object appears in console but is not added to list

Question

When added in the list, the objects appear in the console. however the list is not updated and no obj sent appears.

    import 'package:flutter/material.dart';

void main() => runApp(Home());

class Home extends StatelessWidget {
  const Home({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: ListaTransferencias(),
        // body: FormularioTransferencia(),
      ),
    );
  }
}

class FormularioTransferencia extends StatelessWidget {
  final TextEditingController _controllerNumeroConta = TextEditingController();
  final TextEditingController _controllerValor = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Criando uma transferência'),
      ),
      body: Column(
        children: [
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: Editor(
              controlador: _controllerNumeroConta,
              rotulo: 'Número da conta',
              dica: '0000',
            ),
          ),
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: Editor(
              controlador: _controllerValor,
              rotulo: 'Valor',
              dica: '0.0',
              icone: Icons.monetization_on,
            ),
          ),
          Padding(
            padding: const EdgeInsets.fromLTRB(0.0, 32.0, 0.0, 0.0),
            child: ElevatedButton(
              style: ElevatedButton.styleFrom(
                  textStyle: const TextStyle(fontSize: 24),
                  padding: EdgeInsets.fromLTRB(
                      30.0, 20.0, 30.0, 20.0) // aumentar o tamanho do botão
                  ),
              onPressed: () => _criaTransferencia(context),
              child: Text('Salvar'),
            ),
          )
        ],
      ),
    );
  }

  void _criaTransferencia(BuildContext context) {
    final int? numeroConta = int.tryParse(_controllerNumeroConta.text);
    final double? valor = double.tryParse(_controllerValor.text);
    if (numeroConta != null && valor != null) {
      final transferenciaCriada = Transferencia(numeroConta, valor);
      Navigator.pop(context, transferenciaCriada);
    }
  }
}

class Editor extends StatelessWidget {
  final TextEditingController? controlador;
  final String? rotulo;
  final String? dica;
  final IconData? icone;

  Editor({this.controlador, this.rotulo, this.dica, this.icone});

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: TextField(
        controller: controlador,
        keyboardType: TextInputType.number,
        style: TextStyle(
          fontSize: 24.0,
        ),
        decoration: InputDecoration(
          prefixIcon: Icon(icone),
          hintText: dica,
          labelText: rotulo,
        ),
      ),
    );
  }
}

// tela que lista todas as transferencias
class ListaTransferencias extends StatefulWidget {
  final List<Transferencia> _list = [];

  @override
  State<StatefulWidget> createState() {
    return ListaTransferenciaState();
  }
}

class ListaTransferenciaState extends State<ListaTransferencias> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Transferências'),
      ),
      body: ListView.builder(
        itemCount: widget._list.length,
        itemBuilder: (context, index) {
          final transferencia = widget._list[index];

          return ItemTransferencia(transferencia);
        },
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          final Future<Transferencia?> future = Navigator.push(
            context,
            MaterialPageRoute(builder: (context) {
              return FormularioTransferencia();
            }),
          );

          future.then((value) {
            if (value != null) {
              setState(() {
                () => widget._list.add(value);
              });
            }
          });
        },
      ),
    );
  }
}

class ItemTransferencia extends StatelessWidget {
  final Transferencia _transferencia;

  ItemTransferencia(this._transferencia);

  @override
  Widget build(BuildContext context) {
    return Card(
      child: ListTile(
        leading: Icon(Icons.attach_money),
        title: Text(_transferencia.valor.toString()),
        subtitle: Text(_transferencia.conta.toString()),
      ),
    );
  }
}

class Transferencia {
  final int conta;
  final double valor;

  Transferencia(this.conta, this.valor);

  @override
  String toString() {
    return 'Transferencia{conta: $conta, valor: $valor}';
  }
}

Answer 1

Come on, there are 2 points that are causing your problem...

first If your list will be mutable, ie, will have its status changed, the variable _list must be declared within the state and not in the widget itself, it follows the correct form:

class ListaTransferenciaState extends State<ListaTransferencias> {
  final List<Transferencia> _list = [];

  [...]
}

Obs.: That way no more need to do widget._list, may just be _list

2nd The setState do not need to return a function, you can do only:

  future.then((value) {
    if (value != null) {
      setState(() {
        widget._list.add(value);
      });
    }
  });

Note: I only removed the Arrow-Function () => { }