How to add new element in exist element of ListView or create new ListView element?

I have the following situation: I want to create a ListView that contains collections are sorted by date. So, I have a question: how to add a new element of list to an existing widget or create a new widget (if widget that should store elements with this date is not exist) and add an element to it?

I know how to use ChangeNotifierProvider, ChangeNotifier and Consumer for updating UI, but I have no idea how to use it in my situation.
It can be easy or I have not correct idea, but I am new in Flutter, so I am sorry for it.

I tried to add dividers to my ListView, but they depend on the index, so when I scroll the ListView it doesnt work correctly, because I want it to depend on date, not only index.

If i understand your problem correctly, you need to display list of elements with Date and other information, something like this:

20.02.2024
  element A
  element B
  element C
------------
21.02.2024
  element A
  element B
  ...
------------
...

To do this you will need several classed, for example Element and DateWithElements:

class Element {
  final String name;

  Element(this.name);
}

class DateWithElements {
  final DateTime date;
  final List<Element> elements;

  DateWithElements(this.date, this.elements);
}

Then you can use them to store your data in State of your Statefull Widget(don't forget to initialize the variable with data)

final list = <DateWithElements>[];

and use variable for build widgets:

ListView.separated(
  shrinkWrap: true,
  itemCount: list.length,
  itemBuilder: (_, index) {
    final current = list[index];
    return Column(
      children: [
        Text(current.date.toString()),
        ...current.elements.map((e) => Text(e.name)),
      ],
    );
  },
 separatorBuilder: (_, __) => const Divider(),

),

For adding new items to list you will need a function:

Future<void> addItem(DateTime date, List<String> elements) async {
    final newItem = DateWithElements(
      date, 
      elements.map(Element.new).toList(),
    );
    list.add(newItem);
    setState(() {});
  }