如何让用户在flutter应用中输入她的生日

import 'package:flutter/material.dart';

class BirthdayForm extends StatefulWidget {
  @override
  _BirthdayFormState createState() => _BirthdayFormState();
}

class _BirthdayFormState extends State<BirthdayForm> {
  DateTime selectedDate;

  Future<void> _selectDate(BuildContext context) async {
    final DateTime picked = await showDatePicker(
      context: context,
      initialDate: DateTime.now(),
      firstDate: DateTime(1900),
      lastDate: DateTime.now(),
    );
    if (picked != null && picked != selectedDate) {
      setState(() {
        selectedDate = picked;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Birthday Form'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            RaisedButton(
              onPressed: () => _selectDate(context),
              child: Text('Select Birthday'),
            ),
            SizedBox(height: 20),
            Text(
              selectedDate != null
                  ? 'Selected Birthday: ${selectedDate.toString()}'
                  : 'No birthday selected',
            ),
          ],
        ),
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: BirthdayForm(),
  ));
}