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) {
    this(null, value);
  }
  
  *(Meta *owner, A *value) {
    index();
    registerRef();
  }
  
  // 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); }
  
  // 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);
  }
  
  void change {}

  toString {
    ret
      ifdef RegisteredReference_markInToString
        "RegisteringReference " +
      endifdef
        str(value);
  }
}