将 FutureBuilder 与 Local JSON 用作 Future 时出错

Question

我正在尝试加载一些 json 数据并使用 futurebuilder 等待数据首先可用然后显示它。当我尝试在 DetailsView 中加载数据时，我收到两条日志消息说没有数据和 null，屏幕出现错误，然后 returns 恢复正常并记录 json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.

示例代码：

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：

我没有妥善处理未来
我没有正确解析 json，在下面附上了它的副本。

示例数据：

[{
    "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"
}]

我正在尝试通过使用字符串化数字作为键来解析它，以访问值。如何在 Futurebuilder 中为这个 or/and 制作模型 class 解析这个 json？谢谢。

----------------------------编辑------------ -------------------------- 这是经过编辑的构建器方法，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，它出错并显示以下错误消息：

════════ 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，但由于某种原因无法将其设置为文本小部件？谢谢。

Answer 1

创建一个单独的 Future 方法（在此处添加所有数据处理）并从 Futurebuilder 中调用它。这可能是导致问题的原因。还要检查 API 是否正常工作，使用 Postman

Answer 2

您在 if 条件之外调用 data。像这样将其包装在else条件中

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 wont be logged to console anymore
    final parsedData = json.decode(data);
    print('pd ${parsedData[1]}');
}

Answer 3

您正在尝试解码未加载的 JSON。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();

解法：

if(!snapshot.hasData) {
  return LoadingStrengths(); // return a loader when there is not data
} else {
  // Do stuff when it is loaded;
}

Answer 4

这是有问题的：

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

这需要在变量中。想象一下这个 build() 被调用了 30 次。 .loadString 会被调用 30 次，你的 Future 永远不会完成！

始终将您的 Future 放入一个简单的 var 中，并在您的 FutureBuilder 中引用该简单的 var。不要在那里放一个动作。

将 FutureBuilder 与 Local JSON 用作 Future 时出错

Error Using FutureBuilder With Local JSON as Future

json

flutter

assets

flutter-futurebuilder