sclass RegisteredReference<A extends Concept> implements IRef<A> {
  Meta owner; // all we require is a meta field
  A value;
  
  *() {
    if (!dynamicObjectIsLoading()) registerRef();
  }
  
  void registerRef {
    vmBus_send registeringReference(this);
  }
  
  *(A *value) {
    registerRef();
    index();
  }
  
  // get owning object (source)
  Meta owner() { ret owner;

  // get target
  public A get() { ret value; }
  bool has() { ret value != null; }
  
  bool set(A a) {
    if (a == value) false;
    unindex();
    value = a;
    index();
    true;
  }
  
  void setIfEmpty(A a) {
    if (!has()) set(a);
  }
  
  void set(IRef<A> ref) { set(ref == null ?: ref.get()); }
  void clear() { set((A) null); }
  
  bool validRef() {
    ret value != null && _concepts != null && _concepts == value._concepts;
  }
  
  // TODO: sync all the indexing and unindexing!?
  void index() { 
    if (validRef()) {
      value._addBackRef(this);
      change();
    }
  }

  Ref<A> unindex() {
    if (validRef()) {
      value._removeBackRef(this);
      change();
    }
    this;
  }
  
  void unindexAndDrop {
    unindex();
    _removeRef(this);
  }
  
  void change() {
    owner?.change();
  }

  toString { ret ifdef ConceptRef_markInToString "Concept.Ref " + endifdef
  str(value); }
}