_TypeError 在构建 FutureBuilder<dynamic>(dirty, state: _FutureBuilderState<dynamic> type 'N

Posted 2023-02-23

【中文标题】_TypeError 在构建 FutureBuilder<dynamic>(dirty, state: _FutureBuilderState<dynamic> type \'Null\' is not a subtype of type \'List<dynamic>\'

【英文标题】：_TypeError was thrown building FutureBuilder<dynamic>(dirty, state: _FutureBuilderState<dynamic> type 'Null' is not a subtype of type 'List<dynamic>'_TypeError 在构建 FutureBuilder<dynamic>(dirty, state: _FutureBuilderState<dynamic> type 'Null' is not a subtype of type 'List<dynamic>'

【发布时间】：2021-09-08 17:21:26

【问题描述】：

# 我已经处理这个问题很长时间了。 我正在尝试解析 JSON 并将其转换为列表视图。 我得到了响应正文，它也被转换为列表，但它向未来的构建器发送 null，我收到此错误： #

小部件库捕获的异常 =========================================== ============== 在构建 FutureBuilder(dirty, state: _FutureBuilderState#a289a) 时引发了以下 _TypeError： “Null”类型不是“List”类型的子类型

The relevant error-causing widget was: 
  FutureBuilder<dynamic> file:///Users/metehanmacbook/StudioProjects/flutter_mysql/lib/main.dart:37:13
When the exception was thrown, this was the stack: 
#0      _MyHomePageState.build.<anonymous closure> (package:flutter_mysql/main.dart:40:14)
#1      _FutureBuilderState.build (package:flutter/src/widgets/async.dart:775:55)
#2      StatefulElement.build (package:flutter/src/widgets/framework.dart:4691:27)
#3      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4574:15)
#4      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4746:11)
...

**# Codes: #**

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';

void main() 
  runApp(MyApp());
  


class MyApp extends StatelessWidget 
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) 
    return MaterialApp(
      title: 'Flutter MySql',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.lime,
      ),
      home: MyHomePage(title: 'Flutter MYSQL'),
    );
  

class MyHomePage extends StatefulWidget 
  MyHomePage(Key? key, required this.title) : super(key: key);
  final String title;
  @override
  _MyHomePageState createState() => _MyHomePageState();

class _MyHomePageState extends State<MyHomePage> 
  @override
  Widget build(BuildContext context) 
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: FutureBuilder(
      future: Getmethod(),
        builder: (BuildContext context, AsyncSnapshot snapshot)
        List snap = snapshot.data;
        if(snapshot.connectionState == ConnectionState.waiting)
          return Center(
            child: CircularProgressIndicator(),
          );
        
        if(snapshot.hasError)
          return Center(
            child: Text('error fatch'),
          );
        

        return ListView.builder(
            itemCount: snap.length,
            itemBuilder: (context, index)
              return ListTile(
                title: Text("Product Name: $snap[index]['pro_name']"),
                subtitle: Text("Product Desc: $snap[index]['pro_desc']"),
              );
          
        );
        ,

      ),

    );
  



  Getmethod()async
    String theurl = ('http://10.0.2.2/flutter-demo/getdata.php');

    var res = await http.get(Uri.parse(Uri.encodeFull(theurl)),headers: "Accept":"application/json");
    var responseBody = json.decode(res.body);
    print(responseBody);
    return responseBody;
  

我该如何解决这个问题？

【参考方案1】：

似乎错误是List snap = snapshot.data;snapshot.data 为空，您想将其转换为列表。

解决方案：

您可以将snap 变量设为nullable: List? snap = snapshot.data; 如果snap返回null，您可以返回默认值（例如空列表）：List snap = snapshot.data ?? []; 调查您的Getmethod 为何返回null；