommandRef       inComand,    
                         XPLMCommandCallback_f inHandler,    
                         int                  inBefore,    
                         void *               inRefcon);    

XPLMRegisterCommandHandler registers a callback to be called when a command is executed. You provide a callback with a reference pointer.

If inBefore is true, your command handler callback will be executed before X-Plane executes the command, and returning 0 from your callback will disable X-Plane’s processing of the command. If inBefore is false, your callback will run after X-Plane. (You can register a single callback both before and after a command.)