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);
  }
}