Flutter - Providers 和 Future 调用，如何共享同一个实例？

Question

I'm learning Flutter and there is something I cannot grasp my head around. I implemented a Infinite scroll pagination, with a package ( infine_scroll_pagination ), it works fine, but the data this Package is getting, comes from a Future call, which takes data from the WEB , and parses it in my Provider Class .

My issue is, the data that is loaded by the Infinite Scroll widget , cannot be accessed, in its state , anywhere else.

Example: Let's take a contact list , that loads 10 contacts at a time:

class ContactsBody extends StatefulWidget {
  @override
  _ContactsBodyState createState() => _ContactsBodyState();
}

class _ContactsBodyState extends State<ContactsBody> {
  static const _pageSize = 10;
  final PagingController<int, Contact> pagingController =
      PagingController(firstPageKey: 0);

  @override
  void initState() {
    super.initState();
    pagingController.addPageRequestListener((pageKey) {
      _fetchPage(pageKey);
    });
  }

  Future<void> _fetchPage(int pageKey) async {
    try {
      final newItems = await ContactsService().fetchContactsPaged(pageKey, _pageSize);
      final isLastPage = newItems.length < _pageSize;

      if (isLastPage) {
        pagingController.appendLastPage(newItems.contacts);
      } else {
        final nextPageKey = pageKey + 1;
        pagingController.appendPage(newItems.contacts, nextPageKey);
      }
    } catch (error) {
      pagingController.error = error;
    }
  }
  @override
  Widget build(BuildContext context) {
    return ContactsList(pagingController);
  }

  @override
  void dispose() {
    pagingController.dispose();
    super.dispose();
  }

So basically this Infinite Scroll package, will fetch my contacts, 10 at a time, and here my ContactsService call:

 Future<Contacts> fetchContactsPaged(int pageKey, int pageSize) async {
    final response = await http.get(.....);

    if (response.statusCode == 200) {
    return Contacts.fromJson(jsonDecode(response.body));
    } else {
      throw Exception('Failed to load contacts');
    }
  }

And finally, as you can see here above, it initializes my Provider class (Contacts), using its factory method, " fromJson() ", and returns the parsed data .

Now my Provider class :

class Contacts extends ChangeNotifier {
      List<Contact> _contacts = <Contact>[];
    
      Contacts();
    
      factory Contacts.fromJson(final Map<String, dynamic> json) {
        final Contacts contacts = Contacts();
        if (json['data'] != null) {
          json['data'].forEach((contact) {
            contacts.add(Contact.fromJson(contact));
          });
        }
        return contacts;
      }

  void add(final Contact contact) {
    this._contacts.add(contact);
    this.notifyListeners();
  }

The problem I'm having here is, when the Inifinite Scroll listView is loaded, and for example I change the state of a single contact (contacts can be set as favorite for example),

How can I access the SAME instanc e of the Contacts() class , that the FUTURE call initialized, so that I can access the current state of the data in that class?

Of course if I were to POST my changes onto the API, and refetch the new values where I need them, I would get the updated state of my data, but I want to understand how to access the same instance here and make the current data available inside the app everywhere

Answer 1

EDIT: I removed the original answer to give a better sample of what the OP wants to achieve.

I made a repo on GitHub to try to show you what you want to achieve: https://github.com/Kobatsu/stackoverflow_66578191

There are a few confusing things in your code:

• When to create instances of your objects (ContactsService, Contacts)
• Provider usage
• (Accessing the list of the pagingController?)
• Parsing a JSON / using a factory method

The repository results in the following:

When you update the list (by scrolling down), the yellow container is updated with the number of contacts and the number of favorites. If you click on a Contact, it becomes a favorite and the yellow container is also updated.

I commented the repository to explain you each part.

Note: the Contacts class in your code became ContactProvider in mine.

The ContactsService class to make the API call:

class ContactsService {
  static Future<List<Contact>> fetchContactsPaged(
      int pageKey, int pageSize) async {
    // Here, you should get your data from your API

    // final response = await http.get(.....);
    // if (response.statusCode == 200) {
    //   return Contacts.fromJson(jsonDecode(response.body));
    // } else {
    //   throw Exception('Failed to load contacts');
    // }

    // I didn't do the backend part, so here is an example
    // with what I understand you get from your API:
    var responseBody =
        "{\"data\":[{\"name\":\"John\", \"isFavorite\":false},{\"name\":\"Rose\", \"isFavorite\":false}]}";
    Map<String, dynamic> decoded = json.decode(responseBody);
    List<dynamic> contactsDynamic = decoded["data"];

    List<Contact> listOfContacts =
        contactsDynamic.map((c) => Contact.fromJson(c)).toList();

    // you can return listOfContacts, for this example, I will add 
    // more Contacts for the Pagination plugin since my json only has 2 contacts
    for (int i = pageKey + listOfContacts.length; i < pageKey + pageSize; i++) {
      listOfContacts.add(Contact(name: "Name $i"));
    }
    return listOfContacts;
  }
}

Usage of Provider:

Consumer<ContactProvider>(
        builder: (_, foo, __) => Container(
              child: Text(
                  "${foo.contacts.length} contacts - ${foo.contacts.where((c) => c.isFavorite).length} favorites"),
              padding: EdgeInsets.symmetric(
                  horizontal: 20, vertical: 10),
              color: Colors.amber,
            )),
    Expanded(child: ContactsBody())
  ]),
)

Fetch page method in the ContactsBody class, where we add the contact to our ContactProvider:

  Future<void> _fetchPage(int pageKey) async {
    try {
      // Note : no need to make a ContactsService, this can be a static method if you only need what's done in the fetchContactsPaged method
      final newItems =
          await ContactsService.fetchContactsPaged(pageKey, _pageSize);
      final isLastPage = newItems.length < _pageSize;
      if (isLastPage) {
        _pagingController.appendLastPage(newItems);
      } else {
        final nextPageKey = pageKey + newItems.length;
        _pagingController.appendPage(newItems, nextPageKey);
      }

      // Important : we add the contacts to our provider so we can get
      // them in other parts of our app
      context.read<ContactProvider>().addContacts(newItems);
    } catch (error) {
      print(error);
      _pagingController.error = error;
    }
  }

ContactItem widget, in which we update the favorite statuts and notify the listeners:

class ContactItem extends StatefulWidget {
  final Contact contact;
  ContactItem({this.contact});

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

class _ContactItemState extends State<ContactItem> {
  @override
  Widget build(BuildContext context) {
    return InkWell(
        child: Padding(child: Row(children: [
          Expanded(child: Text(widget.contact.name)),
          if (widget.contact.isFavorite) Icon(Icons.favorite)
        ]), padding: EdgeInsets.symmetric(vertical: 8, horizontal: 10),),
        onTap: () {
          // the below code updates the item
          // BUT others parts of our app won't get updated because
          // we are not notifying the listeners of our ContactProvider !
          setState(() {
            widget.contact.isFavorite = !widget.contact.isFavorite;
          });

          // To update other parts, we need to use the provider
          context.read<ContactProvider>().notifyContactUpdated(widget.contact);
        });
  }
}

And the ContactProvider:

class ContactProvider extends ChangeNotifier {
  final List<Contact> _contacts = [];
  List<Contact> get contacts => _contacts;

  void addContacts(List<Contact> newContacts) {
    _contacts.addAll(newContacts);
    notifyListeners();
  }

  void notifyContactUpdated(Contact contact) {
    // You might want to update the contact in your database,
    // send it to your backend, etc...
    // Here we don't have these so we just notify our listeners :
    notifyListeners();
  }
}