flutter - 如何修复颤振中的“用于空值的空检查运算符”？

Question

我正在制作一个待办事项列表应用程序。我编写了代码并使用了dart migrate 特性来保证空值安全。我不知道如何解决这个问题，有人可以帮助我。

在设备上运行应用程序时显示错误

主要.dart

import 'package:flutter/material.dart';
import 'package:todo_list/screens/todo_list_screen.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'TODO List',
      theme: ThemeData(
        primarySwatch: Colors.red,
      ),
      home: TodoListScreen(),
    );
  }
}

database_helpers.dart

import 'dart:io';

import 'package:path_provider/path_provider.dart';
import 'package:sqflite/sqflite.dart';
import 'package:todo_list/models/task_model.dart';

class DatabaseHelper {
  static final DatabaseHelper instance = DatabaseHelper._instance();
  static Database? _db;

  DatabaseHelper._instance();

  String tasksTable = 'task_table';
  String colId = 'id';
  String colTitle = 'title';
  String colDate = 'date';
  String colPriority = 'priority';
  String colStatus = 'status';

  //Task Table
  //id | Title | Date | Priority | Status

  Future<Database?> get db async {
    if (_db == null) {
      _db = await _initDb();
    }
    return _db;
  }

  Future<Database> _initDb() async {
    Directory dir = await getApplicationDocumentsDirectory();
    String path = dir.path + 'todo_list.db';
    final todoListDb =
        await openDatabase(path, version: 1, onCreate: _createDb);
    return todoListDb;
  }

  void _createDb(Database db, int version) async {
    await db.execute(
        'CREATE TABLE $tasksTable($colId INTEGER PRIMARY KEY AUTOINCREMENT, $colTitle TEXT, $colDate TEXT, $colPriority TEXT, $colStatus INTEGER');
  }

  Future<List<Map<String, dynamic>>> getTaskMapList() async {
    Database db = await (this.db as FutureOr<Database>);
    final List<Map<String, dynamic>> result = await db.query(tasksTable);
    return result;
  }

  Future<List<Task>> getTaskList() async {
    final List<Map<String, dynamic>> taskMapList = await getTaskMapList();
    final List<Task> taskList = [];
    taskMapList.forEach((taskMap) {
      taskList.add(Task.fromMap(taskMap));
    });
    taskList.sort((taskA, taskB) => taskA.date!.compareTo(taskB.date!));
    return taskList;
  }

  Future<int> insertTask(Task task) async {
    Database db = await (this.db as FutureOr<Database>);
    final int result = await db.insert(tasksTable, task.toMap());
    return result;
  }

  Future<int> updateTask(Task task) async {
    Database db = await (this.db as FutureOr<Database>);
    final int result = await db.update(
      tasksTable,
      task.toMap(),
      where: '$colId=',
      whereArgs: [task.id],
    );
    return result;
  }

  Future<int> deleteTask(int? id) async {
    Database db = await (this.db as FutureOr<Database>);
    final int result = await db.delete(
      tasksTable,
      where: '$colId = ',
      whereArgs: [id],
    );
    return result;
  }
}

task_model.dart

class Task {
  int? id;
  String? title;
  DateTime? date;
  String? priority;
  int? status;

  Task({this.date, this.priority, this.status, this.title});
  Task.withId({this.id, this.date, this.priority, this.status, this.title});

  Map<String, dynamic> toMap() {
    final map = Map<String, dynamic>();
    // if (id = null) {
    //   map['id'] = id;
    // }
    map['id'] = id;
    map['title'] = title;
    map['date'] = date!.toIso8601String();
    map['priority'] = priority;
    map['status'] = status;
    return map;
  }

  factory Task.fromMap(Map<String, dynamic> map) {
    return Task.withId(
      id: map['id'],
      date: DateTime.parse(map['date']),
      priority: map['priority'],
      status: map['status'],
      title: map['title'],
    );
  }
}

add_task_screen.dart

import 'package:flutter/material.dart';
import 'package:intl/intl.dart';
import 'package:todo_list/helpers/database_helpers.dart';
import 'package:todo_list/models/task_model.dart';

class AddTaskScreen extends StatefulWidget {
  final Function? updateTaskList;
  final Task? task;
  AddTaskScreen({this.task, this.updateTaskList});

  @override
  _AddTaskScreenState createState() => _AddTaskScreenState();
}

class _AddTaskScreenState extends State<AddTaskScreen> {
  final _formKey = GlobalKey<FormState>();
  String? _title = '';
  String? _priority = '';
  DateTime? _date = DateTime.now();
  int? _status = 0;
  TextEditingController _dateController = TextEditingController();

  final DateFormat _dateFormatter = DateFormat('MMM dd, yyyy');
  final List<String> _priorities = ['Low', 'Medium', 'High'];

  @override
  void initState() {
    super.initState();

    _title = widget.task!.title;
    _date = widget.task!.date;
    _priority = widget.task!.priority;
    _status = widget.task!.status;

    _dateController.text = _dateFormatter.format(_date!);
  }

  @override
  void dispose() {
    _dateController.dispose();
    super.dispose();
  }

  _handleDatePicker() async {
    final DateTime? date = await showDatePicker(
      context: context,
      initialDate: _date!,
      firstDate: DateTime.now(),
      lastDate: DateTime(2100),
      // currentDate: DateTime.now(),
    );
    if (date != null && date != _date) {
      setState(() {
        _date = date;
      });
      _dateController.text = _dateFormatter.format(date);
    }
  }

  _delete() {
    DatabaseHelper.instance.deleteTask(widget.task!.id);
    widget.updateTaskList!();
    Navigator.pop(context);
  }

  _submit() {
    if (_formKey.currentState!.validate()) {
      _formKey.currentState!.save();

      Task task = Task(
          date: _date, title: _title, priority: _priority, status: _status);

      if (task.status == 0)
        DatabaseHelper.instance.insertTask(task);
      else {
        task.id = widget.task!.id;
        task.status = widget.task!.status;
        DatabaseHelper.instance.updateTask(task);
      }
      widget.updateTaskList!();
      Navigator.pop(context);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: GestureDetector(
        onTap: () => FocusScope.of(context).unfocus(),
        child: SingleChildScrollView(
          child: Padding(
            padding: EdgeInsets.symmetric(horizontal: 40, vertical: 80),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: [
                GestureDetector(
                  onTap: () => Navigator.pop(context),
                  child: Icon(
                    Icons.arrow_back_ios_new,
                    size: 30,
                    color: Theme.of(context).primaryColor,
                  ),
                ),
                SizedBox(height: 20),
                Text(
                  widget.task == null ? 'Add Task' : 'Update Task',
                  style: TextStyle(
                    fontSize: 40,
                    fontWeight: FontWeight.bold,
                    color: Colors.black,
                  ),
                ),
                SizedBox(height: 10),
                Form(
                  key: _formKey,
                  child: Column(
                    children: [
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 20),
                        child: TextFormField(
                          style: TextStyle(fontSize: 18),
                          decoration: InputDecoration(
                            labelText: 'Title',
                            labelStyle: TextStyle(fontSize: 18),
                            border: OutlineInputBorder(
                              borderRadius: BorderRadius.circular(10),
                            ),
                          ),
                          validator: (input) =>
                              input != null && input.trim().isEmpty
                                  ? 'Please enter a task title!'
                                  : null,
                          onSaved: (input) {
                            if (input != null) _title = input;
                          },
                          initialValue: _title,
                        ),
                      ),
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 20),
                        child: TextFormField(
                          style: TextStyle(fontSize: 18),
                          controller: _dateController,
                          onTap: _handleDatePicker,
                          readOnly: true,
                          decoration: InputDecoration(
                            labelText: 'Date',
                            labelStyle: TextStyle(fontSize: 18),
                            border: OutlineInputBorder(
                              borderRadius: BorderRadius.circular(10),
                            ),
                          ),
                        ),
                      ),
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 20),
                        child: DropdownButtonFormField(
                          isDense: true,
                          icon: Icon(Icons.arrow_drop_down_circle),
                          iconSize: 22,
                          iconEnabledColor: Theme.of(context).primaryColor,
                          items: _priorities.map((String priority) {
                            return DropdownMenuItem(
                              value: priority,
                              child: Text(
                                priority,
                                style: TextStyle(
                                  color: Colors.black,
                                  fontSize: 18,
                                ),
                              ),
                            );
                          }).toList(),
                          style: TextStyle(fontSize: 18),
                          decoration: InputDecoration(
                            labelText: 'Priority',
                            labelStyle: TextStyle(fontSize: 18),
                            border: OutlineInputBorder(
                              borderRadius: BorderRadius.circular(10),
                            ),
                          ),
                          validator: (dynamic input) => _priority == null
                              ? 'Please select a priority level!'
                              : null,
                          onChanged: (dynamic value) {
                            setState(() {
                              _priority = value.toString();
                            });
                          },
                        ),
                      ),
                      Container(
                        margin: EdgeInsets.symmetric(vertical: 20),
                        height: 60,
                        width: double.infinity,
                        decoration: BoxDecoration(
                          color: Theme.of(context).primaryColor,
                          borderRadius: BorderRadius.circular(30),
                        ),
                        child: TextButton(
                          onPressed: _submit,
                          child: Text(
                            widget.task == null ? 'Add' : 'Update',
                            style: TextStyle(
                              color: Colors.white,
                              fontSize: 20,
                            ),
                          ),
                        ),
                      ),
                      widget.task != null
                          ? Container(
                              margin: EdgeInsets.symmetric(vertical: 20),
                              height: 60,
                              width: double.infinity,
                              decoration: BoxDecoration(
                                color: Theme.of(context).primaryColor,
                                borderRadius: BorderRadius.circular(30),
                              ),
                              child: TextButton(
                                onPressed: _delete,
                                child: Text(
                                  'Delete',
                                  style: TextStyle(
                                    color: Colors.white,
                                    fontSize: 20,
                                  ),
                                ),
                              ),
                            )
                          : SizedBox(height: 0),
                    ],
                  ),
                )
              ],
            ),
          ),
        ),
      ),
    );
  }
}

todo_list_screen.dart

import 'package:flutter/material.dart';
import 'package:intl/intl.dart';
import 'package:todo_list/helpers/database_helpers.dart';
import 'package:todo_list/models/task_model.dart';
import 'package:todo_list/screens/add_task_sreen.dart';

class TodoListScreen extends StatefulWidget {
  @override
  _TodoListScreenState createState() => _TodoListScreenState();
}

class _TodoListScreenState extends State<TodoListScreen> {
  Future<List<Task>>? _taskList;
  final DateFormat _dateFormatter = DateFormat('MMM dd, yyyy');

  @override
  void initState() {
    super.initState();
    _updateTaskList();
  }

  _updateTaskList() {
    setState(() {
      _taskList = DatabaseHelper.instance.getTaskList();
    });
  }

  Widget _buildTask(Task task) {
    return Padding(
      padding: EdgeInsets.symmetric(horizontal: 25),
      child: Column(
        children: [
          ListTile(
            title: Text(
              task.title!,
              style: TextStyle(
                  fontSize: 18,
                  decoration: task.status == 0
                      ? TextDecoration.none
                      : TextDecoration.lineThrough),
            ),
            subtitle: Text(
              '${_dateFormatter.format(task.date!)}·${task.priority}',
              style: TextStyle(
                  fontSize: 15,
                  decoration: task.status == 0
                      ? TextDecoration.none
                      : TextDecoration.lineThrough),
            ),
            trailing: Checkbox(
              value: task.status == 1 ? true : false,
              onChanged: (value) {
                // task.status = value ? 1 : 0;
                if (value == false)
                  task.status = 0;
                else
                  task.status = 1;
                DatabaseHelper.instance.updateTask(task);
                _updateTaskList();
              },
              activeColor: Theme.of(context).primaryColor,
            ),
            onTap: () => Navigator.push(
              context,
              MaterialPageRoute(
                builder: (_) => AddTaskScreen(
                  updateTaskList: _updateTaskList(),
                  task: task,
                ),
              ),
            ),
          ),
          Divider(),
        ],
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        backgroundColor: Theme.of(context).primaryColor,
        child: Icon(Icons.add),
        onPressed: () => Navigator.push(
          context,
          MaterialPageRoute(
            builder: (_) => AddTaskScreen(
              updateTaskList: _updateTaskList(),
            ),
          ),
        ),
      ),
      body: FutureBuilder(
        future: _taskList,
        builder: (context, AsyncSnapshot<List<Task>> snapshot) {
          // if (!snapshot.hasData) {
          //   return Center(
          //     child: CircularProgressIndicator(),
          //   );
          // }
          final int completedTaskCount = snapshot.data!
              .where((Task task) => task.status == 1)
              .toList()
              .length;

          return ListView.builder(
            padding: EdgeInsets.symmetric(
              vertical: 80,
            ),
            itemCount: 1 + snapshot.data!.length,
            itemBuilder: (BuildContext context, int index) {
              if (index == 0) {
                return Padding(
                  padding: EdgeInsets.symmetric(horizontal: 40, vertical: 20),
                  child: Column(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: [
                      Text(
                        'My Tasks',
                        style: TextStyle(
                          color: Colors.black,
                          fontSize: 40,
                          fontWeight: FontWeight.bold,
                        ),
                      ),
                      SizedBox(
                        height: 10,
                      ),
                      Text(
                        '$completedTaskCount of ${snapshot.data!.length}',
                        style: TextStyle(
                          color: Colors.grey,
                          fontSize: 20,
                          fontWeight: FontWeight.w600,
                        ),
                      ),
                    ],
                  ),
                );
              }
              return _buildTask(snapshot.data![index - 1]);
            },
          );
        },
      ),
    );
  }
}

Answer 1

FutureBuilder设计的方式是Future从您那里获取一个对象，并在其状态发生变化时Widget使用该builder函数构建您的对象。Future

由于Futures 是异步的，因此您的数据不会立即可用。这就是它可能崩溃的地方。

您使用了，snapshot.data!.where没有检查数据是否实际存在，并且由于snapshot.data在 Future 完成之前将为空，因此您将遇到此错误。

取消注释此代码，因为这是执行检查数据是否实际存在的代码。

// if (!snapshot.hasData) {
//   return Center(
//     child: CircularProgressIndicator(),
//   );
// }