我的未来构建器在屏幕上给出了几秒钟的错误，然后显示结果，这里是错误日志

Question

下面是我在IDE上拥有的错误日志。

在1,172 in中重新启动应用程序。I/flutter (24036)：由小部件库╞═══════════════════════════════════════════════════════════I/flutter (24036)捕获的══╡异常: NoSuchMethodError抛出构建FutureBuilder(脏，状态:I/颤振(24036)：_FutureBuilderState#bf7da)：i/颤振(24036)：getter 'length‘被调用为null。I/颤振(24036)：接收器:空I/颤振(24036)：尝试调用:长度I/颤振(24036)：I/颤振(24036)：相关的导致错误的部件是：[38；5；248 53/颤振(24036)：FutureBuilder[39;49m I/颤振(24036)：I/颤振(24036)：当异常被抛出时，这是堆栈：[38；5；244mI/颤振(24036)：#0 Object.noSuchMethod (dart:core-Object.noSuchMethod/object_patch.dart:53:5)][39；49m [38;5；248 38/颤振(24036)：#1 _JsonDataState.build.][39；49m[38；5；244 38/颤振(24036)：#2 _FutureBuilderState.build[39;49m [38；5；5；244 38/颤振(24036)]：#3 StatefulElement.build[39;49m；5；244 38/颤振(24036)：#4 ComponentElement.performRebuild[39;49m [38；5；244 38/248mI(24036)]：#5 StatefulElement.performRebuild[39;49m [38；38；5]；244mI/颤振(24036)：#6 Element.rebuild[39;49m [38；5；244 38/颤振(24036)]：#7 ComponentElement._firstBuild[39;49m [38；5；244 38/颤振(24036)：#8 StatefulElement._firstBuild[39;49m [38；5；5；244 38/颤振(24036)]：#9 ComponentElement.mount[39;49m I/颤振(24036)]：.标准元件安装(22帧)[38；5；244 32/颤振(24036)：#31 Element.inflateWidget[39;49m [38；5；244 32/颤振(24036)：#32 MultiChildRenderObjectElement.mount[39;49m I/颤振(24036)：.]普通元件安装(193个框架)[38；5；244 39/颤振(24036)：#225 Element.inflateWidget[39;49m [38；5；244 39/颤振(24036)：#226 MultiChildRenderObjectElement.mount[39;49m I/颤振(24036)：.]法向元件安装(253帧) [38;5；244 39/颤振(24036)：#479 Element.inflateWidget[39;49m [38；5；5；244 39/颤振(24036)：#480 Element.updateChild][39；49m[38；5；244 39/颤振(24036)]：#481 RenderObjectToWidgetElement._rebuild[39;49m [38；5；244 39/颤振(24036)]：#482 RenderObjectToWidgetElement.mount[39;49m [38；5；244 39/颤振(24036)]：#483 Element.updateChild[38；5；244 39/颤振(24036)：#484 RenderObjectToWidgetAdapter.attachToRenderTree[39;49m [39；49m[38；5；244 39/颤振(24036)]：#485 BuildOwner.buildScope[38；5；244 39/颤振(24036)：#486 WidgetsBinding.attachRootWidget[39;49m [38；5；244 39/颤振(24036)]：#487 WidgetsBinding.scheduleAttachRootWidget。49米I/颤振(24036)：(从_RawReceivePortImpl类，_Timer类，dart:异步，11帧和飞镖:异步补丁)I/颤振(24036)：I/颤振(24036)：════════════════════════════════════════════════════════════════════════════════════════════════════

[38；5；248 m NoSuchMethodError异常被小部件库═══════════════════════════════════捕获[39；49m[38；5；244 m]下面的════════被抛出构建FutureBuilder(脏，状态：

_FutureBuilderState#bf7da):[39;49m，getter 'length‘被调用为null。接收方:空尝试调用:长度

[38;5；244 m]相关的导致错误的小部件是[39；49m[38；5；248 248mFutureBuilder][39；49m[38；5；244 m]当异常抛出时，这是堆栈[39；49m] [38;5;244m#0 Object.noSuchMethod _JsonDataState.build [38;5;248m#1 _JsonDataState.build]。[39；49m]38；5；244m#2 _FutureBuilderState.build[39;49m [38；5]；[38;5;248m════════════════════════════════════════════════════════════════════════════════[39;49m 244m#3 StatefulElement.build[39;49m ] [38;5;244m#4 ComponentElement.performRebuild][38；5；244米][39；49m]

下面是运行的代码。该应用程序正在运行，但在启动时出现了错误和几秒钟的红色屏幕。

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

class JsonData extends StatefulWidget {
  @override
  _JsonDataState createState() => _JsonDataState();
}

class _JsonDataState extends State<JsonData> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home Page Data Visualization'),
      ),
      body: Center(
        child: FutureBuilder(
            builder: (context, snapshot) {
              var myData = json.decode(snapshot.data.toString());
              return ListView.builder(
                itemBuilder: (BuildContext context, int index) {
                  return Card(
                    child: ListTile(
                      leading: CircleAvatar(
                          child: Text(myData[index]['trade_code'][0])),
                      title: Text(myData[index]['trade_code']),
                      subtitle: Column(
                        crossAxisAlignment: CrossAxisAlignment.stretch,
                        children: <Widget>[
                          Text(
                            "\nHigh:" + myData[index]['high'],
                            style: TextStyle(fontWeight: FontWeight.bold),
                            textAlign: TextAlign.left,
                          ),
                          Text(
                            "Low:" + myData[index]['low'],
                            style: TextStyle(fontWeight: FontWeight.bold),
                            textAlign: TextAlign.left,
                          ),
                          Text(
                            "Open:" + myData[index]['open'],
                            style: TextStyle(fontWeight: FontWeight.bold),
                            textAlign: TextAlign.left,
                          ),
                          Text(
                            "Close:" + myData[index]['close'],
                            style: TextStyle(fontWeight: FontWeight.bold),
                            textAlign: TextAlign.left,
                          ),
                          Text(
                            "Volume:" + myData[index]['volume'],
                            style: TextStyle(fontWeight: FontWeight.bold),
                            textAlign: TextAlign.left,
                          ),
                          Text(
                            "\nDate:" + myData[index]['date'],
                            style: TextStyle(fontWeight: FontWeight.bold),
                            textAlign: TextAlign.center,
                          ),
                        ],
                      ),
                      isThreeLine: true,
                    ),
                    elevation: 3,
                  );
                },
                itemCount: myData.length,
              );
            },
            future: DefaultAssetBundle.of(context)
                .loadString("assets/stock_market_data.json")),
      ),
    );
  }
}
`

Answer 1

构建器函数执行几次。首先，没有数据。稍后，用数据。

但是，通过这样做，您可以假设它总是有数据：

              var myData = json.decode(snapshot.data.toString());

在这里，snapshot.data最初是空的。这就是为什么你得到了一个例外。

只需遵循文档中的模式即可。https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html

        if (snapshot.hasData) {
          var myData = json.decode(snapshot.data.toString());
        } else if (snapshot.hasError) {
          // show error widget
        } else {
          // show loading widget
        }

我最近谈到的一些具体例子：

https://youtu.be/p8dY_tsAbx8?t=5382

https://youtu.be/p8dY_tsAbx8?t=5633

而且，这种用法也不好，每次执行构建函数时，您都要创建一个未来：

            future: DefaultAssetBundle.of(context)
                .loadString("assets/stock_market_data.json")),

相反，您应该将未来缓存在iniState中。在这里解释：https://youtu.be/p8dY_tsAbx8?t=5683