Change DropdownButtonFormField value programmatically

Question

I'm trying to change the DropdownButtonFormField value on event (button press for example) using setState. But it's not working.

Note: it works in case I use DropdownButton, but with DropdownButtonFormField it's not responding.

Here is a simple code showing what I'm trying to implement.

import 'package:flutter/material.dart';

void main() {
runApp(MyApp());
}

class MyApp extends StatelessWidget {
 @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Testing(),
    );
  }
}

class Testing extends StatefulWidget {
  @override
  _TestingState createState() => _TestingState();
}

class _TestingState extends State<Testing> {
  String selectedValue;
  @override
  Widget build(BuildContext context) {
    return Material(
      child: Column(
        children: <Widget>[
          DropdownButtonFormField(
            value: selectedValue,
            items: ['one', 'two'].map((value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
            onChanged: (value) {
              setState(() {
                selectedValue = value;
              }); 
            },
          ),
          RaisedButton(
            child: Text('test'),
            onPressed: (){
              setState(() {
                selectedValue = 'two';
              });
            },
          ),
        ],
      ),
    );
  }
}

Answer 1

Define instance variable from Global Key and pass it to DropdownButtonFormField

final dropdownState = GlobalKey<FormFieldState>();

You can change the value of dropDownFieldItem by calling this method

dropdownState.currentState.didChange('two');

final code:

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Testing(),
    );
  }
}

class Testing extends StatefulWidget {
  @override
  _TestingState createState() => _TestingState();
}

class _TestingState extends State<Testing> {
  String selectedValue;
  final dropdownState = GlobalKey<FormFieldState>();

  @override
  Widget build(BuildContext context) {
    return Material(
      child: Column(
        children: <Widget>[
          DropdownButtonFormField(
            key: dropdownState,
            value: selectedValue,
            items: ['one', 'two'].map((value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
            onChanged: (value) {
              setState(() {
                selectedValue = value;
              });
            },
          ),
          RaisedButton(
            child: Text('test'),
            onPressed: () {
              dropdownState.currentState.didChange('one');
            },
          ),
        ],
      ),
    );
  }
}

Answer 2

Here working normally with DropdownButtonFormField and DropdownButton.

flutter --version

Flutter 1.12.13+hotfix.9 • channel stable •