Checking from a preemptive evaluation whether a main evaluation is ongoing

Question

How can I programmatically check from a preemptive evaluation whether a main evaluation is currently ongoing?

I need a function mainEvaluationOngoingQ[] so that

Button["Evaluating?", Print@mainEvaluationOngoingQ[], Method -> "Preemptive"]

will print True only if a main evaluation is ongoing.

Answer 1

If you control the launch of the main evaluation process, then a very simple way is to wrap your code in a dynamic environment (Block), which would set some flag:

mainEvaluationOngoingQ[] := TrueQ@mainEvaluationQ

Block[{mainEvaluationQ = True}, Do[i^2, {i, 1, 10^8}]]

You can automate this by creating an environment:

SetAttributes[withMainEvaluation, HoldAll];
withMainEvaluation[code_] :=
    Block[{mainEvaluationQ = True}, code]

You can further automate this with $Pre, if needed: $Pre = withMainEvaluation.

Answer 2

The following works with Mathematica 8.0.0.0:

Button["Evaluating?",Print@(Drop[Stack[],-7]!={}),Method->"Preemptive"]

However the -7 here is an empirical factor which removes all those entries generated from executing the button code itself; it changes depending on the exact form of the code the Stack[] is embedded in, and might even depend on the Mathematica version (I don't think the inner mechanics of Button is documented, let alone guaranteed never to change).