Catch the return of a Alertdialog to treat willpopscope

Question

I have a function called showAlertDialogSimNao() and this receives the name parameter and content. I would like to take the return of this function and treat at the end of the application. However I’m not getting. The function call it happens however the function already gives the return without even the user have chosen the option "YES" or "NO" to terminate the application.

Follows the code:

    main.dart

        import 'package:flutter/material.dart';
        import 'package:flutter_app01/principal.dart';

        import 'biblioteca.dart';

        void main() => runApp(MaterialApp(
            theme: ThemeData.dark(),
            debugShowCheckedModeBanner: false,
            home: GestaoComercial()));

        class GestaoComercial extends StatefulWidget {
          @override
          _GestaoComercialState createState() => _GestaoComercialState();
        }

        class _GestaoComercialState extends State<GestaoComercial> {
          TextEditingController _loginEditingController = TextEditingController(text: 'usuario');
          TextEditingController _pwdEditingController = TextEditingController(text: 'senha');

          bool _loading = false;



          @override
          Widget build(BuildContext context) {
            return Scaffold(
              appBar: AppBar(
                title: Text("Gestão Comercial"),
                centerTitle: true,
              ),


              body: Column(
                children: <Widget>[

                  SizedBox(height: 20.0),

                  Text(
                    'Autenticação',
                    style: TextStyle(fontSize: 20),
                  ),

                  SizedBox(height: 20.0),

                  TextFormField(
                      controller: _loginEditingController,
                      keyboardType: TextInputType.emailAddress,
                      decoration: InputDecoration(
                        labelText: "Identificação",
                        icon: Icon(Icons.person),
                      )),


                  TextFormField(
                      obscureText: true,
                      controller: _pwdEditingController,
                      decoration: InputDecoration(
                          labelText: "Senha", icon: Icon(Icons.lock))),


                  SizedBox(height: 20.0),


                  RaisedButton(
                    child: _loading
                        ? linearProgressIndicator(context)
                        : Text("Autenticar"),
                    onPressed: () async {
                      setState(() {
                        _loading = true;
                      });
                      await Future.delayed(Duration(seconds: 1));
                          Navigator.push(context, SlideRightRoute(page: Principal()));

                      setState(() {
                        _loading = false;
                      });
                    },
                    shape: new RoundedRectangleBorder(
                        borderRadius: new BorderRadius.circular(20.0),
                        side: BorderSide(color: Colors.green)),
                    color: Colors.green,
                    textColor: Colors.white,
                  ),

                ],
              ),
            );
          }
        }


biblioteca.dart

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

Widget linearProgressIndicator(BuildContext context) {
  return Container(
      width: 45.0,
      height: 45.0,
      alignment: Alignment.center,
      child: CircularProgressIndicator(
          backgroundColor: Colors.green[900],
          valueColor: AlwaysStoppedAnimation<Color>(Colors.white),
          strokeWidth: 3.0));
}

class SlideRightRoute extends PageRouteBuilder {
  final Widget page;

  SlideRightRoute({this.page})
      : super(
          pageBuilder: (
            BuildContext context,
            Animation<double> animation,
            Animation<double> secondaryAnimation,
          ) =>
              page,
          transitionsBuilder: (
            BuildContext context,
            Animation<double> animation,
            Animation<double> secondaryAnimation,
            Widget child,
          ) =>
              SlideTransition(
            position: Tween<Offset>(
              begin: const Offset(-1, 0),
              end: Offset.zero,
            ).animate(animation),
            child: child,
          ),
        );
}



bool showAlertDialogSimNao(BuildContext context, String titulo, String conteudo) {
  // configura o button

  Widget btnSim = FlatButton(
    child: Text("Sim"),
    onPressed: () {
      Navigator.of(context).pop(true);
    },
  );


  Widget btnNao = FlatButton(
    child: Text("Não"),
    onPressed: () {
      Navigator.pop(context,false);
    },
  );

  // configura o  AlertDialog
  AlertDialog alerta = AlertDialog(
    title: Text("$titulo"),
    content: Text("$conteudo"),
    actions: [
      btnSim,
      btnNao
    ],
  );
  // exibe o dialog
  showDialog(
    barrierDismissible: false,
    context: context,
    builder: ( context )  => alerta);


}



principal.dart

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'biblioteca.dart';

class Principal extends StatefulWidget {
  @override
  _PrincipalState createState() => _PrincipalState();
}

class _PrincipalState extends State<Principal> {


  Future<bool> _onWillPop() async {
    var resposta = showAlertDialogSimNao(
        context, "Encerrar Aplicativo", "Deseja encerrar?");

    print(resposta);
    return resposta;
  }

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: _onWillPop,
      child: Scaffold(
          appBar: AppBar(
            backgroundColor: Colors.green,
            leading: Text("AÇÃO 1"),
            centerTitle: true,
            title: Text("APLICATIVO - MODELO"),
          ),
          body: Padding(
            padding: const EdgeInsets.all(8.0),
            child: GridView.count(
                crossAxisCount: 2,
                children: List.generate(choices.length, (index) {
                  return ChoiceCard(choice: choices[index]);
                })),
          )),
    );
  }
}

class ChoiceCard extends StatelessWidget {
  const ChoiceCard({Key key, this.choice}) : super(key: key);
  final Choice choice;

  @override
  Widget build(BuildContext context) {
    final TextStyle textStyle = Theme.of(context).textTheme.display1;
    return Card(
      color: Colors.green[900],
      child: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: <Widget>[
            Icon(
              choice.icon,
              color: textStyle.color,
              size: 50.0,
            ),
            Text(
              choice.title,
              style: textStyle,
            )
          ],
        ),
      ),
    );
  }
}

class Choice {
  const Choice({this.title, this.icon});

  final String title;
  final IconData icon;
}

const List<Choice> choices = const <Choice>[
  const Choice(title: 'Venda', icon: Icons.attach_money),
  const Choice(title: 'Cliente', icon: Icons.people),
  const Choice(title: 'Produto', icon: Icons.wallpaper),
  const Choice(title: 'Venda', icon: Icons.monetization_on),
  const Choice(title: 'Cliente', icon: Icons.people),
  const Choice(title: 'Produto', icon: Icons.wallpaper),
  const Choice(title: 'Venda', icon: Icons.monetization_on),
  const Choice(title: 'Cliente', icon: Icons.people),
  const Choice(title: 'Produto', icon: Icons.wallpaper),
  const Choice(title: 'Venda', icon: Icons.monetization_on),
  const Choice(title: 'Cliente', icon: Icons.people),
  const Choice(title: 'Produto', icon: Icons.wallpaper),
];

Answer 1

You are having problems working with asynchronous processes, the changes below will probably solve your problem...

Modify your method showAlertDialogSimNao() to make it asynchronous

Future<bool> showAlertDialogSimNao(BuildContext context, String titulo, String conteudo) async{
  // configura o button

  Widget btnSim = FlatButton(
    child: Text("Sim"),
    onPressed: () {
      Navigator.of(context).pop(true);
    },
  );


  Widget btnNao = FlatButton(
    child: Text("Não"),
    onPressed: () {
      Navigator.pop(context,false);
    },
  );

  // configura o  AlertDialog
  AlertDialog alerta = AlertDialog(
    title: Text("$titulo"),
    content: Text("$conteudo"),
    actions: [
      btnSim,
      btnNao
    ],
  );

  // exibe o dialog
  return = await showDialog(
    barrierDismissible: false,
    context: context,
    builder: ( context )  => alerta);

}

And then modify the method _onWillPop() also to function properly, because you are using it asynchronously, but it has nothing asynchronous

Future<bool> _onWillPop() async {
   resposta = await showAlertDialogSimNao(
    context, "Encerrar Aplicativo", "Deseja encerrar?");

  print(resposta);
  return resposta;
}

At the moment I am without access to Flutter to test, maybe you need to fix a little thing here or there in my example.