Erro no FutureBuilder,

3
respostas

por Anderson Alves Duarte

| 20.8k xp | 3 posts

Olá,

Estou tendo este erro ao abrir a lista de contatos:

The getter 'length' was called on null.
Receiver: null
Tried calling: length

The relevant error-causing widget was
FutureBuilder<List<Contact>>
package:bytebank02/screens/contacts_lista.dart:19
When the exception was thrown, this was the stack
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      ContactsList.build.<anonymous closure>
package:bytebank02/screens/contacts_lista.dart:28
#2      _FutureBuilderState.build
package:flutter/…/widgets/async.dart:751
#3      StatefulElement.build
package:flutter/…/widgets/framework.dart:4744
#4      ComponentElement.performRebuild
package:flutter/…/widgets/framework.dart:4627

Código contacts_list:

class ContactsList extends StatelessWidget {
  final List<Contact> contacts = List();

  @override
  Widget build(BuildContext context) {
    // contacts.add(Contact(name: "Anderson", accountNumber: 12));
    // contacts.add(Contact(name: "Duarte", accountNumber: 45));
    // contacts.add(Contact(name: "Alves", accountNumber: 78));
    return Scaffold(
      appBar: AppBar(
        title: Text("Contacts"),
      ),
      body: FutureBuilder(
        future: findAll(),
        builder: (context, snapshot) {
          final List<Contact> contacts = snapshot.data;
          return ListView.builder(
            itemBuilder: (context, index) {
              final Contact contact = contacts[index];
              return _ContactItem(contact: contact);
            },
            itemCount: contacts.length,
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context)
              .push(
            MaterialPageRoute(
              builder: (context) => ContactForm(),
            ),
          )
              .then((newContact) {
            debugPrint("Contato");
            debugPrint(newContact.toString());
          });
        },
        child: Icon(Icons.add),
      ),
    );
  }
}

class _ContactItem extends StatelessWidget {
  final Contact contact;
  const _ContactItem({
    Key key,
    this.contact,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Card(
      child: ListTile(
        title: Text(
          contact.name,
          style: TextStyle(
            fontSize: 24.0,
          ),
        ),
        subtitle: Text(
          contact.accountNumber.toString(),
          style: TextStyle(
            fontSize: 16.0,
          ),
        ),
      ),
    );
  }
}

Código app_database:


Future<Database> createDatabase() {
  return getDatabasesPath().then((dbPath) {
    final String path = join(dbPath, 'bytebank02.db');

    return openDatabase(path, onCreate: (db, version) {
      db.execute('CREATE TABLE contacts('
          'id INTEGER PRIMARY KEY, '
          'name TEXT, '
          'account_number INTEGER)');
    }, version: 1);
  });
}

Future<int> save(Contact contact) {
  return createDatabase().then((db) {
    final Map<String, dynamic> contactMap = Map();

    contactMap['id'] = contact.id;
    contactMap['name'] = contact.name;
    contactMap['account_number'] = contact.accountNumber;

    return db.insert('contacts', contactMap);
  });
}

Future<List<Contact>> findAll() {
  return createDatabase().then((db) {
    return db.query('contacts').then((maps) {
      final List<Contact> contacts = List();
      for (Map<String, dynamic> map in maps) {
        final Contact contact = Contact(
          id: map['id'],
          name: map['name'],
          accountNumber: map['account_number'],
        );
        contacts.add(contact);
      }
      return contacts;
    });
  });
}

3 respostas

por Renato Correa Bento

| 75.3k xp | 1 posts

13/03/2021

Olá, Anderson.

Substitua a parte do código:

itemCount: contacts.length por itemCount: snapshot.data.length

Comigo funcionou.

por Danilo Carlos Ribeiro

| 118.2k xp | 5 posts

Senior iOS Developer

18/03/2021

Assim Funciona...

import 'package:bytebank/database/databaseApp.dart';
import 'package:bytebank/models/contact.dart';
import 'package:bytebank/screens/contact_form.dart';
import 'package:flutter/material.dart';

class ContactsList extends StatefulWidget {
  //final List<Contact> contacts = List();

  @override
  _ContactsListState createState() => _ContactsListState();
}

class _ContactsListState extends State<ContactsList> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Contacts"),
      ),
      body: FutureBuilder<List<Contact>>(
        initialData: List(),
        future: Future.delayed(Duration(seconds: 1)).then((value) => findAll()),
        builder: (context, snapshot) {
          if (snapshot.data != null) {
            final List<Contact> contacts = snapshot.data;
            return ListView.builder(
              itemBuilder: (context, index) {
                final Contact contact = contacts[index];
                return _ContactItem(contact: contact);
              },
              itemCount: snapshot.data.length,
            );
          }
          return Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              crossAxisAlignment: CrossAxisAlignment.center,
              children: <Widget>[
                CircularProgressIndicator(),
                Text('Loading'),
              ],
            ),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context)
              .push(
            MaterialPageRoute(
              builder: (context) => ContactForm(),
            ),
          )
              .then((newContact) {
            debugPrint("Contato");
            debugPrint(newContact.toString());
          });
        },
        child: Icon(Icons.add),
      ),
    );
  }
}

class _ContactItem extends StatelessWidget {
  final Contact contact;

  const _ContactItem({
    Key key,
    this.contact,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Card(
      child: ListTile(
        title: Text(
          contact.name,
          style: TextStyle(
            fontSize: 24.0,
          ),
        ),
        subtitle: Text(
          contact.accountNumber.toString(),
          style: TextStyle(
            fontSize: 16.0,
          ),
        ),
      ),
    );
  }
}

por Danilo Carlos Ribeiro

| 118.2k xp | 5 posts

Senior iOS Developer

08/04/2021

import 'package:flutter/material.dart';
import 'package:newtopbytebank/components/progress.dart';
import 'package:newtopbytebank/database/dao/contact_dao.dart';
import 'package:newtopbytebank/models/contact.dart';
import 'package:newtopbytebank/screens/contact_form.dart';
import 'package:newtopbytebank/screens/transaction_form.dart';
import 'package:newtopbytebank/screens/update_contact_form.dart';

class ContactsList extends StatefulWidget {
  @override
  _ContactsListState createState() => _ContactsListState();
}

class _ContactsListState extends State<ContactsList> {
  final ContactDao _dao = ContactDao();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Transfer'),
      ),
      body: FutureBuilder<List<Contact>>(
        initialData: [],
        future: _dao.findAllDB(),
        builder: (context, snapshot) {
          if (snapshot.data != null)
            switch (snapshot.connectionState) {
              case ConnectionState.none:
                break;
              case ConnectionState.waiting:
                return Progress();
                break;
              case ConnectionState.active:
                break;
              case ConnectionState.done:
                final List<Contact> contacts = snapshot.data;
                return ListView.builder(
                  itemBuilder: (context, index) {
                    final Contact contact = contacts[index];
                    return _ContactItem(
                      contact,
                      onClick: () {
                        Navigator.of(context).push(
                          MaterialPageRoute(
                            builder: (context) => TransactionForm(contact),
                          ),
                        );
                      },
                    );
                  },
                  itemCount: contacts.length,
                );
                break;
            }
          return Text('Unknown error');
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context)
              .push(
                MaterialPageRoute(builder: (context) => ContactForm()),
              )
              .then((value) => setState(() {}));
        },
        child: Icon(
          Icons.add,
        ),
      ),
    );
  }
}

class _ContactItem extends StatelessWidget {
  final Contact contact;
  final Function onClick;

  final ContactDao _dao = ContactDao();

  _ContactItem(
    this.contact, {
    @required this.onClick,
  });

  @override
  Widget build(BuildContext context) {
    final avatar = contact.avatarUrl == null || contact.avatarUrl.isEmpty
        ? CircleAvatar(child: Icon(Icons.person))
        : CircleAvatar(backgroundImage: NetworkImage(contact.avatarUrl));

    return ListTile(
      leading: avatar,
      onTap: () => onClick(),
      title: Text(
        contact.name,
      ),
      subtitle: Text(
        contact.email,
      ),
      trailing: Container(
        width: 100,
        child: Row(
          children: [
            IconButton(
              icon: Icon(Icons.edit),
              color: Colors.green,
              onPressed: () {
                Navigator.of(context);
              },
            ),
            IconButton(
              icon: Icon(Icons.delete),
              color: Colors.red,
              onPressed: () {
                _dao.deleteDB(contact.id).then((id) => Navigator.pop(context));
              },
            ),
          ],
        ),
      ),
    );
  }
}
//
// void _showContactForm(BuildContext context) {
//   Navigator.of(context).push(
//     MaterialPageRoute(
//       builder: (context) => ContactForm(),
//     ),
//   );
// }

void _showUpdateContactForm(BuildContext context) {
  Navigator.of(context).push(
    MaterialPageRoute(
      builder: (context) => UpdateContactForm(),
    ),
  );
}

Erro no FutureBuilder, "The getter 'length' was called on null"

Chegou a hora de transformar