颤振错误“方法'toLowerCase（）'在null上被调用”

Question

我正在颤振中创建一个 Skype 克隆应用程序，并在创建“搜索”功能时遇到错误。我想通过在您键入时更新的列表以小写形式返回的用户名或名称在我的 firebase 数据库中搜索其他用户。但是，在第一次击键时，我遇到了这个错误：

The method 'toLowerCase' was called on null.
Receiver: null
Tried calling: toLowerCase()

这是搜索屏幕的代码（问题在buildSuggestions下）：

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:gradient_app_bar/gradient_app_bar.dart';
import 'package:skypeclone1/models/user.dart';
import 'package:skypeclone1/resources/firebase_repository.dart';
import 'package:skypeclone1/utils/universal_variables.dart';
import 'package:skypeclone1/widgets/custom_tile.dart';

class SearchScreen extends StatefulWidget {
  @override
  _SearchScreenState createState() => _SearchScreenState();
}

class _SearchScreenState extends State<SearchScreen> {
  FirebaseRepository _repository = FirebaseRepository();

  List<User> userList;
  String query = "";
  TextEditingController searchController = TextEditingController();

  @override
  void initState() {
    // TODO: implement initState
    super.initState();

    _repository.getCurrentUser().then((FirebaseUser user) {
      _repository.fetchAllUsers(user).then((List<User> list) {
        setState(() {
          userList = list;
        });
      });
    });
  }

  searchAppBar(BuildContext context) {
    return GradientAppBar(
      backgroundColorStart: UniversalVariables.gradientColorStart,
      backgroundColorEnd: UniversalVariables.gradientColorEnd,
      leading: IconButton(
        icon: Icon(Icons.arrow_back, color: Colors.white),
        onPressed: () => Navigator.pop(context),
      ),
      elevation: 0,
      bottom: PreferredSize(
        preferredSize: const Size.fromHeight(kToolbarHeight + 20),
        child: Padding(
          padding: EdgeInsets.only(left: 20),
          child: TextField(
            controller: searchController,
            onChanged: (val) {
              setState(() {
                query = val;
              });
            },
            cursorColor: UniversalVariables.blackColor,
            autofocus: true,
            style: TextStyle(
              fontWeight: FontWeight.bold,
              color: Colors.white,
              fontSize: 35,
            ),
            decoration: InputDecoration(
              suffixIcon: IconButton(
                icon: Icon(Icons.close, color: Colors.white),
                onPressed: () {
                  WidgetsBinding.instance
                      .addPostFrameCallback((_) => searchController.clear());
                },
              ),
              border: InputBorder.none,
              hintText: "Search",
              hintStyle: TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 35,
                color: Color(0x88ffffff),
              ),
            ),
          ),
        ),
      ),
    );
  }

  buildSuggestions(String query) {
    final List<User> suggestionList = query.isEmpty
        ? []
        : userList.where((User user) {
            String _getUsername = user.username.toLowerCase();
            String _query = query.toLowerCase();
            String _getName = user.name.toLowerCase();
            bool matchesUsername = _getUsername.contains(_query);
            bool matchesName = _getName.contains(_query);

            return (matchesUsername || matchesName);


          }).toList();

    return ListView.builder(
      itemCount: suggestionList.length,
      itemBuilder: ((context, index) {
        User searchedUser = User(
            uid: suggestionList[index].uid,
            profilePhoto: suggestionList[index].profilePhoto,
            name: suggestionList[index].name,
            username: suggestionList[index].username);

        return CustomTile(
          mini: false,
          onTap: () {},
          leading: CircleAvatar(
            backgroundImage: NetworkImage(searchedUser.profilePhoto),
            backgroundColor: Colors.grey,
          ),
          title: Text(
            searchedUser.username,
            style: TextStyle(
              color: Colors.white,
              fontWeight: FontWeight.bold,
            ),
          ),
          subtitle: Text(
            searchedUser.name,
            style: TextStyle(color: UniversalVariables.greyColor),
          ),
        );
      }),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: UniversalVariables.blackColor,
      appBar: searchAppBar(context),
      body: Container(
        padding: EdgeInsets.symmetric(horizontal: 20),
        child: buildSuggestions(query),
      ),
    );
  }
}

如果我从 buildSuggestions 中省略了 toLowerCase() 小部件，我会收到类似的错误：

The method 'contains' was called on null.
Receiver: null
Tried calling: contains("h")

*h 是我的按键

Answer 1

你的构建函数应该是这样的，它只在 userList 不为空时运行 buildSuggestions。

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: UniversalVariables.blackColor,
      appBar: searchAppBar(context),
      body: Container(
        padding: EdgeInsets.symmetric(horizontal: 20),
        child: userList.isEmpty?Container():buildSuggestions(query),
      ),
    );
  }

Answer 2

您需要验证您的输入数据。因为您的某些输入数据可以为空，所以会出现错误 你应该检查这个代码：

final List<User> suggestionList = query.isEmpty
        ? []
        : userList.where((User user) {
            String _getUsername = user.username.toLowerCase();
            String _query = query.toLowerCase();
            String _getName = user.name.toLowerCase();
            bool matchesUsername = _getUsername.contains(_query);
            bool matchesName = _getName.contains(_query);

            return (matchesUsername || matchesName);


          }).toList();

或者你可以如下修复：

          userList.where((User user) {
            if (query != null || user.username != null || user.name != null) {
              String _getUsername = user.username.toLowerCase();
              String _query = query.toLowerCase();
              String _getName = user.name.toLowerCase();
              bool matchesUsername = _getUsername.contains(_query);
              bool matchesName = _getName.contains(_query);

            return (matchesUsername || matchesName);
            } else { return false; }

          }).toList()

Answer 3

嘿，在上面的代码中，错误 方法 'toLowerCase' 在 null 上被调用。

接收者：null 尝试调用：toLowerCase()

是由于数据库中的用户数据不规则造成的

要解决这个问题，请尝试删除数据库中的用户集合或删除不规则的集合并重新登录