FutureBuilder with future method issue

I am trying to fetch contacts from the phonebook

List<Contact> contacts = [];

For that I am using futureBuilder method

_buildContactsCardList(BuildContext context) {
    return Container(
      child: FutureBuilder(
        future: getContacts(),
        builder: (context, AsyncSnapshot snapshot) {
          if (snapshot.hasError) {
            return const Center(
              child: Text("Error Fetching Contacts",
                  style: TextStyle(
                    color: Colors.grey,
                    fontSize: 16,
                    fontWeight: FontWeight.bold,
                  )),
            );
          }
          if (snapshot.data == null) {
            return const Center(
              child: SizedBox(height: 50, child: CircularProgressIndicator()),
            );
          }
          if (snapshot.data.length == 0) {
            return const Center(
              child: SizedBox(height: 50, child: Text("No Contacts Found")),
            );
          }
          return ListView.separated(
              physics: const NeverScrollableScrollPhysics(),
              shrinkWrap: true,
              itemCount: snapshot.data!.length,
              separatorBuilder: (context, index) {
                return Divider(
                  color: Colors.grey.shade300,
                  thickness: 1,
                );
              },
              itemBuilder: (context, index) {
                Contact contact = snapshot.data![index];
                return ListViewItem(contact.phones.first.number);
              });
        },
      ),
    );
  }

To fetch the contacts from the system, which is being fetching correctly sometimes as I am using fast_contacts plugin.

Future<List<Contact>> getContacts() async {
    bool isGranted = await ContactsController.getPermission();

    if (isGranted) {
      try {
        return await FastContacts.getAllContacts();
      } catch (e) {
        log.d(RouteName.AddFriendsScreen + ": Failed to get contacts");
      }
    }
    log.d(RouteName.AddFriendsScreen + ": Failed to get contacts");
    return [];
  }

Now the challenge is that, Future Builder calls future method more than one time, and if getAllContacts are tried to be read more than one time, then it throws an StateError.
So statement

return await FastContacts.getAllContacts();

is throwing error i.e.
StateError (Bad state: getAllContacts is already in progress.)"
due to which I am getting an empty array as per my logic. Which is not what I want.

I want either the async statement to return full contacts list or give empty array only is zero contacts exists in system.

Kindly help.

Yes, the FutureBuilder builds itself on every state change and calls the future function.

You can do a things, make a nullable Future<List<Contact>> type variable. Firstly, assign it with your future function and pass it to the future of the FutureBuilder. At the execution of the future function make that variable null. As the the future of FutureBuilder gets null it will not call the future function again and will not rebuild itself.

You can refer to the below code:

  Future<List<Contact>>? getContactsVar;

  void initState() {
    super.initState();
    getContactsVar = getContacts();
  }
  
  Future<List<Contact>> getContacts() async {
    getContactsVar = null;
    bool isGranted = await ContactsController.getPermission();

    if (isGranted) {
      try {
        return await FastContacts.getAllContacts();
      } catch (e) {
        log.d(RouteName.AddFriendsScreen + ": Failed to get contacts");
      }
    }
    log.d(RouteName.AddFriendsScreen + ": Failed to get contacts");
    return [];
  }

  _buildContactsCardList(BuildContext context) {
    return Container(
      child: FutureBuilder(
        future: getContactsVar,
        builder: (context, AsyncSnapshot snapshot) {
          if (snapshot.hasError) {
            return const Center(
              child: Text("Error Fetching Contacts",
                  style: TextStyle(
                    color: Colors.grey,
                    fontSize: 16,
                    fontWeight: FontWeight.bold,
                  )),
            );
          }
          if (snapshot.data == null) {
            return const Center(
              child: SizedBox(height: 50, child: CircularProgressIndicator()),
            );
          }
          if (snapshot.data.length == 0) {
            return const Center(
              child: SizedBox(height: 50, child: Text("No Contacts Found")),
            );
          }
          return ListView.separated(
              physics: const NeverScrollableScrollPhysics(),
              shrinkWrap: true,
              itemCount: snapshot.data!.length,
              separatorBuilder: (context, index) {
                return Divider(
                  color: Colors.grey.shade300,
                  thickness: 1,
                );
              },
              itemBuilder: (context, index) {
                Contact contact = snapshot.data![index];
                return ListViewItem(contact.phones.first.number);
              });
        },
      ),
    );
  }