Where can I place the function that should always execute or listen?

Question

I want to show the warning notification when the expenses amount exceed the set budget amount for a particular category of expense or all categories. I've created a function. but I don't know where I should call this function. The function should always execute or listen to changes without clicking anything. Please help me. Where can I call it?

The Function

Future<void> fetchData() async {
  try {
    // Fetch budget data
    List<Map<String, dynamic>> budgetDataList = [];
    List<Map<String, dynamic>> expenseDataList = [];
    QuerySnapshot budgetSnapshot = await budgetCollectionRef.get();
    if (budgetSnapshot.docs.isNotEmpty) {
      List<DocumentSnapshot> documents = budgetSnapshot.docs;
      for (var document in documents) {
        String documentId = document.id;
        Map<String, dynamic> data = document.data() as Map<String, dynamic>;
        data['documentId'] = documentId;
        budgetDataList.add(data);
      }
    } else {
      // Handle no data case
    }

    // Fetch expense data
    QuerySnapshot expenseSnapshot = await expenseCollectionRef.get();
    if (expenseSnapshot.docs.isNotEmpty) {
      List<DocumentSnapshot> documents = expenseSnapshot.docs;
      for (var document in documents) {
        Map<String, dynamic> data = document.data() as Map<String, dynamic>;
        expenseDataList.add(data);
      }
    } else {
      // Handle no data case
    }

    // Process data as needed
    List<Amount> sample = [];

    for (var budget in budgetDataList) {
      String documentId = budget['documentId'];
      String category = budget['CategoryName'];
      Timestamp fromDate = budget['FromDate'];
      Timestamp tillDate = budget['TillDate'];
      double percentage = budget['Percentage'].toDouble();
      double budgetAmount = budget['Amount'].toDouble();
      List<Map<String, dynamic>> expenseFilteredList;

      if (category == 'All Categories') {
        expenseFilteredList = expenseDataList.where((expense) {
          Timestamp expenseDate = expense['UserSelectedDate'];
          return expenseDate.compareTo(fromDate) >= 0 &&
              expenseDate.compareTo(tillDate) <= 0;
        }).toList();
      } else {
        expenseFilteredList = expenseDataList.where((expense) {
          Timestamp expenseDate = expense['UserSelectedDate'];
          return expenseDate.compareTo(fromDate) >= 0 &&
              expenseDate.compareTo(tillDate) <= 0 &&
              expense['Category'] == category;
        }).toList();
      }
      double expenseTotalAmount = expenseFilteredList.fold(
        0.0,
        (previous, expense) => previous + (expense['Amount']),
      );
    // seperate function for calculate the percentage
      calPercentage = calculatePercentage(expenseTotalAmount, budgetAmount);
      print('$expenseTotalAmount');

      if (calPercentage * 100 >= percentage) {
        return NotificationService().showNotification(
            title: 'Budget Exceed !',
            body: 'You Exceed the budget for $category');

      } else {}
    }
  } catch (e) {
    // Handle any errors during data retrieval
    print('Error fetching data: $e');
  }
}

Answer 1

You can put this in the init state. It will listen data and you will be able to show notification. But if you want to show notification while app is close then you will need firebase function which listen data and send push notification to the device.