Allow moving bodies to another parent in mjSpec

[jjyyxx]

The feature, motivation and pitch

I'm experimenting with modifying the kinematic tree during simulation. I understand that, for a single mjModel, the kinematic tree and the number of DOFs have to be fixed. So I tried out the recent mjSpec API, but found that it might not be flexible enough to achieve this seemingly simple functionality (discussed in #2327 (comment)).

Except for keyframe management, I could implement this feature when building from source as follows:

class mjCBody : public mjCBody_, private mjsBody {
  // ...
  void MoveTo(mjCBody* new_parent) {
    mjCBody* old_parent = parent;
    for (int i=0; i<old_parent->bodies.size(); i++) {
      if (old_parent->bodies[i] == this) {
        old_parent->bodies.erase(old_parent->bodies.begin() + i);
        break;
      }
    }
    new_parent->bodies.push_back(this);
    parent = new_parent;
  }
  // ...
}

void mjs_moveBody(mjsBody* parent, mjsBody* child) {
  mjCBody* child_body = static_cast<mjCBody*>(child->element);
  mjCBody* new_parent = static_cast<mjCBody*>(parent->element);
  try {
    child_body->MoveTo(new_parent);
  } catch (mjCError& e) {
    new_parent->model->SetError(e);
  }
}

Alternatives

No response

Additional context

No response