How to make dependent multilevel DropDown in flutter?

Question

I am trying to make dependent multilevel dropdown first contains states list and second contains cities list, all the data fetched from API. Initially, I load state dropdown

Answer 1

You have to make cityModel = null in onChanged callback of State dropdown.

setState(() {
  cityModel = null;
  stateModel = selectedState;
  _city = _fetchCities(stateModel.billstateid);
});

There should be exactly one item with [DropdownButton]'s value: Instance of 'City'. Either zero or 2 or more [DropdownMenuItem]s were detected with the same value

This error occurs here, because the value you are passing not in the items of DropdownButtonFormField(city dropdown).

When you select a State, you are fetching new list of city list and passing it to CityDropDown but forgot to clear the previously selected city(cityModel).

You can also refer this example: DartPad