A predefined callback function that prints one or more callback arguments in the Trace window


This function can be used as a debugging tool to verify that a reactor has fired.


Define a command reactor and assign vlr-trace-reaction as the callback function:

_$  (VLR-Reaction-Set
(VLR-Command-Reactor) :VLR-commandWillStart 'VLR-trace-reaction)

At the AutoCAD Command prompt, enter the following:


Respond to the command prompts, then activate the VLISP window and open the Trace window. You should see the following in the Trace window:

; "Reaction": :VLR-commandWillStart; "argument list": (#<VLR-COMMAND-REACTOR> ("LINE"))

The output from vlr-trace-reaction identifies the type of trigger event, the reactor type, and the command that triggered the reactor.