snapshot.data 是 Flutter 中的 null

Question

My snapshot.data is null. When I print the response it is displaying the retrieved data. But still snapshot.data is null.

    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

You should use the format given in the documentation for FutureBuilder . You're not checking for the state of the future, so when the FutureBuilder is first built, it will display "no data" . You haven't implemented your else branch, so by the time you have data, your build will probably not refresh anyway. Try this code instead:

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

with Flutter 2.2, this one returns an error

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,);

but this one dosen't

builder: (BuildContext context, AsyncSnapshot snapshot) {

When similar things happen, take the type "var" not "String" or other non-nullable type. (If it was not Flutter, the compilers will do?)

Answer 3

Since i cannot see your complete code, i am assuming you are parsing your json data incorrectly after receiving it inside FutureBuilder. Below is an example which is similar to what you are doing. This example retrieves Date json data and displays using FutureBuilder,

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;
  }
}

Test screenshot:

Hope this helps.

Answer 4

Because your async function doesnt return anything.. Change it like this:

 Future _getUsers() async {
         
        return 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;
       }  
    
       }