Can Anyone Please Tell me that how to use the function without the button. Function should be triggered automatically

Question

Hey There I am trying to display the flutter local notification in my app. The functionality that I want is that whenever a certain condition is met, The notification function should trigger and then it should display... Here is the source code below please help me out ( From this point onwards I cannot understand what to do... please help me out) NOTE:- The functionality should be done without the use of any button's on Press. The _show Notification() method should run whenever the condition is met using if else statement..

***code***
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:intl/intl.dart';
import 'package:flutter_local_notifications/flutter_local_notifications.dart';

final _firestore = FirebaseFirestore.instance;

class NotificationPage extends StatefulWidget {
  static String id = 'notification_screen';
  @override
  _NotificationPageState createState() => _NotificationPageState();
}

class _NotificationPageState extends State<NotificationPage> {
  FlutterLocalNotificationsPlugin fltrNotification;

  @override
  void initState() {
    super.initState();
    var androidInitilize = new AndroidInitializationSettings('app_icon');
    var initilizationsSettings =
        new InitializationSettings(android: androidInitilize);
    fltrNotification = new FlutterLocalNotificationsPlugin();
    fltrNotification.initialize(initilizationsSettings);
    // onSelectNotification: notificationSelected
  }

  // ignore: missing_return
  Future _showNotification() async {
    var androidDetails = new AndroidNotificationDetails(
        "Channel ID", "Expilert", "This is my channel",
        importance: Importance.max);

    var generalNotificationDetails =
        new NotificationDetails(android: androidDetails);

    var scheduledTime = DateTime.now().add(Duration(seconds: 1));

    fltrNotification.schedule(1, "Your product has expired!!!", 'Task',
        scheduledTime, generalNotificationDetails);
  }

  notificationScreen() async {
    await for (var snapshot
        in _firestore.collection('entereditemsdata').snapshots()) {
      for (var itemsPresent in snapshot.docs) {
        print(itemsPresent.data());
        _showNotification();
      }
    }
  }

  static final DateTime now = DateTime.now();
  static final DateFormat formatter = DateFormat('dd/MM/yyyy');
  final String formattedDate = formatter.format(now);

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Center(
        child: NotificationStream(
          todayDate: formattedDate,
          function:
              notificationScreen(), // expected error occuring at this point
        ),
      ),
    );
  }
}

class NotificationStream extends StatelessWidget {
  final Function function;
  final String todayDate;
  NotificationStream({this.todayDate, this.function});

  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
      stream: _firestore.collection('entereditemsdata').snapshots(),
      // ignore: missing_return

      builder: (context, snapshot) {
        List<NotificationCard> notificationCards = [];
        if (!snapshot.hasData) {
          return Center(
            child: CircularProgressIndicator(),
          );
        }

        final items = snapshot.data.docs;

        for (var item in items) {
          final itemData = item.data();
          final productName = itemData['productName'];
          final expiryDate = itemData['expiryDate'];

          final notificationCard = NotificationCard(
            pName: productName,
            eDate: expiryDate,
          );

          // This is the part which you told me to put in the code
          final streamUser =
              _firestore.collection("users").snapshots().asBroadcastStream();
          streamUser.listen((event) {
            event.docs.forEach((user) {
               print(user.data);
              // add condition here and invoke PushNotification
              // ignore: unnecessary_statements
              
              // This is where i am calling the function being passed at constructor
               () => function;
            });
          });

          if (todayDate == expiryDate) {
            notificationCards.add(notificationCard);
          }
        }

        return Expanded(
          child: ListView(
            children: notificationCards,
          ),
        );
      },
    );
  }
}

class NotificationCard extends StatelessWidget {
  final String pName;
  final String eDate;

  NotificationCard({this.pName, this.eDate});

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: Container(
        decoration: BoxDecoration(
          borderRadius: BorderRadius.circular(24.0),
          gradient: LinearGradient(colors: [
            Color(0xffFDDB27),
            Color(0xffFDDB27),
          ], begin: Alignment.bottomLeft, end: Alignment.topRight),
          boxShadow: [
            BoxShadow(
              color: Colors.grey,
              blurRadius: 8,
              offset: Offset(0, 3),
            ),
          ],
        ),
        child: Center(
          child: Padding(
            padding: const EdgeInsets.all(8.0),
            child: Text(
              "$pName has expired on $eDate",
              style: TextStyle(
                color: Color(0xff00B1D2),
                fontFamily: 'Avenir',
                fontSize: 15.0,
                fontWeight: FontWeight.w700,
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Answer 1

You might have written a code to fetch the document in your application, match the values of date and invoke this PushNotification.

If not, you can add listener to firebase collection and listen to change in value like this,

final streamUser = Firestore.instance.collection("users").snapshots().asBroadcastStream();
streamUser.listen((event) {
  event.documents.forEach((user) {
    print(user.data);
    // add condition here and invoke PushNotification
  });
});