How to get flutter datepicker value to the textfield?

I fix your code as below (you can select date from DatePicker and you can customise theme of DatePicker):

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

class TestPickerWidget extends StatefulWidget {
  @override
  _TestPickerWidgetState createState() => _TestPickerWidgetState();
}

class _TestPickerWidgetState extends State<TestPickerWidget> {
  DateTime _selectedDate;
  TextEditingController _textEditingController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: TextField(
          focusNode: AlwaysDisabledFocusNode(),
          controller: _textEditingController,
          onTap: () {
            _selectDate(context);
          },
        ),
      ),
    );
  }

  _selectDate(BuildContext context) async {
    DateTime newSelectedDate = await showDatePicker(
        context: context,
        initialDate: _selectedDate != null ? _selectedDate : DateTime.now(),
        firstDate: DateTime(2000),
        lastDate: DateTime(2040),
        builder: (BuildContext context, Widget child) {
          return Theme(
            data: ThemeData.dark().copyWith(
              colorScheme: ColorScheme.dark(
                primary: Colors.deepPurple,
                onPrimary: Colors.white,
                surface: Colors.blueGrey,
                onSurface: Colors.yellow,
              ),
              dialogBackgroundColor: Colors.blue[500],
            ),
            child: child,
          );
        });

    if (newSelectedDate != null) {
      _selectedDate = newSelectedDate;
      _textEditingController
        ..text = DateFormat.yMMMd().format(_selectedDate)
        ..selection = TextSelection.fromPosition(TextPosition(
            offset: _textEditingController.text.length,
            affinity: TextAffinity.upstream));
    }
  }
}

class AlwaysDisabledFocusNode extends FocusNode {
  @override
  bool get hasFocus => false;
}

and also you must add Intl dependency in your pubspec.yaml :

dev_dependencies:
  flutter_test:
    sdk: flutter
  intl: ^0.16.1

Finally call TestPickerWidget in your main for testing it.

Try this. First of all, you need to define a TextEditing controller. Then you can access that controller to set the selected date.

This is your customizable date picker.

showDatePicker(
  context: context,
  initialDate: DateTime.now(),
  firstDate: DateTime(2019, 1),
  lastDate: DateTime(2021,12),
  builder: (context,picker){
    return Theme(
    //TODO: change colors
    data: ThemeData.dark().copyWith(
      colorScheme: ColorScheme.dark(
        primary: Colors.deepPurple,
        onPrimary: Colors.white,
        surface: Colors.pink,
        onSurface: Colors.yellow,
      ),
      dialogBackgroundColor:Colors.green[900],
     ),
     child: picker!,);
   })
   .then((selectedDate) {
     //TODO: handle selected date
     if(selectedDate!=null){
       _controller.text = selectedDate.toString();
     }
 });

You can try this live preview.

You can use like this

showDatePicker(
                context: context,
                initialDate: DateTime.now(),
                firstDate: DateTime(2019, 1),
                lastDate: DateTime(2021, 12),
              ).then((pickedDate) {
                //do whatever you want
              });