/// <summary>
/// Register a system with the entity manager
/// </summary>
/// <param name="system"></param>
public void RegisterSystem(ISystem system)
{
if (system.entityManager == null && !_systems.Contains(system))
{
system.SetEntityManager(this);
_systems.Add(system);
foreach (UID e in _entities.Keys)
{
system.EntityModified(e);
}
}
}