ReactiveUI - DynamicData switching between streams to a single ReadOnlyObservableCollection - android-recyclerview

Question

I have a ReadOnlyObservableCollection<ItemViewModel> that I want to toggle between showing all the items and limiting to top 2. I have the following sample code that works.
public class MainViewModel : ReactiveObject
{
private readonly SourceCache<ItemViewModel, string> _sourceCache = new SourceCache<ItemViewModel, string>(x => x.Text);
private readonly BehaviorSubject<bool> _toggleSubject = new BehaviorSubject<bool>(false);
public ReadOnlyObservableCollection<ItemViewModel> ObsCollection;
public ReactiveCommand<Unit, Unit> ToggleCommand { get; }
public MainViewModel()
{
ToggleCommand = ReactiveCommand.Create(() =>
{
_toggleSubject.OnNext(!_toggleSubject.Value);
});
_sourceCache.AddOrUpdate(new ItemViewModel() { Text = "a" });
_sourceCache.AddOrUpdate(new ItemViewModel() { Text = "c" });
_sourceCache.AddOrUpdate(new ItemViewModel() { Text = "b" });
IComparer<ItemViewModel> comparer = Comparer<ItemViewModel>.Create((x, y) => x.Text.CompareTo(y.Text));
var observableChanges = _sourceCache.Connect();
var limitedStream = _toggleSubject.Where(x => !x).Select(_ => observableChanges.Top(comparer: comparer, size: 2));
var fullStream = _toggleSubject.Where(x => x).Select(_ => observableChanges.Sort(comparer));
Observable.Merge(limitedStream, fullStream)
.Switch()
.Bind(out ObsCollection)
.Subscribe();
}
}
Using the ToggleCommand, the ObsCollection emits the expected result. But when I try to observe the ChangeSets (ViewModel.ObsCollection.ToObservableChangeSet()) in a ReactiveRecyclerViewAdapter<ItemViewModel> the wrong results are displayed on toggle. Instead of displaying a,b,c the list displays a,b,b. Debuging through the implementation of ReactiveRecyclerViewAdapter I see that the change.Range.Index is set to -1 instead of what I expected 0. I think that is why the last value is not updated.
case ListChangeReason.AddRange:
NotifyItemRangeInserted(change.Range.Index, change.Range.Count);
break;
case ListChangeReason.RemoveRange:
case ListChangeReason.Clear:
NotifyItemRangeRemoved(change.Range.Index, change.Range.Count);
break;
Why would the any change start at index -1? Is this type of implementation valid. I have the full project to reproduce this.

Answer 1

Combining change sets in dynamic data using the rx merge operator will not work. This is because there is no way for subsequent operators to understand that the changes come from different sources which leads to unpredictable results.
Instead dynamic data provides it's own operators which enables various means to join changes sets. Examples of this are the And and Or operators which apply a logical join across the entire collection. However in this case we are swapping the result out so correct way is to use the Dynamic Data overload of Switch which unloads previous results and loads the new results when the toggle is fired.
var toggled = _toggleSubject
//when the toggle subject changes select top values or all accordingly
.Select(b => b ? observableChanges.Top(comparer: comparer, size: 2) : observableChanges.Sort(comparer))
//this is a dynamic data overload of switch which flattens the nested observable
.Switch();