As getting started working on Geant4, I wonder, why people want to use SensitiveDetector vs SteppingAction.
To me, SD is very similar to a SteppingAction if you confirm the step is in a specific volume.
One can easily implement the same logic in the SteppingAction with a big switch case condition.
The only add-on to the SD that I can think of is the internal handle of Hits and Hits collections.
However, the Hit data structure is defined by the user, and hard to generalize.
One can also implement the private hit collections in SteppingAction as well.
So, why SD is used at all?
SD allows the code to be well encapsulated. You don’t have to include the “if volume” check. If you want different selection criteria for different things (here’s my APD for optical photons, here’s my silicon pixel detector, here’s my hadronic calorimeter module), you can have separate SD classes for each of them, with just their specific code. If you throw everything into a SteppingAction, then you have a giant block of unreadable spaghetti code with a bunch of if-blocks.
If you decide to “encapsulate” the stuff in such an overblown SteppingAction, with each volume’s action in a separate member function, and separate classes for the different hit data, you’re basically reinventing what SD’s do.
Additionally there is functionality to support “digitization” – taking hits recorded by SDs and applying user code to convert them to detector readout. That functionality is leveraged on the existence (and registration) of SDs and their associated HitCollections.