6.2. Optional User Actions

6.2. Optional User Actions
	Chapter 6. User Actions

There are five virtual classes whose methods the user may override in order to gain control of the simulation at various stages. Each method of each action class has an empty default implementation, allowing the user to inherit and implement desired classes and methods.

Objects of user action classes must be registered with G4RunManager (Section 3.4.1.2), which takes ownership of them. The user must not delete these objects directly, and they must be created using 'new'.

6.2.1. Usage of User Actions

`G4UserRunAction`

This class has three virtual methods which are invoked by G4RunManager for each run:

GenerateRun(): This method is invoked at the beginning of BeamOn. Because the user can inherit the class G4Run and create his/her own concrete class to store some information about the run, the GenerateRun() method is the place to instantiate such an object. It is also the ideal place to set variables which affect the physics table (such as production thresholds) for a particular run, because GenerateRun() is invoked before the calculation of the physics table.
BeginOfRunAction(): This method is invoked before entering the event loop. A typical use of this method would be to initialize and/or book histograms for a particular run. This method is invoked after the calculation of the physics tables.
EndOfRunAction(): This method is invoked at the very end of the run processing. It is typically used for a simple analysis of the processed run.

Example 6.8. G4UserRunAction

     class G4UserRunAction
     {
       public:
         G4UserRunAction();
         virtual ~G4UserRunAction();

       public:
         virtual G4Run* GenerateRun();
         virtual void BeginOfRunAction(const G4Run*);
         virtual void EndOfRunAction(const G4Run*);
     };

`G4UserEventAction`

This class has two virtual methods which are invoked by G4EventManager for each event:

beginOfEventAction(): This method is invoked before converting the primary particles to G4Track objects. A typical use of this method would be to initialize and/or book histograms for a particular event.
endOfEventAction(): This method is invoked at the very end of event processing. It is typically used for a simple analysis of the processed event. If the user wants to keep the currently processing event until the end of the current run, the user can invoke fpEventManager->KeepTheCurrentEvent(); so that it is kept in G4Run object. This should be quite useful if you simulate quite many events and want to visualize only the most interest ones after the long execution. Given the memory size of an event and its contents may be large, it is the user's responsibility not to keep unnecessary events.

Example 6.9. G4UserEventAction

     class G4UserEventAction 
     {
       public:
           G4UserEventAction() {;}
           virtual ~G4UserEventAction() {;}
           virtual void BeginOfEventAction(const G4Event*);
           virtual void EndOfEventAction(const G4Event*);
       protected:
           G4EventManager* fpEventManager;
     };

`G4UserStackingAction`

This class has three virtual methods, ClassifyNewTrack, NewStage and PrepareNewEvent which the user may override in order to control the various track stacking mechanisms. ExampleN04 could be a good example to understand the usage of this class.

ClassifyNewTrack() is invoked by G4StackManager whenever a new G4Track object is "pushed" onto a stack by G4EventManager. ClassifyNewTrack() returns an enumerator, G4ClassificationOfNewTrack, whose value indicates to which stack, if any, the track will be sent. This value should be determined by the user. G4ClassificationOfNewTrack has four possible values:

fUrgent - track is placed in the urgent stack
fWaiting - track is placed in the waiting stack, and will not be simulated until the urgent stack is empty
fPostpone - track is postponed to the next event
fKill - the track is deleted immediately and not stored in any stack.

These assignments may be made based on the origin of the track which is obtained as follows:

  G4int parent_ID = aTrack->get_parentID();

where

parent_ID = 0 indicates a primary particle
parent_ID > 0 indicates a secondary particle
parent_ID < 0 indicates postponed particle from previous event.

NewStage() is invoked when the urgent stack is empty and the waiting stack contains at least one G4Track object. Here the user may kill or re-assign to different stacks all the tracks in the waiting stack by calling the stackManager->ReClassify() method which, in turn, calls the ClassifyNewTrack() method. If no user action is taken, all tracks in the waiting stack are transferred to the urgent stack. The user may also decide to abort the current event even though some tracks may remain in the waiting stack by calling stackManager->clear(). This method is valid and safe only if it is called from the G4UserStackingAction class. A global method of event abortion is

  G4UImanager * UImanager = G4UImanager::GetUIpointer();
  UImanager->ApplyCommand("/event/abort");

PrepareNewEvent() is invoked at the beginning of each event. At this point no primary particles have been converted to tracks, so the urgent and waiting stacks are empty. However, there may be tracks in the postponed-to-next-event stack; for each of these the ClassifyNewTrack() method is called and the track is assigned to the appropriate stack.

Example 6.10. G4UserStackingAction

     #include "G4ClassificationOfNewTrack.hh"

     class G4UserStackingAction 
     {
       public:
           G4UserStackingAction();
           virtual ~G4UserStackingAction();
       protected:
           G4StackManager * stackManager;

       public:
     //---------------------------------------------------------------
     // virtual methods to be implemented by user
     //---------------------------------------------------------------
     //
           virtual G4ClassificationOfNewTrack 
             ClassifyNewTrack(const G4Track*);
     //
     //---------------------------------------------------------------
     //
           virtual void NewStage();
     //
     //---------------------------------------------------------------
     //
           virtual void PrepareNewEvent();
     //
     //---------------------------------------------------------------

     };

`G4UserTrackingAction`

Example 6.11. G4UserTrackingAction

     //---------------------------------------------------------------
     //
     // G4UserTrackingAction.hh
     //
     // Description:
     //   This class represents actions taken place by the user at each
     //   end of stepping. 
     //
     //---------------------------------------------------------------

     ///////////////////////////
     class G4UserTrackingAction 
     ///////////////////////////
     {
     
     //--------
        public:
     //--------
     
     // Constructor & Destructor
        G4UserTrackingAction(){};
        virtual ~G4UserTrackingAction(){}
     
     // Member functions
        virtual void PreUserTrackingAction(const G4Track*){}
        virtual void PostUserTrackingAction(const G4Track*){}
     
     //----------- 
        protected:
     //----------- 
     
     // Member data
        G4TrackingManager* fpTrackingManager;
     
     };

`G4UserSteppingAction`

Example 6.12. G4UserSteppingAction

     //---------------------------------------------------------------
     //
     //  G4UserSteppingAction.hh
     //
     //  Description:
     //    This class represents actions taken place by the user at each
     //    end of stepping. 
     //
     //---------------------------------------------------------------
     
     ///////////////////////////
     class G4UserSteppingAction 
     ///////////////////////////
     {
     
     //--------
        public:
     //--------
     
     // Constructor and destructor
        G4UserSteppingAction(){}
        virtual ~G4UserSteppingAction(){}
     
     // Member functions
        virtual void UserSteppingAction(const G4Step*){}
     
     //----------- 
        protected:
     //----------- 
     
     // Member data
        G4SteppingManager* fpSteppingManager;
     
     };

6.2.2. Killing Tracks in User Actions and Energy Conservation

In either of user action classes described in the previous section, the user can implement an unnatural/unphysical action. A typical example is to kill a track, which is under the simulation, in the user stepping action. In this case the user have to be cautious of the total energy conservation. The user stepping action itself does not take care the energy or any physics quantity associated with the killed track. Therefore if the user want to keep the total energy of an event in this case, the lost track energy need to be recorded by the user.

The same is true for user stacking or tracking actions. If the user has killed a track in these actions the all physics information associated with it would be lost and, for example, the total energy conservation be broken.

If the user wants the Geant4 kernel to take care the total energy conservation automatically when he/she has killed artificially a track, the user has to use a killer process. For example if the user uses G4UserLimits and G4UserSpecialCuts process, energy of the killed track is added to the total energy deposit.