我正在尝试加载一些json数据，并已使用afuturebuilder等待数据首先可用，然后显示它。当我尝试将数据加载到时DetailsView，我收到两条日志消息，说没有数据且为null，屏幕错误消失，之后又恢复正常，并且json被记录到控制台。
抛出的异常是The following NoSuchMethodError was thrown building FutureBuilder<String>(dirty, state: _FutureBuilderState<String>#cc31b): The method '[]' was called on null. Receiver: null。
样例代码：
body: Container(
margin: const EdgeInsets.symmetric(horizontal: 8.0, vertical: 15.0),
child: FutureBuilder(
future: DefaultAssetBundle.of(context).loadString("assets/my_data.json"), // validly referenced in pubspec and in right folder
builder: (context, snapshot){
if(!snapshot.hasData){
print('no data'); // I understand it will be empty for now
} else {
print('hd');
}
var data = snapshot.data.toString();
print(data); // null logged to console, is the future still empty at this point?
final parsedData = json.decode(data);
print('pd ${parsedData[1]}'); // this line is where it shows the method [] was called on null
return data == null || data.isEmpty
? LoadingStrengths()
: MyDataText();
// : DailyStrengthText(day: widget.day, data: parsedData);
},
),
),
问题似乎是：
样本数据：
[{
"1": "Each day is an opportunity to start over again"
},
{
"2": "Every man must at one point be tempted to hoist the black"
},
{
"3": "Be kind to everyone you meet"
}]
我试图通过使用字符串化的数字作为键来解析它，以访问值。我如何为此或/和在futurebuilder中解析此json制作模型类？谢谢。
-------------------------------编辑------------------ ----------------------这是经过编辑的生成器方法，我可以从中解析并打印出数据：
if(!snapshot.hasData){
print('no data yet');
return LoadingStrengths();
} else {
print('hd');
var data = snapshot.data.toString();
parsedData = json.decode(data);
print('pd ${parsedData[widget.day - 1]}');
}
return Text('${data[widget.day - 1]}'); // it errors out here
当我将值分配给“文本”小部件时，它会显示以下错误消息：
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building FutureBuilder<String>(dirty, state: _FutureBuilderState<String>#cc19b):
The method '[]' was called on null.
Receiver: null
Tried calling: [](4)
The relevant error-causing widget was:
FutureBuilder<String> file:///E:/FlutterProjects/daily_strength/lib/screens/DetailsView.dart:30:18
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1 _DetailsViewState.build.<anonymous closure>
(package:daily_strength/screens/DetailsView.dart:47:34)
#2 _FutureBuilderState.build
(package:flutter/src/widgets/async.dart:751:55)
#3 StatefulElement.build
(package:flutter/src/widgets/framework.dart:4744:28)
#4 ComponentElement.performRebuild
(package:flutter/src/widgets/framework.dart:4627:15)
请问这是什么意思？我正在获取数据，但是由于某种原因将其设置为文本小部件不起作用？谢谢。
您正在尝试在未加载JSON时对其进行解码。添加了一些注释以进行解释。
if(!snapshot.hasData) {
// Problem is here, Data has not arrived yet. You are just printing there is
// no data, but still moving on the next step
print('no data');
} else {
print('hd');
}
// data is null right now, toString() just return 'null' in string form
var data = snapshot.data.toString();
// null logged to console, is the future still empty at this point? yes exactly
print(data);
// 'null' is parsed has null is returned
final parsedData = json.decode(data);
// parsedData is null, you can't access using [], when it's null, that why exception is thrown
print('pd ${parsedData[1]}');
return data == null || data.isEmpty
? LoadingStrengths()
: MyDataText();
解：
if(!snapshot.hasData) {
return LoadingStrengths(); // return a loader when there is not data
} else {
// Do stuff when it is loaded;
}
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