无法在 Flutter 的 listview.builder 中获取长度

Posted 2023-03-07

【中文标题】无法在 Flutter 的 listview.builder 中获取长度

【英文标题】：Cant get length in listview.builder in Flutter

【发布时间】：2020-04-19 12:16:05

【问题描述】：

我正在学习颤振和飞镖语言，并尝试通过 rxdart 应用 bloc 模式并在页面中显示数据。

无法获取length 或在页面流生成器和listview.builder 中显示数据，因为我收到错误：

class 'Future<List<User>>' has no instance getter 'length'.
Receiver: Instance of 'Future<List<User>>'
Tried calling: length

我的班级user.dart

class User 
 int id;
 String name;
 String username;
 String email;

 User(this.id, this.name, this.username, this.email);

 User.fromJson(Map<String, dynamic> parsedJson) 
   User(
     id: parsedJson['id'],
     name: parsedJson['name'],
     username: parsedJson['username'],
     email: parsedJson['email'],
   );
 

// user_repo.dart

import 'package:learn_flutter_bloc/bloc_rx_example/User.dart';
import 'package:http/http.dart' as http;
import 'dart:async';
import 'dart:convert';

import 'package:learn_flutter_bloc/url.dart';

class UserRepo 
 static Future<List<User>> getUsers() async 
   final res = await http.get("https://jsonplaceholder.typicode.com/users");
   List<User> users = [];
   if (res.statusCode == 200) 
     var data = jsonDecode(res.body);
     for (var user in data) 
       users.add(User.fromJson(user));
     
     return users;
    else 
     throw Exception("Error In Calling APi");
   
 

// 用户块

import 'package:learn_flutter_bloc/bloc_base.dart';
import 'package:learn_flutter_bloc/bloc_rx_example/user_repo.dart';
import 'package:rxdart/subjects.dart';

class UserBloc extends BlocBase 
 Subject _usersSubject = BehaviorSubject();

 Stream get users => _usersSubject.stream;

 getUsers() async 
   var usersData = UserRepo.getUsers();
   _usersSubject.add(usersData);
 

 @override
 dispose() 
   _usersSubject.close();
 


final bloc = UserBloc();

// user_list_app.dart

import 'package:flutter/material.dart';
import 'package:learn_flutter_bloc/bloc_rx_example/user_bloc.dart';

class UserListApp extends StatelessWidget 
 @override
 Widget build(BuildContext context) 
   return MaterialApp(
     debugShowCheckedModeBanner: false,
     title: 'Bloc Rx Example',
     theme: ThemeData(
       primaryColor: Colors.teal,
     ),
     home: Scaffold(
       appBar: AppBar(
         title: Text('Bloc Rx Example'),
         leading: IconButton(
             icon: Icon(Icons.arrow_back),
             onPressed: () 
               Navigator.of(context).pop();
             ),
       ),
       body: UserList(),
     ),
   );
 


class UserList extends StatefulWidget 
 @override
 _UserListState createState() => _UserListState();


class _UserListState extends State<UserList> 
 @override
 void dispose() 
   bloc.dispose();
   super.dispose();
 

 @override
 Widget build(BuildContext context) 
   bloc.getUsers();
   return Padding(
     padding: EdgeInsets.all(10.0),
     child: StreamBuilder(
       stream: bloc.users,
       builder: (context, snapshot) 
         if (snapshot.hasData) 
           return ListView.builder(
               itemCount: snapshot.data.length,
               itemBuilder: (context, index) 
                 return ListTile(
                   leading: Icon(Icons.weekend),
                   title: Text('$snapshot.data[index].name'),
                 );
               );
          else if (!snapshot.hasError) 
           return Text(snapshot.error.toString());
          else 
           return Center(
             child: CircularProgressIndicator(),
           );
         
       ,
     ),
   );
 

【参考方案1】：

当您在 UI 中使用 StreamBuilder 时，您的 getUsers() 正在返回 Future<List<User>> 。从getUsers()返回Stream<<List<User>>。

【讨论】：

感谢 Newaj 先生的回答

【参考方案2】：

修复了我在 bloc_user.dart 中缺少等待的问题

 Future<List<User>> getUsers() async 
    var usersData = UserRepo.getUsers();
    print(usersData);
    _usersSubject.sink.add(usersData);
  

到

 Future<List<User>> getUsers() async 
    var usersData = await UserRepo.getUsers();
    print(usersData);
    _usersSubject.sink.add(usersData);