Question 125652 in Android Studio Question posted in
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Android Studio – Null check operator

Hi guys I’m new to flutter . I was working with a follow along tutorial
and this were working out fine until I ran my code and got a null check operator used on a null value and when I remove the ! the code just stays on a loading mode and doesn’t return the response from thee api?
please advice

below is my main.dart


    import 'package:flutter/material.dart';
    import 'package:c3mobiredo/presentation/LoginScreen.dart';
    
    void main() => runApp(const MyApp());
    
    class MyApp extends StatelessWidget {
      const MyApp({Key? key}) : super(key: key);
    
      @override
      Widget build(BuildContext context) {
        return const MaterialApp(
          title: 'Material App',
          home: Home(),
        );
      }
    }

below is my LoginScreen.dart file


    import 'package:flutter/material.dart';
    import 'package:c3mobiredo/connectivity/apiConfig.dart';
    import 'package:c3mobiredo/connectivity/models/getTimesheetForUserDay.dart';
    
    import '../connectivity/Services/api_service.dart';
    
    class Home extends StatefulWidget {
      const Home({Key? key}) : super(key: key);
    
      @override
      _HomeState createState() => _HomeState();
    }
    
    class _HomeState extends State<Home> {
      late List<UserProjectSpecificDay>? _userModel = [];
      @override
      void initState() {
        super.initState();
        _getData();
      }
    
      void _getData() async {
        _userModel = (await ApiService().GetUserProjectSpecificDay())!;//where I'm getting the error but when I remove the ! the code stays in a loading state which is another issue
        
      }
    
      @override
      Widget build(BuildContext context) {
        return Scaffold(
          appBar: AppBar(
            title: const Text('REST API Example'),
          ),
          body: _userModel == null || _userModel!.isEmpty
              ? const Center(
            child: CircularProgressIndicator(),
          )
              : ListView.builder(
            itemCount: _userModel!.length,
            itemBuilder: (context, index) {
              return Card(
                child: Column(
                  children: [
                    Row(
                      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                      children: [
                        Text(_userModel![index].id.toString()),
                        Text(_userModel![index].project.projectName),
                      ],
                    ),
                    const SizedBox(
                      height: 20.0,
                    ),
                    Row(
                      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                      children: [
                        Text(_userModel![index].hours),
                        Text(_userModel![index].desc),
                      ],
                    ),
                  ],
                ),
              );
            },
          ),
        );
      }
    }
Tags:

2

Answers


  1. 0

    since someone downvote, i update with reference from official documentation:

    https://dart.dev/null-safety/understanding-null-safety

    https://stackoverflow.com/a/69313493/12838877

    when I remove the ! the code just stays on a loading mode
    yes it because your _userModel == null

    and here you set the condition like that

    _userModel == null || _userModel!.isEmpty
              ? const Center(
            child: CircularProgressIndicator(),)

    here need to fix

    • use late means, the value is non-null variable and you will set the value later. remove late when you initilize nullable value.

    when you use ? sign, it means, the variable is nullable

    List<UserProjectSpecificDay>? _userModel; // since its nullable, no need to set  initial value

    then on API call:

     void _getData() async {
        _userModel = await ApiService().GetUserProjectSpecificDay()); 
      }

    no need ! anymore, because we had remove late above

    • last in your body:
     body: _userModel == null // only check if null 
       ? const Center(
          child: CircularProgressIndicator(),)
       : ListView.builder(
       itemCount: _userModel.length ?? 0 // if null, set the list length = 0
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  2. 0

    It means your reponse.body is null. You cn use use a FutureBuiler to build your widget and you can also set a timeout for the request after what you show the user that there is no data.

    Eg:

    List<UserProjectSpecificDay>? _userModel;
bool isTimeout = false;
String errorMsg = '';

void _getData() async {
  ApiService().GetUserProjectSpecificDay().then((response) {
    // I suppose you have a "fromMap" in your model
    if (response.statusCode == 200) {
      var data = jsonDecode(response.body);
      _userModel = UserProjectSpecificDay.fromMap(data);
    } else if (response.statusCode == 408) {
      setState(() {
        isTimeout = true;
        errorMsg = 'There was a network error';
      });
    } else setState(() => errorMsg = 'An error occured');
  });
}
/* ... */
body: FutureBuilder(
  builder: (_, __) {
    if (_userModel != null) {
      // return your widget
    }
    if (isTimeout) {
       return Center(child: Text(errorMsg);
    }
    return Center(child: CircularProgressIndicator());

}
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