对于我的初学者问题，我很抱歉，我正在尝试自学 Flutter，所以我正在做一些来自 youtube 的教程，但不知何故，我得到了 Invalid argument(s) 错误，即使我输入的完全一样它在教程中，我尝试通过打印进行调试，首先打印 null，然后打印带有 json 对象的列表，但是当我打印 mydata[0] 时，它打印 The method ' []' 在 null 时被调用。。

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

void main() {
  runApp(new MaterialApp(
    home: new Home(),
    theme: ThemeData(
      primarySwatch: Colors.indigo,
    ),
  ));
}

class Home extends StatefulWidget {
  @override
  HomeState createState() => new HomeState();
}

class HomeState extends State<Home> {
  List data;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(
        title: Text("Some App"),
      ),
      body: Container(
        child: Center(
          child: FutureBuilder(
            future: DefaultAssetBundle.of(context).loadString('assets/data.json'),
            builder: (context, snapshot) {
              var mydata = json.decode(snapshot.data.toString());
              print(mydata);
              return ListView.builder(
                itemBuilder: (BuildContext context, int index) {
                  return Card(
                    child: Column(
                      crossAxisAlignment: CrossAxisAlignment.stretch,
                      children: <Widget>[
                        Text("Name " + mydata[index]['Name']),
                        Text("Age " + mydata[index]['Age']),
                        Text("Gender " + mydata[index]['Gender']),
                      ],
                    ),
                  );
                },
                itemCount: mydata == null ? 0 : mydata.length,
              );
            },
          ),
        ),
      ),
    );
  }
}

最佳答案

首先，不要忘记pubspec.yaml:

  assets:
   - assets/data.json

这是json:

[
  {
    "Name": "John Doe",
    "Age": "30",
    "Gender": "Male"
  },
  {
    "Name": "Jane Doe",
    "Age": "25",
    "Gender": "Female"
  }
]

您需要在 FutureBuilder 的构建器方法中检查 Future 是否已交付。喜欢:

这是构建方法:

class HomeState extends State<Home> {
  List data;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(
        title: Text("Some App"),
      ),
      body: Container(
        child: Center(
          child: FutureBuilder(
            future:
                DefaultAssetBundle.of(context).loadString('assets/data.json'),
            builder: (context, snapshot) {
              if(!snapshot.hasData) { /// CRITICAL POINT
                return CircularProgressIndicator();
              }
              var myData = json.decode(snapshot.data);
              print(myData);
              return ListView.builder(
                itemBuilder: (BuildContext context, int index) {
                  return Card(
                    child: Column(
                      crossAxisAlignment: CrossAxisAlignment.stretch,
                      children: <Widget>[
                        Text("Name " + myData[index]['Name']),
                        Text("Age " + myData[index]['Age']),
                        Text("Gender " + myData[index]['Gender']),
                      ],
                    ),
                  );
                },
                itemCount: myData == null ? 0 : myData.length,
              );
            },
          ),
        ),
      ),
    );
  }
}

它的工作，但我有更好的解决方案，检查它:

Load and read data from Json file

关于Flutter:加载本地 json 文件返回 null，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/56202645/