flutter：RangeError（索引）：無效值：有效值范圍為空：

Question

class PictureBox extends StatefulWidget {
  int num;
  PictureBox({Key key, this.num}): super(key:key);

  @override
  _PictureBoxState createState() => _PictureBoxState();
}

var _writeprofile = writeprofileState();

class _PictureBoxState extends State<PictureBox> {
  @override
  void initState() {
    super.initState();
  }


  @override
  Widget build(BuildContext context) {
    return Container(
      child: InkWell(
        onTap: () async {
          print('asddd1');
          var getimage =
              await ImagePicker.pickImage(source: ImageSource.gallery);
          setState(() {
            tmpimage = getimage;
            imageBytes = getimage.readAsBytesSync();
            image64 = base64Encode(imageBytes);
            print(tmp64list.length);
            print('Image Path $tmpimage');
          });
          tmp64list[widget.num] = image64;
        },

'tmp64list[widget.num] = image64;' 從那以后一直是錯誤的。

我想通過列表的索引

像這樣：

PictureBox pictureBox1 = PictureBox(num:1);
PictureBox pictureBox2 = PictureBox(num:2);
PictureBox pictureBox3 = PictureBox(num:3);
PictureBox pictureBox4 = PictureBox(num:4);
PictureBox pictureBox5 = PictureBox(num:5);
PictureBox pictureBox6 = PictureBox(num:6);

但我的意圖不是運行，而是出現錯誤。

錯誤：

Unhandled Exception: RangeError (index): Invalid value: Valid value range is empty: 3
#0      List._setIndexed (dart:core-patch/growable_array.dart:153:72)
#1      List.[]= (dart:core-patch/growable_array.dart:150:5)

我是這么認為的，因為 initState 在有狀態

正確的？

但我無法解決，因為我對 flutter 了解不夠

你能幫助我嗎？

Answer 1

tmp64list 未正確初始化
使用 tmp64list[widget.num] 時，該元素尚不存在
在您的情況下，列表從 0 而不是 1 開始

如果你用 3 個元素初始化

List<String> tmp64list = ['','',''];

你可以使用 0,1,2 而不是 1,2,3
如果想從 1 開始，您可以在小部件的邏輯中執行 num -1 以匹配列表索引

PictureBox(num:0);
PictureBox(num:1);
PictureBox(num:2);

完整的示例代碼

import 'package:flutter/material.dart';
import 'package:image_picker/image_picker.dart';
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 Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            PictureBox(num:1),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

class PictureBox extends StatefulWidget {
  int num;
  PictureBox({Key key, this.num}): super(key:key);

  @override
  _PictureBoxState createState() => _PictureBoxState();
}

List<String> tmp64list = ['','',''];
String image64;

class _PictureBoxState extends State<PictureBox> {
  @override
  Widget build(BuildContext context) {
    return Container(
        height: 100,
        width: 100,
        color: Colors.blue,
        child: InkWell(
        onTap: () async {
      print('asddd1');
      var getimage =
          await ImagePicker.pickImage(source: ImageSource.gallery);
      setState(() {
        var tmpimage = getimage;
        var imageBytes = getimage.readAsBytesSync();
        image64 = base64Encode(imageBytes);
        print(tmp64list.length);
        print('Image Path $tmpimage');
      });
      tmp64list[widget.num] = image64;
      print('${tmp64list[widget.num]}');
    }));
  }
}

如果 tmp64list 正確初始化，則演示圖片打印 base64 字符串。