[Feature]: Group Operator with a Dynamic Grouping Key Selector

[dwcullop]

Describe the functionality desired

A Grouping operator that supports the Group Key Selector changing dynamically, such as:

IObservable<IGroupChangeSet<TObject, TKey, TGroupKey>> Group(
    this IObservable<IChangeSet<TObject, TKey>> source,
    IObservable<Func<TObject, TKey, TGroupKey>> grouperObservable);

// Also maybe
IObservable<IGroupChangeSet<TObject, TKey, TGroupKey>> Group(
    this IObservable<IChangeSet<TObject, TKey>> source,
    IObservable<Func<TObject, TKey, TGroupKey>> grouperObservable,
    IObservable<Unit> regrouper);

The steps the functionality will provide

When groupObservable fires, it will regroup all the items according to the new Grouping Key selector function.

If regrouper is provided, it will re-evaluate the grouping using the current Grouping Key.

Considerations

Current Group operators don't support changing of the Group Key selector. The Group Key selector can be dynamically re-applied, but it can't be changed.

IObservable<IGroupChangeSet<TObject, TKey, TGroupKey>> Group<TObject, TKey, TGroupKey>(
this IObservable<IChangeSet<TObject, TKey>> source, Func<TObject, TGroupKey> groupSelectorKey, IObservable<Unit> regrouper);

GroupOnObservable allows for dynamic changing of the Group Key Value but on a per item basis.

[github-actions]

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