Question

我的snapshot.data为空。当我打印响应时，它显示检索到的数据。但是快照.data仍然为空。请帮忙。

    Future _getUsers() async {

        var data = await http.post("http://10.0.2.2/Flutter/abreport.php", body: {
        {
          "date": mydt,
         });
         var jsonData = json.decode(data.body); //edited
         print(jsonData); // the data is printing here

         return jsonData;
       }  

       }  

    FutureBuilder(
            future: _getUsers(),
            builder: (BuildContext context, AsyncSnapshot snapshot) {
                  debugPrint(snapshot.data);
              if (snapshot.data == null) {
                return Container(
                  child: Center(

                    child:Text("no data"),
                    )
                    );
              } else {
                 //some code
             }

              )

Answer 1

您应该使用FutureBuilder文档中提供的格式。您无需检查未来的状态，因此在首次构建FutureBuilder时，它将显示"no data"。您尚未实现else分支，因此，在拥有数据时，您的构建可能仍将无法刷新。请尝试以下代码：

FutureBuilder(
  future: _getUsers(),
  builder: (BuildContext context, AsyncSnapshot snapshot) {
    if (snapshot.connectionState == ConnectionState.done) {
      if (snapshot.data == null) {
        return Text('no data');
      } else {
        return Text('data present');
      }
    } else if (snapshot.connectionState == ConnectionState.error) {
      return Text('Error'); // error
    } else {
      return CircularProgressIndicator(); // loading
    }
  }
)

Answer 2

由于我看不到完整的代码，因此我假设您在FutureBuilder中接收到json数据后，错误地解析了它。以下是与您正在执行的操作类似的示例。此示例检索Date json数据并使用FutureBuilder显示，

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() {
  runApp(new MyApp());
}

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() => MyAppState();
}

class MyAppState extends State<MyApp> {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
            body: Center(
                child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: [
                      Padding(
                          padding: const EdgeInsets.only(left: 10.0, right: 10.0),
                          child: FutureBuilder(
                            future: _getDate(),
                            builder: (BuildContext context, AsyncSnapshot snapshot) {
                              if (snapshot.hasData) {
                                return Text('Date: ' + snapshot.data['date']
                                    + '\nMilliseconds Since Epoch: ' + snapshot.data['milliseconds_since_epoch'].toString()
                                + '\nTime: ' + snapshot.data['time'],
                                style: TextStyle(fontSize: 18.0, fontWeight: FontWeight.bold, color: Colors.grey));
                              } else {
                                return Center(child: CircularProgressIndicator());
                              }
                            },
                          ))
        ]))));
  }

  Future _getDate() async {
    var data = await http.post("http://date.jsontest.com/");
    var jsonData = json.decode(data.body);
    print(jsonData);

    return jsonData;
  }
}

测试屏幕截图：

希望这会有所帮助。

Answer 3

使用 Flutter 2.2，这个返回错误

builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
  if (snapshot.hasData) {
    return Text(snapshot.data,);

Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't.
return Text(snapshot.data,);

但是这个没有

builder: (BuildContext context, AsyncSnapshot snapshot) {

当类似的事情发生时，取类型“var”而不是“String”或其他不可为空的类型。（如果不是 Flutter，编译器会做？）

flutter中的snapshot.data为null

3 个答案: