问题描述
这是我在IDE上遇到的错误日志。
Restarted application in 1,172ms. I/flutter (24036): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════ I/flutter (24036): The following NoSuchMethodError was thrown building FutureBuilder<String>(dirty,state: I/flutter (24036): _FutureBuilderState<String>#bf7da): I/flutter (24036): The getter 'length' was called on null. I/flutter (24036): Receiver: null I/flutter (24036): Tried calling: length I/flutter (24036): I/flutter (24036): The relevant error-causing widget was: [38;5;248mI/flutter (24036): FutureBuilder<String>[39;49m I/flutter (24036): I/flutter (24036): When the exception was thrown,this was the stack: [38;5;244mI/flutter (24036): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)[39;49m [38;5;248mI/flutter (24036): #1 _JsonDataState.build.<anonymous closure>[39;49m [38;5;244mI/flutter (24036): #2 _FutureBuilderState.build[39;49m [38;5;244mI/flutter (24036): #3 StatefulElement.build[39;49m [38;5;244mI/flutter (24036): #4 ComponentElement.performRebuild[39;49m [38;5;244mI/flutter (24036): #5 StatefulElement.performRebuild[39;49m [38;5;244mI/flutter (24036): #6 Element.rebuild[39;49m [38;5;244mI/flutter (24036): #7 ComponentElement._firstBuild[39;49m [38;5;244mI/flutter (24036): #8 StatefulElement._firstBuild[39;49m [38;5;244mI/flutter (24036): #9 ComponentElement.mount[39;49m I/flutter (24036): ... Normal element mounting (22 frames) [38;5;244mI/flutter (24036): #31 Element.inflateWidget[39;49m [38;5;244mI/flutter (24036): #32 MultiChildRenderObjectElement.mount[39;49m I/flutter (24036): ... Normal element mounting (193 frames) [38;5;244mI/flutter (24036): #225 Element.inflateWidget[39;49m [38;5;244mI/flutter (24036): #226 MultiChildRenderObjectElement.mount[39;49m I/flutter (24036): ... Normal element mounting (253 frames) [38;5;244mI/flutter (24036): #479 Element.inflateWidget[39;49m [38;5;244mI/flutter (24036): #480 Element.updateChild[39;49m [38;5;244mI/flutter (24036): #481 RenderObjectToWidgetElement._rebuild[39;49m [38;5;244mI/flutter (24036): #482 RenderObjectToWidgetElement.mount[39;49m [38;5;244mI/flutter (24036): #483 RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous关闭> [39; 49m [38; 5; 244mI / flutter（24036）：＃484 BuildOwner.buildScope [39; 49m [38; 5; 244mI / flutter（24036）：＃485 RenderObjectToWidgetAdapter.attachToRenderTree [39; 49m [38; 5; 244mI / flutter（24036）：＃486 WidgetsBinding.attachRootWidget [39; 49m [38; 5; 244mI / flutter（24036）：＃487 WidgetsBinding.scheduleAttachRootWidget。[39; 49m I / flutter（24036）：（从_RawReceivePortImpl类，_Timer类，dart：async和dart：async-patch中删除了11个帧） I /颤振（24036）： I /颤振（24036）：═══════════════════════════════════════════ ══════════════════════════════════════════════════ ═══════
[38;5;248m════════ Exception caught by widgets library ═══════════════════════════════════[39;49m [38;5;244mThe following NoSuchMethodError was thrown building FutureBuilder<String>(dirty,state:_FutureBuilderState＃bf7da）：[39; 49m getter'length'在null上被调用。 接收者：null 尝试致电：长度
[38;5;244mThe relevant error-causing widget was[39;49m [38;5;248mFutureBuilder<String>[39;49m [38;5;244mWhen the exception was thrown,this was the stack[39;49m [38;5;244m#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)[39;49m [38;5;248m#1 _JsonDataState.build.<anonymous closure>[39;49m [38;5;244m#2 _FutureBuilderState.build[39;49m [38;5;244m#3 StatefulElement.build[39;49m [38;5;244m#4 ComponentElement.performRebuild[39;49m [38;5;244m...[39;49m [38;5;248m════════════════════════════════════════════════════════════════════════════════[39;49m
这是正在运行的代码。该应用程序正在运行，但在启动时出现错误和红屏几秒钟。
`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'],Text(
"Open:" + myData[index]['open'],Text(
"Close:" + myData[index]['close'],Text(
"Volume:" + myData[index]['volume'],Text(
"\nDate:" + myData[index]['date'],textAlign: TextAlign.center,],isThreeLine: true,elevation: 3,);
},itemCount: myData.length,);
},future: DefaultAssetBundle.of(context)
.loadString("assets/stock_market_data.json")),);
}
}
`
解决方法
您的构建器函数执行两次。首先，没有数据。后来，有了数据。
但是，您通过执行以下操作假定它始终有数据：
var myData = json.decode(snapshot.data.toString());
此处，snapshot.data最初为空。这就是为什么您要例外。
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
}
我最近的一次演讲中的一些具体例子：
https://youtu.be/p8dY_tsAbx8?t=5382
https://youtu.be/p8dY_tsAbx8?t=5633
此外，这种用法也不是很好，您每次构建函数执行时都在创建一个未来：
future: DefaultAssetBundle.of(context)
.loadString("assets/stock_market_data.json")),相反，您应该将将来保存在iniState中。在这里解释：https://youtu.be/p8dY_tsAbx8?t=5683
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