如何验证颤振中的下拉

Question

I've created on the dropdown and i want to validate that dropdown. when a user doesn't select an item from drop-down then it will show a validation.

here is some code I've tried so far.

in this code I've created one drop down when I am click submit then it will show a validation when the user doesn't select an item from the dropdown.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Dropdown Validation',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  List<String> _type = <String>[
    'Passport',
    'Driving License',
    'Voter ID card',
    'Ration Card',
    'Aadhar',
    'Other Id'
  ];
  String type = 'Local Goverment';
  Widget _buildlicenseissuingauthority() {
    return FormField(
      builder: (FormFieldState state) {
        return InputDecorator(
          decoration: InputDecoration(
            prefixIcon: Icon(Icons.location_on),
            labelText: 'Select Personal Identification type',
          ),
          isEmpty: type == '',
          child: new DropdownButtonHideUnderline(
            child: new DropdownButton(
              value: type,
              isDense: true,
              onChanged: (String newValue) {
                setState(() {
                  type = newValue;
                });
              },
              items: _type.map(
                (String value) {
                  return new DropdownMenuItem(
                    value: value,
                    child: new Text(value),
                  );
                },
              ).toList(),
            ),
          ),
        );
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[_buildlicenseissuingauthority()],
        ),
      ),
    );
  }
}

Answer 1

please use pacakge https://pub.dev/packages/flutter_form_builder
github https://github.com/danvick/flutter_form_builder
It has more than 10 buildin validators, in your case FormBuilderValidators.required()

code snippet

FormBuilderDropdown(
                      attribute: "gender",
                      decoration: InputDecoration(
                        labelText: "Gender",
                        icon: const Icon(Icons.category),
                        hintText: "Select hint",
                      ),
                      // initialValue: 'Male',
                      onChanged: _onChanged,
                      hint: Text('Select Gender'),
                      validators: [FormBuilderValidators.required()],
                      items: ['Male', 'Female', 'Other']
                          .map((gender) => DropdownMenuItem(
                                value: gender,
                                child: Text('$gender'),
                              ))
                          .toList(),
                    )

full example code

import 'package:datetime_picker_formfield/datetime_picker_formfield.dart';
import 'package:flutter/material.dart';
import 'package:flutter_form_builder/flutter_form_builder.dart';
import 'package:intl/intl.dart';

import './data.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter FormBuilder Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  MyHomePageState createState() {
    return MyHomePageState();
  }
}

class MyHomePageState extends State<MyHomePage> {
  var data;
  bool autoValidate = true;
  bool readOnly = false;
  bool showSegmentedControl = true;
  final GlobalKey<FormBuilderState> _fbKey = GlobalKey<FormBuilderState>();
  final GlobalKey<FormFieldState> _specifyTextFieldKey =
      GlobalKey<FormFieldState>();
  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();

  ValueChanged _onChanged = (val) => print(val);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _scaffoldKey,
      appBar: AppBar(
        title: Text("FormBuilder Example"),
      ),
      body: Padding(
        padding: EdgeInsets.all(10),
        child: SingleChildScrollView(
          child: Column(
            children: <Widget>[
              FormBuilder(
                // context,
                key: _fbKey,
                autovalidate: true,
                // readonly: true,
                child: Column(
                  children: <Widget>[
                    FormBuilderCustomField(
                      attribute: "name",
                      validators: [
                        FormBuilderValidators.required(),
                      ],
                      formField: FormField(
                        // key: _fieldKey,
                        enabled: true,
                        builder: (FormFieldState<dynamic> field) {
                          return InputDecorator(
                            decoration: InputDecoration(
                              icon: const Icon(Icons.laptop),
                              hintText: "Select hint",
                              labelText: "Select option",
                              contentPadding:
                                  EdgeInsets.only(top: 10.0, bottom: 0.0),
                              border: InputBorder.none,
                              errorText: field.errorText,
                            ),
                            child: DropdownButton(
                              isExpanded: true,
                              items: ["One", "Two"].map((option) {
                                return DropdownMenuItem(
                                  child: Text("$option"),
                                  value: option,
                                );
                              }).toList(),
                              value: field.value,
                              onChanged: (value) {
                                field.didChange(value);
                              },
                            ),
                          );
                        },
                      ),
                    ),
                    FormBuilderChipsInput(
                      decoration: InputDecoration(
                        labelText: "Chips",
                        icon: const Icon(Icons.person),
                        hintText: "Select hint",
                      ),
                      attribute: 'chips_test',
                      // readonly: true,
                      onChanged: _onChanged,
                      // valueTransformer: (val) => val.length > 0 ? val[0] : null,
                      initialValue: [
                        Contact('Andrew', 'stock@man.com',
                            'https://d2gg9evh47fn9z.cloudfront.net/800px_COLOURBOX4057996.jpg'),
                      ],
                      maxChips: 5,
                      findSuggestions: (String query) {
                        if (query.length != 0) {
                          var lowercaseQuery = query.toLowerCase();
                          return mockResults.where((profile) {
                            return profile.name
                                    .toLowerCase()
                                    .contains(query.toLowerCase()) ||
                                profile.email
                                    .toLowerCase()
                                    .contains(query.toLowerCase());
                          }).toList(growable: false)
                            ..sort((a, b) => a.name
                                .toLowerCase()
                                .indexOf(lowercaseQuery)
                                .compareTo(b.name
                                    .toLowerCase()
                                    .indexOf(lowercaseQuery)));
                        } else {
                          return const <Contact>[];
                        }
                      },
                      chipBuilder: (context, state, profile) {
                        return InputChip(
                          key: ObjectKey(profile),
                          label: Text(profile.name),
                          avatar: CircleAvatar(
                            backgroundImage: NetworkImage(profile.imageUrl),
                          ),
                          onDeleted: () => state.deleteChip(profile),
                          materialTapTargetSize:
                              MaterialTapTargetSize.shrinkWrap,
                        );
                      },
                      suggestionBuilder: (context, state, profile) {
                        return ListTile(
                          key: ObjectKey(profile),
                          leading: CircleAvatar(
                            backgroundImage: NetworkImage(profile.imageUrl),
                          ),
                          title: Text(profile.name),
                          subtitle: Text(profile.email),
                          onTap: () => state.selectSuggestion(profile),
                        );
                      },
                    ),
                    FormBuilderDateTimePicker(
                      attribute: "date",
                      onChanged: _onChanged,
                      inputType: InputType.date,
                      format: DateFormat("yyyy-MM-dd"),
                      decoration: InputDecoration(
                        labelText: "Appointment Time",
                        icon: const Icon(Icons.calendar_today),
                        hintText: "Select hint",
                      ),
                      // readonly: true,
                    ),
                    FormBuilderSlider(
                      attribute: "slider",
                      validators: [FormBuilderValidators.min(6)],
                      onChanged: _onChanged,
                      min: 0.0,
                      max: 10.0,
                      initialValue: 1.0,
                      divisions: 20,
                      decoration: InputDecoration(
                        labelText: "Number of somethings",
                        icon: const Icon(Icons.snooze),
                        hintText: "Select hint",
                      ),
                    ),
                    FormBuilderCheckbox(
                      decoration: InputDecoration(
                        labelText: "terms",
                        icon: const Icon(Icons.subject),
                        hintText: "Select hint",
                      ),
                      attribute: 'accept_terms',
                      initialValue: false,
                      onChanged: _onChanged,
                      leadingInput: true,
                      label: Text(
                          "I have read and agree to the terms and conditions"),
                      validators: [
                        FormBuilderValidators.requiredTrue(
                          errorText:
                              "You must accept terms and conditions to continue",
                        ),
                      ],
                    ),
                    FormBuilderDropdown(
                      attribute: "gender",
                      decoration: InputDecoration(
                        labelText: "Gender",
                        icon: const Icon(Icons.category),
                        hintText: "Select hint",
                      ),
                      // initialValue: 'Male',
                      onChanged: _onChanged,
                      hint: Text('Select Gender'),
                      validators: [FormBuilderValidators.required()],
                      items: ['Male', 'Female', 'Other']
                          .map((gender) => DropdownMenuItem(
                                value: gender,
                                child: Text('$gender'),
                              ))
                          .toList(),
                    ),
                    FormBuilderTextField(
                      attribute: "age",
                      decoration: InputDecoration(
                        labelText: "Age",
                        icon: const Icon(Icons.accessibility),
                        hintText: "Select hint",
                      ),
                      onChanged: _onChanged,
                      valueTransformer: (text) => num.tryParse(text),
                      validators: [
                        FormBuilderValidators.numeric(),
                        FormBuilderValidators.max(70),
                      ],
                    ),
                    FormBuilderTypeAhead(
                      // initialValue: "Canada",
                      decoration: InputDecoration(
                        labelText: "Country",
                        icon: const Icon(Icons.golf_course),
                        hintText: "Select hint",
                      ),
                      attribute: 'country',
                      onChanged: _onChanged,
                      itemBuilder: (context, country) {
                        return ListTile(
                          title: Text(country),
                        );
                      },
                      suggestionsCallback: (query) {
                        if (query.length != 0) {
                          var lowercaseQuery = query.toLowerCase();
                          return allCountries.where((country) {
                            return country
                                .toLowerCase()
                                .contains(lowercaseQuery);
                          }).toList(growable: false)
                            ..sort((a, b) => a
                                .toLowerCase()
                                .indexOf(lowercaseQuery)
                                .compareTo(
                                    b.toLowerCase().indexOf(lowercaseQuery)));
                        } else {
                          return allCountries;
                        }
                      },
                    ),
                    FormBuilderRadio(
                      decoration: InputDecoration(
                        labelText: 'My chosen language',
                        icon: const Icon(Icons.language),
                        hintText: "Select hint",
                      ),
                      attribute: "best_language",
                      leadingInput: true,
                      onChanged: _onChanged,
                      validators: [FormBuilderValidators.required()],
                      options: [
                        "Dart",
                        "Kotlin",
                        "Java",
                        "Swift",
                        "Objective-C"
                      ]
                          .map((lang) => FormBuilderFieldOption(value: lang))
                          .toList(growable: false),
                    ),
                    FormBuilderSegmentedControl(
                      decoration: InputDecoration(
                        labelText: "Movie Rating (Archer)",
                        icon: const Icon(Icons.crop_rotate),
                        hintText: "Select hint",
                      ),
                      attribute: "movie_rating",
                      options: List.generate(5, (i) => i + 1)
                          .map(
                              (number) => FormBuilderFieldOption(value: number))
                          .toList(),
                      onChanged: _onChanged,
                    ),
                    FormBuilderSwitch(
                      decoration: InputDecoration(
                        labelText: "conditions",
                        icon: const Icon(Icons.dashboard),
                        hintText: "Select hint",
                      ),
                      label: Text('I Accept the tems and conditions'),
                      attribute: "accept_terms_switch",
                      initialValue: true,
                      onChanged: _onChanged,
                    ),
                    FormBuilderStepper(
                      decoration: InputDecoration(
                        labelText: "Stepper",
                        icon: const Icon(Icons.account_box),
                        hintText: "Select hint",
                      ),
                      attribute: "stepper",
                      initialValue: 10,
                      step: 1,
                      validators: [
                        (val) {
                          if (!_fbKey.currentState.fields["accept_terms_switch"]
                                  .currentState.value &&
                              val >= 10) {
                            return "You can only put more than 10 if you've accepted terms";
                          }
                        }
                      ],
                    ),
                    FormBuilderRate(
                      decoration: InputDecoration(
                        labelText: "Rate this form",
                        icon: const Icon(Icons.branding_watermark),
                        hintText: "Select hint",
                      ),
                      attribute: "rate",
                      iconSize: 32.0,
                      initialValue: 1,
                      max: 5,
                      onChanged: _onChanged,
                    ),
                    FormBuilderCheckboxList(
                      decoration: InputDecoration(
                        labelText: "The language of my people",
                        icon: const Icon(Icons.collections),
                        hintText: "Select hint",
                      ),
                      attribute: "languages",
                      initialValue: ["Dart"],
                      leadingInput: true,
                      options: [
                        FormBuilderFieldOption(value: "Dart"),
                        FormBuilderFieldOption(value: "Kotlin"),
                        FormBuilderFieldOption(value: "Java"),
                        FormBuilderFieldOption(value: "Swift"),
                        FormBuilderFieldOption(value: "Objective-C"),
                      ],
                      onChanged: _onChanged,
                    ),
                    FormBuilderCustomField(
                      attribute: 'custom',
                      valueTransformer: (val) {
                        if (val == "Other")
                          return _specifyTextFieldKey.currentState.value;
                        return val;
                      },
                      formField: FormField(
                        builder: (FormFieldState<String> field) {
                          var languages = [
                            "English",
                            "Spanish",
                            "Somali",
                            "Other"
                          ];
                          return InputDecorator(
                            decoration: InputDecoration(
                              labelText: "What's your preferred language?",
                              icon: const Icon(Icons.bug_report),
                              hintText: "Select hint",
                            ),
                            child: Column(
                              children: languages
                                  .map(
                                    (lang) => Row(
                                          children: <Widget>[
                                            Radio<dynamic>(
                                              value: lang,
                                              groupValue: field.value,
                                              onChanged: (dynamic value) {
                                                field.didChange(lang);
                                              },
                                            ),
                                            lang != "Other"
                                                ? Text(lang)
                                                : Expanded(
                                                    child: Row(
                                                      children: <Widget>[
                                                        Text(
                                                          lang,
                                                        ),
                                                        SizedBox(width: 20),
                                                        Expanded(
                                                          child: TextFormField(
                                                            key:
                                                                _specifyTextFieldKey,
                                                          ),
                                                        ),
                                                      ],
                                                    ),
                                                  ),
                                          ],
                                        ),
                                  )
                                  .toList(growable: false),
                            ),
                          );
                        },
                      ),
                    ),
                    FormBuilderSignaturePad(
                      decoration: InputDecoration(
                        labelText: "Signature",
                        icon: const Icon(Icons.card_travel),
                        hintText: "Select hint",
                      ),
                      attribute: "signature",
                      // height: 250,
                      clearButtonText: "Start Over",
                      onChanged: _onChanged,
                    ),
                  ],
                ),
              ),
              Row(
                children: <Widget>[
                  Expanded(
                    child: MaterialButton(
                      color: Theme.of(context).accentColor,
                      child: Text(
                        "Submit",
                        style: TextStyle(color: Colors.white),
                      ),
                      onPressed: () {
                        _fbKey.currentState.save();
                        if (_fbKey.currentState.validate()) {
                          print(_fbKey.currentState.value);
                        } else {
                          print(_fbKey.currentState.value);
                          print("validation failed");
                          _scaffoldKey.currentState.showSnackBar(
                              SnackBar(
                                content: Text('Validation failed'),
                                duration: Duration(seconds: 3),
                              ));
                        }
                      },
                    ),
                  ),
                  SizedBox(
                    width: 20,
                  ),
                  Expanded(
                    child: MaterialButton(
                      color: Theme.of(context).accentColor,
                      child: Text(
                        "Reset",
                        style: TextStyle(color: Colors.white),
                      ),
                      onPressed: () {
                        _fbKey.currentState.reset();
                      },
                    ),
                  ),
                ],
              ),
            ],
          ),
        ),
      ),
    );
  }
}

Data.dart

class Contact {
  final String name;
  final String email;
  final String imageUrl;

  const Contact(this.name, this.email, this.imageUrl);

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
          other is Contact &&
              runtimeType == other.runtimeType &&
              name == other.name;

  @override
  int get hashCode => name.hashCode;

  @override
  String toString() {
    return name;
  }
}

const allCountries = [
  "Afghanistan",
  "Albania",
  "Algeria",
  "American Samoa",
  "Andorra",
  "Angola",
  "Anguilla",

];

const contacts = <Contact>[
  Contact('Andrew', 'stock@man.com',
      'https://d2gg9evh47fn9z.cloudfront.net/800px_COLOURBOX4057996.jpg'),
  Contact('Paul', 'paul@google.com',
      'https://mbtskoudsalg.com/images/person-stock-image-png.png'),
  Contact('Fred', 'fred@google.com',
      'https://media.istockphoto.com/photos/feeling-great-about-my-corporate-choices-picture-id507296326'),
  Contact('Brian', 'brian@flutter.io',
      'https://upload.wikimedia.org/wikipedia/commons/7/7c/Profile_avatar_placeholder_large.png'),

];

demo, after click submit button, you can see gender has red text "This field can not be empty"

Answer 2

Flutter already has its own Form DropDownButtom for this

List<String> typeNeg = [
"One",
"Two",
"Three",];

String dropdownValue = "One";

DropdownButtonFormField<String>(
                    value: dropdownValue,
                    hint: Text("Type of business"),
                    onChanged: (String newValue) {
                      setState(() {
                        dropdownValue = newValue;
                      });
                    },
                    validator: (String value) {
                      if (value?.isEmpty ?? true) {
                        return 'Please enter a valid type of business';
                      }
                    },
                    items: typeNeg
                        .map<DropdownMenuItem<String>>((String value) {
                      return DropdownMenuItem<String>(
                        value: value,
                        child: Text(value),
                      );
                    }).toList(),
                    onSaved: (val) => setState(() => _user.typeNeg = val),
                  ),