WIP: hardcode a uniform magnetic field.

Simulation/DetSimGeom/src/AnExampleDetElemTool.cpp

@@ -19,6 +19,11 @@
 #include "G4PhysicalVolumeStore.hh"
 #include "G4OpticalSurface.hh"
 
+// Field
+#include "G4UniformMagField.hh"
+#include "G4FieldManager.hh"
+#include "G4TransportationManager.hh"
+
 #include "DD4hep/Detector.h"
 #include "DD4hep/Plugins.h"
 #include "DDG4/Geant4Converter.h"
@@ -143,6 +148,26 @@ AnExampleDetElemTool::ConstructSDandField() {
         }
     }
 
+    // =======================================================================
+    // Construct Field
+    // =======================================================================
+    // TODO: integrate the field between DD4hep and Geant4
+    // Note:
+    //   DD4hep provides the parameters of fields
+    //   Geant4 will setup the field based on the DD4hep fields
+    
+    // Related Examples:
+    // - G4: G4GlobalMagFieldMessenger.cc
+
+    G4FieldManager* fieldManager
+        = G4TransportationManager::GetTransportationManager()->GetFieldManager();
+
+    G4ThreeVector value(0,0,3.*tesla);
+    G4UniformMagField* aMagField = new G4UniformMagField(value);
+
+    fieldManager->SetDetectorField(aMagField);
+    fieldManager->CreateChordFinder(aMagField);
+
 
 }