Gets whether Mathematica is currently busy with a computation (that is, whether Compute is currently running).
You can use this property in an event handler method to see whether it is safe to make a call to Compute. With the HandleEvents property set to its default value of true, event handlers can fire while the Compute method is running. To prevent a reentrant call to Compute, you can check this property in an event handler to decide whether it is safe to proceed with a call to Compute.
If you go ahead and try to make a reentrant call to Compute, an InvalidOperationException will be thrown.