sclass Dyn_FieldWatcher {
  DynModule module;
  S field;
  O value;
  Runnable action;
  
  swappable O cloneValue(O o) { ret o; }
  
  *(DynModule *module, S *field, Runnable *action) {
    value = cloneValue(get(module, field));
    
    // Let's do both for safety
    module.onChange(r check);
    module.onFieldChange(voidfunc(S f) {
      if (eq(f, field))
        check();
    });
  }
  
  void check {
    O newValue = cloneValue(get(module, field));
    if (eq(value, newValue)) ret;
    value = newValue;
    dm_q(module, action);
  }
}