public static class ObservableExtensions
{
    public static IDisposable RecordCacheItems<TObject, TKey>(
        this    IObservable<IChangeSet<TObject, TKey>>      source,
        out     CacheItemRecordingObserver<TObject, TKey>   observer,
                IScheduler?                                 scheduler = null);

    public static IDisposable RecordListItems<TObject, TKey>(
        this    IObservable<IChangeSet<TObject, TKey>>      source,
        out     ListItemRecordingObserver<TObject, TKey>    observer,
                IScheduler?                                 scheduler = null);

    public static IDisposable RecordValues<T>(
        this    IObservable<T>              source,
        out     ValueRecordingObserver<T>   observer,
                IScheduler?                 scheduler = null);

    public static IObservable<IChangeSet<T>> ValidateChangeSets<T>(this IObservable<IChangeSet<T>> source);

    public static IObservable<IChangeSet<TObject, TKey>> ValidateChangeSets<TObject, TKey>(
        this    IObservable<IChangeSet<TObject, TKey>>  source,
                Func<TObject, TKey>                     keySelector);

    public static IObservable<T> ValidateSynchronization<T>(this IObservable<T> source);
}

public abstract class RecordingObserverBase<T>
    : IObserver<T>
{
    public Exception? Error { get; }
    public bool HasCompleted { get; }
    public bool HasFinalized { get; }
    public IReadOnlyList<Recorded<Notification<T>>> Notifications { get; }
}

public sealed class ValueRecordingObserver<T>
    : RecordingObserverBase<T>
{
    public IReadOnlyList<T> RecordedValues { get; }
}

public sealed class CacheItemRecordingObserver<TObject, TKey>
        : RecordingObserverBase<IChangeSet<TObject, TKey>>
    where TObject : notnull
    where TKey : notnull
{
    public IReadOnlyList<IChangeSet<TObject, TKey>> RecordedChangeSets { get; }
    public IReadOnlyDictionary<TKey, TObject> RecordedItemsByKey { get; }
    public IReadOnlyList<TObject> RecordedItemsSorted { get; }
}

public sealed class ListItemRecordingObserver<T>
        : RecordingObserverBase<IChangeSet<T>>
    where T : notnull
{
    public IReadOnlyList<IChangeSet<T>> RecordedChangeSets { get; }
    public IReadOnlyList<T> RecordedItems { get; }
}