Flutter: Dropdown需要在文本字段具有焦点时打开两次轻击

import 'package:flutter/material.dart';

class CustomDropdown extends StatefulWidget {
  @override
  _CustomDropdownState createState() => _CustomDropdownState();
}

class _CustomDropdownState extends State<CustomDropdown> {
  bool hasFocus = false;

  FocusNode _focusNode;

  @override
  void initState() {
    super.initState();
    _focusNode = FocusNode();
    _focusNode.addListener(_onFocusChanged);
  }

  @override
  void dispose() {
    _focusNode.removeListener(_onFocusChanged);
    _focusNode.dispose();
    super.dispose();
  }

  void _onFocusChanged() {
    setState(() {
      hasFocus = _focusNode.hasFocus;
    });
  }

  void _openDropdown() {
    if (hasFocus) {
      DropdownButtonHideUnderline(
        child: DropdownButton<String>(
          value: null,
          items: <String>[].map((String value) {
            return DropdownMenuItem<String>(
              value: value,
              child: Text(value),
            );
          }).toList(),
          onChanged: (String newValue) {},
        ),
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return TextField(
      focusNode: _focusNode,
      onTap: _openDropdown,
    );
  }
}