Flutter：在 null 上调用了方法“[]”

Posted 2023-02-23

【中文标题】Flutter：在 null 上调用了方法“[]”

【英文标题】：Flutter: The method '[ ]' was called on null

【发布时间】：2020-11-06 18:52:02

【问题描述】：

我收到以下代码的以下错误。 https://github.com/Avi-nash5/CP-flutter-app/blob/master/lib/PlatformScreen.dart 错误屏幕显示 1 秒，然后按预期加载。请帮忙。 error how it is supposed to be

【问题讨论】：

您能否发布导致错误的相关代码而不是整个文件，因为如果不运行代码就更难调试。错误可能与 data[index]['name'] 相关，因为您在 Text 小部件中调用它，并且它可能在开始时为空，因此您需要执行空检查。

【参考方案1】：

您可以复制粘贴PlatformScreen的完整代码data在使用时未准备好 你可以使用 bool _isLoading 来控制，如果data 没有准备好显示CircularProgressIndicator

bool _isLoading = true;
...
Future<String> getJsonData() async 
    ...
    setState(() 
      _isLoading = false;
    );
... 
child: _isLoading
      ? Center(child: CircularProgressIndicator(backgroundColor: Colors.white,))
      : ListView.builder(   

工作演示

PlatformScreen的完整代码

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

class PlatformScreen extends StatefulWidget 
  final Platform platform;

  PlatformScreen(this.platform);

  @override
  PlatformScreenState createState() => PlatformScreenState();


class PlatformScreenState extends State<PlatformScreen> 
  List data;
  int n;
  bool _isLoading = true;

  @override
  void initState() 
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_)
      getJsonData();
    );
  

  Future<String> getJsonData() async 
    var response = await http.get(Uri.encodeFull(widget.platform.url),
        headers: "Accept": "application/json");
    print(response.body);

    data = jsonDecode(response.body);

    n = data.length;
    print(data);
    convert();
    setState(() 
      _isLoading = false;
    );
    return "Success";
  

  void convert() 
    print(n);
    var now = new DateTime.now();
    now = now.toLocal();
    for (int i = 0; i < n; i++) 
      var start = DateTime.parse(data[i]['start_time']);
      var end = DateTime.parse(data[i]['end_time']);
      start = start.toLocal();
      end = end.toLocal();
      data[i]['start_time'] = start.toString();
      data[i]['end_time'] = end.toString();
    
    print(data);
  

  @override
  Widget build(BuildContext context) 
    return Scaffold(
      backgroundColor: Theme.of(context).accentColor,
      body: Column(
        children: <Widget>[
          Expanded(
            child: Container(
              child: Column(
                children: <Widget>[
                  Padding(
                    padding: const EdgeInsets.only(
                      top: 25.0,
                    ),
                    child: Container(
                      margin: EdgeInsets.all(10.0),
                      height: 125,
                      decoration: BoxDecoration(
                        color: Theme.of(context).primaryColor,
                        borderRadius: BorderRadius.circular(30.0),
                      ),
                      child: Stack(
                        children: <Widget>[
                          Hero(
                            tag: widget.platform.imgurl,
                            child: ClipRRect(
                              borderRadius: BorderRadius.circular(30.0),
                              child: Image(
                                height: 125,
                                width: 125,
                                image: AssetImage(widget.platform.imgurl),
                                fit: BoxFit.cover,
                              ),
                            ),
                          ),
                          Padding(
                            padding: const EdgeInsets.fromLTRB(125, 0, 0, 0),
                            child: Container(
                              child: Center(
                                child: Text(
                                  widget.platform.name,
                                  style: TextStyle(
                                    color: Colors.white,
                                    fontSize: 32,
                                  ),
                                ),
                              ),
                            ),
                          ),
                        ],
                      ),
                    ),
                  ),
                  Expanded(
                    child: Container(
                      decoration: BoxDecoration(
                        color: Theme.of(context).accentColor,
                      ),
                      child: _isLoading
                          ? Center(child: CircularProgressIndicator(backgroundColor: Colors.white,))
                          : ListView.builder(
                              scrollDirection: Axis.vertical,
                              itemCount: n,
                              itemBuilder: (BuildContext context, int index) 
                                return Padding(
                                  padding: const EdgeInsets.fromLTRB(
                                      15.0, 15.0, 10.0, 10.0),
                                  child: Container(
                                    height: 150,
                                    decoration: BoxDecoration(
                                        color: Theme.of(context).accentColor,
                                        borderRadius:
                                            BorderRadius.circular(20.0),
                                        boxShadow: [
                                          BoxShadow(
                                            color: Colors.black54,
                                            offset: Offset(0, 10.0),
                                            blurRadius: 10.0,
                                          )
                                        ]),
                                    child: Column(
                                      children: <Widget>[
                                        Center(
                                          child: Text(
                                            data[index]['name'],
                                            style: TextStyle(
                                              color: Colors.white,
                                              fontSize: 18,
                                            ),
                                          ),
                                        ),
                                      ],
                                    ),
                                  ),
                                );
                              ),
                    ),
                  ),
                ],
              ),
            ),
          ),
        ],
      ),
    );