Implementando o StatefulWidget

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Implementando o StatefulWidget

import 'package:flutter/material.dart';

void main() {
  runApp(BytebankApp());
}

class BytebankApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: ListaTransferencias(),
      ),
    );
  }
}

class FormularioTransferencia extends StatelessWidget {
  final TextEditingController _controladorCampoNumeroConta =
      TextEditingController();
  final TextEditingController _controladorCampoValor = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Criando Transferências'),
      ),
      body: Column(children: [
        Editor(
            controlador: _controladorCampoNumeroConta,
            rotulo: 'Número da Conta',
            dica: '0000'),
        Editor(
            controlador: _controladorCampoValor,
            rotulo: 'Valor',
            dica: '0.00',
            icone: Icons.monetization_on),
        RaisedButton(
          onPressed: () => _criaTransferencia(context),
          child: Text('Confirmar'),
        )
      ]),
    );
  }

  void _criaTransferencia(BuildContext context) {
    final int numeroConta =
        int.tryParse(_controladorCampoNumeroConta.text);
    final double valor = double.tryParse(_controladorCampoValor.text);

    if (numeroConta != null && valor != null) {
      final transferenciaCriada = Transferencia(valor, numeroConta);
      debugPrint('Criando Transferência');
      debugPrint('$transferenciaCriada');
      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}); //parametros opcionais

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

class ListaTransferencias extends StatefulWidget{
  final List<Transferencia> _transferencias = List();

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

class ListaTransferenciasState extends State<ListaTransferencias> {
  @override
  Widget build(BuildContext context) {
    widget._transferencias.add(Transferencia(100.0, 1000));
    widget._transferencias.add(Transferencia(100.0, 1000));
    widget._transferencias.add(Transferencia(100.0, 1000));
    widget._transferencias.add(Transferencia(100.0, 1000));
    return Scaffold(
      appBar: AppBar(
        title: Text('Transferências'),
      ),
      body: ListView.builder(
        itemCount: widget._transferencias.length,
        itemBuilder: (context, indice) {
          final transferencia = widget._transferencias[indice];
          return ItemTransferencia(transferencia);
        },
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add), onPressed: () {
        final Future<Transferencia> future = Navigator.push(context, MaterialPageRoute(builder: (context) {
          return FormularioTransferencia();
        }));
        future.then((transferenciaRecebida) {
          debugPrint('chegou no then do future');
          debugPrint('$transferenciaRecebida');
          widget._transferencias.add(transferenciaRecebida);
        });
      },),
    );
  }
}

class ItemTransferencia extends StatelessWidget {
  final Transferencia _transferencia;

  ItemTransferencia(this._transferencia);

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

class Transferencia {
  final double valor;
  final int numeroConta;

  Transferencia(this.valor, this.numeroConta);

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

Onde estou errando?