flutter - Dart列表未通过forEach循环更新

Question

我正在使用此package来检索设备的联系人。库检索427个联系人，我想循环整个列表，以便创建另一个列表并将其发送到后端。问题是循环无法正常工作，此功能在循环完成之前返回。
这是我使用的功能:

  Future<QueryResult> uploadContacts() async {
    final List<Contact> rawContacts =
        (await ContactsService.getContacts(withThumbnails: false)).toList();
    List<ContactInput> contactsListInput;

    print('contactsListInput length: ${rawContacts.length}');
    
    rawContacts.forEach((contact) {
      print('contact: $contact'); //PRINTED JUST ONCE

      //Contact can have more than 1 number. We need them all
      contact.phones.forEach((phone) {
        final contactInput =
            ContactInput(name: contact.displayName, phone: phone.value);

        contactsListInput.add(contactInput);
      });
    });

    print('contactsListInput length: ${contactsListInput.length}'); //NEVER PRINT ANYTHING

    final ContactsListInput input =
        ContactsListInput(contacts: contactsListInput);

    final MutationOptions _options = MutationOptions(
        document: SyncContactsMutation().document,
        variables: SyncContactsArguments(input: input).toJson());

    return client.mutate(_options);
  }

我也尝试过使用for循环，并且发生了同样的事情。

    for (int i = 0; i < rawContacts.length; i++) {
      final contact = rawContacts[i];

      final contactInput =
      ContactInput(name: contact.displayName, phone: contact.phones.first.value);

      contactsListInput.add(contactInput);
    }
    print('contactsListInput length: ${contactsListInput.length}'); //NEVER CALLED

我也尝试过Future.forEach

    await Future.forEach(rawContacts, (contact) async {
      print('contact: $contact');

      //Since contact can have more than one number we loop them too.
      await Future.forEach(contact.phones, (phone) async {
        final contactInput =
            ContactInput(name: contact.displayName, phone: phone.value);

        contactsListInput.add(contactInput);
      });
    });

如何解决这个问题？任何帮助都感激不尽。

Answer 1

我已将其固定为

  Future<QueryResult> uploadContacts() async {
    final Iterable<Contact> rawContacts =
        (await ContactsService.getContacts(withThumbnails: false));

    final Iterable<ContactInput> contacts = rawContacts.expand((contact) => contact.phones.map(
        (phone) =>
            ContactInput(name: contact.displayName, phone: phone.value)));

    final input = ContactsListInput(contacts: contacts);

    final MutationOptions _options = MutationOptions(
        document: SyncContactsMutation().document,
        variables: SyncContactsArguments(input: input).toJson());

    return client.mutate(_options);
  }

归功于@pskink和@ loganrussell48