sclass Dyn_FieldWatcher {
  DynModule module;
  S field;
  O value;
  Runnable action;
  
  *(DynModule *module, S *field, Runnable *action) {
    value = get(module, field);
    //module.onChange(r check);
    module.onFieldChange(voidfunc(S f) {
      if (eq(f, field))
        check();
    });
  }
  
  void check {
    O newValue = get(module, field);
    if (eq(value, newValue)) ret;
    value = newValue;
    dm_q(module, action);
  }
}