Show a form as custom dialog and collect the field values.

show_form_dialog.dart

import 'package:flutter/material.dart';

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

@immutable
class CategoryModel {
  const CategoryModel({this.name, this.keywords});

  final String name;
  final String keywords;

  String toString() => 'CategoryModel{name:$name, keywords:$keywords}';
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  CategoryModel model = CategoryModel(name: '{Empty}', keywords: '{Empty}');
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Dialog Form'),
      ),
      body: Center(
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            RaisedButton(
              child: Text('1.Show Dialog'),
              onPressed: () async {
                final result = await showDialog<CategoryModel>(
                  context: context,
                  builder: categoryDialogBuilder,
                );
                setState(() => model = result);
              },
            ),
            SizedBox(width: 5),
            RaisedButton(
              child: Text('2.Console Print'),
              onPressed: () => print(model ?? 'User canceled'),
            ),
          ],
        ),
      ),
    );
  }
}

Widget categoryDialogBuilder(BuildContext context) {
  String name = '';
  String keywords = '';
  GlobalKey<FormState> formKey = GlobalKey<FormState>();
  return Dialog(
    child: Form(
      key: formKey,
      child: ListView(
        padding: EdgeInsets.fromLTRB(10, 0, 10, 10),
        shrinkWrap: true,
        children: [
          TextFormField(
            decoration: InputDecoration(labelText: 'Name'),
            onSaved: (value) => name = value,
          ),
          SizedBox(height: 10),
          TextFormField(
            decoration: InputDecoration(labelText: 'Keywords'),
            onSaved: (value) => keywords = value,
          ),
          SizedBox(height: 20),
          Row(
            mainAxisAlignment: MainAxisAlignment.spaceAround,
            children: [
              RaisedButton(
                child: Text('Ok'),
                onPressed: () {
                  formKey.currentState.save();
                  Navigator.of(context).pop<CategoryModel>(
                    CategoryModel(
                      name: name,
                      keywords: keywords,
                    ),
                  );
                },
              ),
              RaisedButton(
                child: Text('Cancel'),
                onPressed: () {
                  Navigator.of(context).pop<CategoryModel>();
                },
              ),
            ],
          )
        ],
      ),
    ),
  );
}