我将你的代码修改如下(你可以从 DatePicker 中选择日期，你可以自定义 DatePicker 的主题):

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;
}

您还必须在 pubspec.yaml 中添加 Intl 依赖项:

dev_dependencies:
  flutter_test:
    sdk: flutter
  intl: ^0.16.1

最后在你的 main 中调用 TestPickerWidget 来测试它.

Finally call TestPickerWidget in your main for testing it.