Flutter cannot get the new value of a variable when called in another class-PHP Chinese Network Q&A

I have this StatefulWidget which is supposed to retrieve data from MySQL database based on conditions and add these retrieved values to an array so that I can display on a ListView using the array values. My problem is that sometimes when the array (_fileValues) and the _custUsername variable are empty or not updated, the exception is not thrown.

This is my stateful widget (main.dart)

String? _custUsername;
List<String> _fileValues = <String>[];

class cakeLogin extends StatefulWidget {
  @override
  cakeLoginPageMain createState() => cakeLoginPageMain(); 
}

class cakeLoginPageMain extends State<cakeLogin> {

  String? _CustEmailInit; 
  BuildContext? loginContext;

  List<String> _fileNameStores = <String>[];

  void callData() {
    // @ Get username based on user email and assign it to _custUsername variable
    UsernameGetter().Connection(_CustEmailInit!).then((UsernameValue) {
      _custUsername = UsernameValue!;
    });
    
    // @ Get item names based on the user username (_custUsername) and add them into
    // _fileNameStores
    NameGetter().Connection(_custUsername).then((NamesValues) {
      for(var _nameItems in NamesValues) {
        setState(() {
          _fileNameStores.add(_nameItems);
        });
      }
    });
    // @ Remove duplicates value and add them into main list (_fileValues)
    var _removeDuplicates = _fileNameStores.toSet().toList();
    for(var item in _removeNamesDupes) {
      setState(() {
       _fileValues.add(item);
      });
     }

  }
  
  @override
  Widget build(BuildContext context) {

    final _emailController = TextEditingController();

    return Scaffold(
      appBar: AppBar(
        title: Text("Login to App",
          style: TextStyle(
            fontSize: 18,
            color: setupFontCol,
            fontWeight: FontWeight.bold
          )),
        backgroundColor: setupThemeBar,
        centerTitle: true,
        shape: RoundedRectangleBorder(
          borderRadius: BorderRadius.circular(20),
        )
      ),
      backgroundColor: setupMainTheme,
      body: Padding(
        padding: EdgeInsets.all(20),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextFormField(
              controller: _emailController,
              style: TextStyle(color: Color.fromARGB(255, 214, 213, 213)),
              decoration: InputDecoration(
                border: OutlineInputBorder(
                  borderRadius: BorderRadius.circular(10),
                ),
                filled: true,
                hintStyle: TextStyle(color: Color.fromARGB(255, 197, 197, 197)),
                hintText: "Enter your email",
                fillColor: setupThemeBar
              ),
            ),

           SizedBox(height: 18),

           SizedBox(height: 25),

          SizedBox(
            height: 45,
            width: 500,
            child: ElevatedButton(
              style: 
                ElevatedButton.styleFrom(
                  primary: setupFontCol,
                  onPrimary: Colors.white,
                  shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(10),
                  )
                ),
              onPressed: () {

                var _CustEmail = _emailController.text;

                if(_CustEmail.contains("@gmail.com") || _CustEmail.contains("@email.com")) {
                   if(_CustEmail != "") {
                          _CustEmailInit = _CustEmail;
                          loginContext = context;

                          _isFromLogin = true;
  
                          _fileValues.clear();
                          callData();

                    } else {
                      alertDialog("Please enter your email address",context);
                    }
                } else {
                    alertDialog("Email address is not valid",context);
                }
              },
              //color: setupFontCol,
              child: Text("Login",
                style: TextStyle(
                  color: Colors.white,
                  fontWeight: FontWeight.normal,
                  fontSize: 14,
                ))
              ),
            ),
          ],
        ),
      ),      
    );
  }
}

UsernameGetter Class (This class will retrieve the user username from the database table based on the email in the text field input)

When I print the _usernameDetected class from the UsernameGetter class it returns an updated version of the returned string, but when I assign _custUsername to When UsernameValue is removed from main.dart, it does not update and sometimes returns null value.

import 'package:mysql_client/mysql_client.dart';
import 'dart:convert';
import 'dart:ffi';
import 'dart:typed_data';
import 'package:signuppage/main.dart' as main;
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

class UsernameGetter {
  Future<String> Connection(String _custEmail) async {
    final conn = await MySQLConnection.createConnection(
      host: "",
      port: ,
      userName: "",
      password: "",
      databaseName: "",
    );

    await conn.connect();

    var _getUsernameQue = await conn.execute("SELECT CUST_USERNAME FROM information WHERE CUST_EMAIL = :email", 
    {
      "email": _custEmail
    });

    String? _usernameDetected;
    for(final _rowsOfEmail in _getUsernameQue.rows) {
      _usernameDetected = _rowsOfEmail.assoc()['CUST_USERNAME']!;
    }
    return await _usernameDetected!;
  }
}

NameGetter Class (Retrieve user item name stored in text column)

For the NameGetter class, the same thing happens with UsernameGetter. When I retrieve the value from main.dart and add the value to the _fileValues list, sometimes the value is empty and sometimes the value is not updated.

import 'package:mysql_client/mysql_client.dart';
import 'dart:convert';
import 'dart:ffi';
import 'dart:typed_data';
import 'package:signuppage/main.dart' as main;
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

class NameGetter {

  Future<List<String>> Connection(String _custUsername) async {
    final conn = await MySQLConnection.createConnection(
      host: "",
      port: ,
      userName: "",
      password: "",
      databaseName: "",
    );
    await conn.connect();

    var _getItemNames= await conn.execute("SELECT ITEM_NAMES FROM item_details WHERE CUST_USERNAME = :username",
      {
        "username": _custUsername
      });

    List<String>? _itemNamesValues= <String>[]; 
    for(var _rowsOfItemNames in _getItemNames.rows) {
      var _retrieveNames = await _rowsOfItemNames.assoc()['ITEM_NAMES']!;
      _itemNamesValues.add(_retrieveNames);
    }

    return await _itemNamesValues;
  }
}

I'm not sure where my mistakes are, but I'm pretty sure I'm making a lot of them and I need someone to help me figure them out. Especially regarding why lists and variables are not updated/sometimes empty.

Reference link:

Flutter ListView not updated

My list in flutter is not updated in the code

Why are variable values not refreshed in flutter?

question

callData() The simplified version of the method is as follows:

var foo; void callData() { someAsyncFunction().then( ... foo = value ... ); anotherAsyncFunction(foo).then( ... ); }

As mentioned above, anotherAsyncFunction(foo) is called immediately after someAsyncFunction() and before then() > assignment The value of foo.

You can observe this behavior yourself by strategically inserting print() statements.

solution

Under the hood, Future.then() and async/await are the same thing. However, it is often easier for people to infer the behavior of asynchronous code when using async/await because then it reads like sequentially executed code.

While you can use any of these, I recommend moving to async/await to make the code easier to understand when reading.

The following is an example of how to refactor callData():

Future callData() async { // @ Get username based on user email and assign it to _custUsername variable var custUsername = await UsernameGetter().Connection(_CustEmailInit!); // @ Get item names based on the user username (_custUsername) and add them into // _fileNameStores var namesValues = await NameGetter().Connection(custUsername); setState(() { _fileNameStores.addAll(namesValues); // @ Remove duplicates value and add them into main list (_fileValues) _fileValues.addAll(_fileNameStores.toSet()); }); }

Written like this, it is obvious that custUsername is assigned before calling NameGetter().Connection(custUsername).