How to produce single update on .Clear() when using .GroupOn()

[Bulgrak]

Let's say I have a list of topics, where topic is declared as:

public class Topic
{
    public string Title { get; set; }
    public string Category { get; set; }
    public DateTime Timestamp { get; set; }
}

I want to get latest topic for each category.
Here is an example that I have made, which achieves that:

using DynamicData;
using DynamicData.Binding;

var topics = new List<Topic>
{
    new Topic { Title = "TestA1", Category = "A", Timestamp = new DateTime(2022, 7, 20, 10, 0, 0, DateTimeKind.Utc) },
    new Topic { Title = "TestA2", Category = "A", Timestamp = new DateTime(2022, 7, 20, 5, 0, 0, DateTimeKind.Utc) },
    new Topic { Title = "TestA3", Category = "A", Timestamp = new DateTime(2022, 7, 20, 7, 0, 0, DateTimeKind.Utc) },
    new Topic { Title = "TestB1", Category = "B", Timestamp = new DateTime(2022, 7, 20, 11, 0, 0, DateTimeKind.Utc) },
    new Topic { Title = "TestB2", Category = "B", Timestamp = new DateTime(2022, 7, 20, 12, 0, 0, DateTimeKind.Utc) },
    new Topic { Title = "TestC1", Category = "C", Timestamp = new DateTime(2022, 7, 20, 12, 0, 0, DateTimeKind.Utc) },
    new Topic { Title = "TestC2", Category = "C", Timestamp = new DateTime(2022, 7, 20, 13, 0, 0, DateTimeKind.Utc) },
};

var topicSourceList = new SourceList<Topic>();
topicSourceList.AddRange(topics);
var latestTopicByCategoryObservable = topicSourceList.Connect()
                                                     .GroupOn(x => x.Category)
                                                     .TransformMany(x => x.List.Connect()
                                                                               .Sort(SortExpressionComparer<Topic>.Descending(y => y.Timestamp))
                                                                               .Top(1)
                                                                               .AsObservableList());
latestTopicByCategoryObservable.ToCollection().Subscribe(x =>
{
    Console.WriteLine($"Topic count: {x.Count}");
    foreach (var topic in x)
    {
        Console.WriteLine($"Tile: {topic.Title}, Category: {topic.Category}, Timestamp: {topic.Timestamp}");
    }
    Console.WriteLine();
});
Console.WriteLine("Clearing topics");
topicSourceList.Clear();
Console.WriteLine("Cleared topics");
Console.ReadLine();

However, the problem here is that I seem to get an update for each group cleared when executing topicSourcelist.Clear().
The above example produces the following output:

Topic count: 3
Tile: TestA1, Category: A, Timestamp: 20-07-2022 10:00:00
Tile: TestB2, Category: B, Timestamp: 20-07-2022 12:00:00
Tile: TestC2, Category: C, Timestamp: 20-07-2022 13:00:00

Clearing topics
Topic count: 2
Tile: TestB2, Category: B, Timestamp: 20-07-2022 12:00:00
Tile: TestC2, Category: C, Timestamp: 20-07-2022 13:00:00

Topic count: 1
Tile: TestC2, Category: C, Timestamp: 20-07-2022 13:00:00

Topic count: 0

Topic count: 0

Cleared topics

Is it possible to get a single update when executing topicSourcelist.Clear()?

[RolandPheasant]

The reason is you have a nested observable list and each one of these is producing an extra notification. You can achieve the desired result and simplify the code by using GroupWithImmutableState instead on GroupOn. The former produces an array of items instead of a nested observable cache for each group category.

var latestTopicByCategoryObservable = topicSourceList.Connect()
    .GroupWithImmutableState(x => x.Category)
    .Transform(grouping => grouping.Items.OrderByDescending(t => t.Timestamp).First());

latestTopicByCategoryObservable.Subscribe(x =>
{
    Console.WriteLine(x.TotalChanges);
});

The above code produces a single change set for Clear.

[Bulgrak]

That makes sense! Thank you for your explanation!