RangeError（索引）：无效值：有效值范围为空：0（http请求）

Question

我是初学者，我的http请求有问题，当我想在我的DropdownButton中添加此http请求（我将其放在名为deviceGet的列表中）时，甚至打印它以查看它：我有此错误：

RangeError(index) : Invalid value : Valid value range is empty : 0

这是所有错误消息：

 Exception caught by widgets library ═══════════════════════════════════
The following RangeError was thrown building Builder:
RangeError (index): Invalid value: Valid value range is empty: 0

The relevant error-causing widget was
    MaterialApp 
package:chat/main.dart:180
When the exception was thrown, this was the stack
#0      List.[]  (dart:core-patch/growable_array.dart:149:60)
#1      _MyListScreenState.initState 
package:chat/main.dart:226
#2      StatefulElement._firstBuild 
package:flutter/…/widgets/framework.dart:4428
#3      ComponentElement.mount 
package:flutter/…/widgets/framework.dart:4274
#4      Element.inflateWidget 
package:flutter/…/widgets/framework.dart:3269
...
════════════════════════════════════════════════════════════════════════════════
Reloaded 1 of 591 libraries in 363ms.
flutter: 0

════════ Exception caught by widgets library ═══════════════════════════════════
The following RangeError was thrown building Builder:
RangeError (index): Invalid value: Valid value range is empty: 0

The relevant error-causing widget was
    MaterialApp 
package:chat/main.dart:180
When the exception was thrown, this was the stack
#0      List.[]  (dart:core-patch/growable_array.dart:149:60)
#1      _MyListScreenState.initState 
package:chat/main.dart:225
#2      StatefulElement._firstBuild 
package:flutter/…/widgets/framework.dart:4428
#3      ComponentElement.mount 
package:flutter/…/widgets/framework.dart:4274
#4      Element.inflateWidget 

我不明白此错误，因为如果我把它放在体内，我就能看到我的要求

ListView.builder(
        itemCount: deviceGet.length,
        itemBuilder: (context, index) {
        return ListTile(
                title: Text("Num $index " + device[index].commands[0].id));
         title: Text("Num $index " + deviceGet[index].name));

或者如果我放

Text(deviceGet[0].name)

但有警告

这是我的代码：

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

import 'Get.dart';
import 'Device.dart';
import 'Commands.dart';
import 'deviceData.dart';
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  build(context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'My App',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyListScreen(),
    );
  }
}

class MyListScreen extends StatefulWidget {
  @override
  createState() => _MyListScreenState();
}

class _MyListScreenState extends State {
  //
  List<deviceData> dataDevice = deviceData.getDevicesData();

  List<DropdownMenuItem<deviceData>> listDropDevice;
  deviceData deviceSelection;
  //

  var deviceGet = new List<Device>();

  GetDevice() {
    GET.getDevice().then((response) {
      setState(() {
        Iterable list = json.decode(response.body);
        deviceGet = list.map((model) => Device.fromJson(model)).toList();
      });
    });
  }

  void initState() {
    super.initState();
    GetDevice();

    //print(deviceGet[0].name);

    // add in the dropDown
    for (int i = 0; i < 5;) {
      print(i);
      dataDevice.add(deviceData("Device" + i.toString()));
      dataDevice.add(deviceData(deviceGet[0].name));
      i = i + 1;
    }

    listDropDevice = buildDropdownMenuItems(dataDevice);
    deviceSelection = listDropDevice[0].value;
  }

  List<DropdownMenuItem<deviceData>> buildDropdownMenuItems(List devices) {
    List<DropdownMenuItem<deviceData>> items = List();
    for (deviceData device1 in devices) {
      items.add(
        DropdownMenuItem(
          value: device1,
          child: Text(device1.name),
        ),
      );
    }
    return items;
  }

  onChange(deviceData selectionUtilisateur) {
    setState(() {
      deviceSelection = selectionUtilisateur;
    });
  }

  @override
  build(context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Device List"),
        ),
        body: Column(children: <Widget>[
          //Text(deviceGet[0].name),
          DropdownButton(
            value: deviceSelection,
            items: listDropDevice,
            onChanged: onChange,
          ),
        ]));
  }
}

谢谢您的帮助

Answer 1

GetDevice（）是异步的方法，因此从json获取数据需要花费时间，并且在请求中[[deviceGet为空，因此会产生范围错误。

您可以使用await和async *使GetDevice（）方法异步，并在调用该方法时使用.then方法，然后访问值。

此外，请确保您正在GetDevice方法中获取数据。