尝试FormWidget，将此文件 Package 在Form内，
像这样

import 'package:flutter/material.dart';

class DemoDropDownCreate extends StatefulWidget {
  @override
  _DemoDropDownCreate createState() =>
      _DemoDropDownCreate();
}

class _DemoDropDownCreate
    extends State<DemoDropDownCreate> {
  final _formKey = GlobalKey<FormState>();
  bool _autovalidate = false;
  String selectedCountry;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Form(
        key: _formKey,
        autovalidate: _autovalidate,
        child:
        Column(
          children: <Widget>[
            DropdownButtonFormField<String>(
              value: selectedCountry,
              hint: Text(
                'Select Country',
              ),
              onChanged: (salutation) =>
                  setState(() => selectedCountry = salutation),
              validator: (value) =>  value.isEmpty ? 'field required' : null,
              items:
              ['India', 'Japan'].map<DropdownMenuItem<String>>((String value) {
                return DropdownMenuItem<String>(
                  value: value,
                  child: Text(value),
                );
              }).toList(),
            ),
            FlatButton(
              child: Text('PROCEED'),
              color: Colors.green,
              onPressed: () {
                if (_formKey.currentState.validate()) {
                  //form is valid, proceed further
                  _formKey.currentState.save();//save once fields are valid, onSaved method invoked for every form fields
                  print("success");
                } else {
                  setState(() {
                    _autovalidate = true; //enable realtime validation
                  });
                  print("Please select all field");

                }
              },
            )
          ],
        ),
      ),
    );
  }
}

您确实可以为DropdownButtonFormField使用验证器

DropdownButtonFormField(
    validator: (dynamic value) => value.isEmpty ? "An error happened or please pick something" : null,
    decoration: InputDecoration(....

您可以将value.isEmpty更改为您想要的任何条件，与"An error happened"消息相同。