sclass RegisteredReference<A> implements IRef<A> {

  Meta owner; // all we require is a meta field

  A value;

  *() {

    //if (!dynamicObjectIsLoading()) registerRef();

  }

  /*void registerRef {

    vmBus_send registeringReference(this);

  }*/

  // Note that the single-argument constructor takes an owner, not a value

  *(Meta owner) {

    this(owner, null);

  }

  *(Meta *owner, A *value) {

    index();

    //registerRef();

  }

  // get owning object (source)

  Meta owner() { ret owner; }

  // get target

  public A get() { ret value; }

  public bool has() { ret value != null; }

  public bool set_trueIfChanged(A a) {

    if (eq(a, value)) false;

    unindex();

    value = a;

    index();

    true;

  }

  public void set(A a) {

    set_trueIfChanged(a);

  }

  void setIfEmpty(A a) {

    if (!has()) set(a);

  }

  public void set(IRef<A> ref) { set(ref == null ?: ref.get()); }

  public void clear() { set((A) null); }

  // can override

  bool validRef() { true; }

  // TODO: sync all the indexing and unindexing!?

  void index() { 

    if (!validRef()) ret;

    var br = lookupInterface IHasBackRefs(get());

    br?._registerBackRef(this);

  }

  void unindex() {

    if (!validRef()) ret;

    var br = lookupInterface IHasBackRefs(get());

    br?._unregisterBackRef(this);

  }

  // not used yet

  void change {}

  toString {

    ret

      ifdef RegisteredReference_markInToString

        "RegisteringReference " +

      endifdef

        str(value);

  }

}