[Paraview] How to know that the filter was applied?

[Artem Bodrin]

Hello, dear community.

Does anyone can help me with this question: I'd like to know, if a filter in a pipeline have already been applied/calculated. Maybe there are some observers exists (e.g. vtkSMPProgressObserver)? Or I have to dig into vtkDemandDrivenPipeline?

The overall task is to modify some GUI elements depending on a pipeline execution state...

Can anyone push me to a right direction please?