question

In order for FlexSim to call the onStartSimulation() method on your object, you need to call SimulationStartEvent::addObject() in the onReset method:

class MyClass : public FlexSimEventHandler /*or any subclass of FlexSimHandler*/
{
public:
  virtual double onReset() override { 
    print("onReset"); 
    SimulationStartEvent::addObject(this);
    return 0; 
  }
  virtual double onStartSimulation() override { print("onStartSimulation"); return 0; }
};

The SimulationStart mechanism adds a single event at time zero. When that event executes, it calls the onStartSimulation() method for all the objects in its list. You can see this in MAIN:/project/exec/globals/includebody, around line 550.

There is also a PostReset mechanism, but FlexSimEventHandler doesn't implement that method. So you'd need to add the OnPostReset eventfunction to your class in the library, which calls your class' onPostReset method. Finally, the onReset can call SimulationStartEvent::addPostResetObject(). PostReset is basically a second pass during the model's reset, and the simulation start event happens at time zero.