Question: SkiaScroll - how to maintain the ScrollPosition when reloading Items?

[bnSonic]

I'm trying to replace a CollectionView with the SkiaScroll and have a problem that the SkiaScroll is not maintaining its scroll position when Items being refreshed/reloaded.
I've searched the Demo Code and Dokumentation but haven't found a hint if this can be solved and how. Maybe I was searching for the wrong keywords?!

I have a ObservableCollection<IMyData> Items in a ViewModel that I use with {Binding Items}
(See the XAML part of my TestPage below)

This works.

But I need to reload my Items and whenever I reload it, the SkiaScroll jumps to the top 😔
How can I make it, that the SkiaScroll holds its scrolling position when the Items are filled again? (the CollectionView has a property for that and the ListView remembers its scrolling position too)

if someone could provide me a short how to / demo code how to do this correctly - that would be wonderful 😊❤️

Currently I'm using this to refill the Items:

// -- BulkObservableCollection to be able to clear + fill the collection without UI refresh after the .Clear (useful for CollectionView / ListView)
public class BulkObservableCollection<T> : ObservableCollection<T>
{
    private bool _suppressNotification = false;

    public BulkObservableCollection() : base()
    {

    }
    public BulkObservableCollection(IEnumerable<T> collection) : base(new List<T>(collection ?? throw new ArgumentNullException(nameof(collection))))
    {

    }

    public void AddRange(IEnumerable<T> items, bool clearBeforeAdding = false)
    {
        _suppressNotification = true;
        try
        {
            if (clearBeforeAdding)
                Clear();

            if (items == null)
                return;

            foreach (var item in items)
                Add(item);
        }
        finally
        {
            _suppressNotification = false;
            OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
        }
    }


    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        if (!_suppressNotification)
            base.OnCollectionChanged(e);
    }
}

And then in my ViewModel:

public BulkObservableCollection<IMyData> Items { get; set; } = new();
...
    public void RefreshItems()
    {
        var newItems = new List<IMyData>();

        //when this function is called, Data is filled with new ViewModels from a Data Source like Database or Web etc.
        foreach (var i in Data)
        {
            if (i.IsVisible)
                newItems.Add(i);
        }

        Items.AddRange(newItems, clearBeforeAdding: true); //is making a .clear() once and .add() for each item with a refresh at the end
    }

This is the Canvas Part of my TestPage so far

        <draw:Canvas
            Gestures="Enabled"
            RenderingMode = "Accelerated"
            BackgroundColor="WhiteSmoke"
            HorizontalOptions="Fill"
            VerticalOptions="Start"
            HeightRequest="500">

            <draw:SkiaLayout
                HorizontalOptions="Fill"
                VerticalOptions="Fill">

                <draw:SkiaScroll
                    Orientation="Vertical"
                    FrictionScrolled="0.4"
                    IgnoreWrongDirection="True"
                    HorizontalOptions="Fill"
                    VerticalOptions="Fill"
                    Bounces="True"
                    >

                    <draw:SkiaLayout
                        HorizontalOptions="Fill"
                        MeasureItemsStrategy="MeasureAll"
                        ItemsSource="{Binding Items}"
                        RecyclingTemplate="Enabled"
                        Type="Column"
                        Spacing="0">
                        
                        <draw:SkiaLayout.ItemTemplate>
                            <DataTemplate>
                                <cells:MyCellTemplate />
                            </DataTemplate>
                        </draw:SkiaLayout.ItemTemplate>

                    </draw:SkiaLayout>
                    
                </draw:SkiaScroll>
            </draw:SkiaLayout>

        </draw:Canvas>

[taublast]

Hi!

The drawn scroll always keeps its position, from what i can fast-remember it would reset it only if:

• observable collection sent NotifyCollectionChangedAction.Reset - your specific case, this is what causing this.
• scroll property ResetScrollPositionOnContentSizeChanged was set to True (not default) and content size changed

You could refresh the recycled scroll without changing position:

• call myCellsStack.ApplyItemsSource(); with latest preview nuget (xxx.9-pre) - just refresh all, same source
• Difference source: Observablecollection should send Replace instead of Reset
• Different source: change ItemSource to another reference

[bnSonic]

What the … 🤩 Thank you!

For the record if someone is finding this:

In my RefreshItems Instead of:

Items.AddRange(newItems, clearBeforeAdding: true);

I'm doing this:

        Items = new BulkObservableCollection<IMyData>(newItems);
        OnPropertyChanged(nameof(Items));

(I could be a regular ObservableCollection now because no need for the bulk-update-workaround anymore)

Now the scrolling position doesn't change a single Pixel 👍