public void PreSolve(Contact contact, ref Manifold oldManifold)
 {
     var unitA = (Unit) contact.GetFixtureA().GetBody().GetUserData();
     var unitB = (Unit) contact.GetFixtureB().GetBody().GetUserData();
     if (!(unitA.EnableCollision && unitB.EnableCollision
           && GroupManager.Instance.ShouldContact(unitA.Group, unitB.Group)))
     {
         contact.SetEnabled(false);
     }
 }