Question

For an Rx based change tracking solution I am in need of an operator which can get me the first and most recent item in an observable sequence.

How would I write an Rx operator that produces the following marble diagram (Note: the brackets are used just to lineup the items…I’m not sure how best to represent this in text):

     xs:---[a  ]---[b  ]-----[c  ]-----[d  ]---------|
desired:---[a,a]---[a,b]-----[a,c]-----[a,d]---------| 

Answer 1

Using the same naming as @Wilka you can use the below extension which is somewhat self-explanatory:

public static IObservable<TResult> FirstAndLatest<T, TResult>(this IObservable<T> source, Func<T,T,TResult> func)
{
    var published = source.Publish().RefCount();
    var first = published.Take(1);        
    return first.CombineLatest(published, func);
}

Note that it doesn’t necessarily return a Tuple, but rather gives you the option of passing a selector function on the result. This keeps it in line with the underlying primary operation (CombineLatest). This is obviously easily changed.

Usage (if you want Tuples in the resulting stream):

Observable.Interval(TimeSpan.FromSeconds(0.1))
          .FirstAndLatest((a,b) => Tuple.Create(a,b))
          .Subscribe(Console.WriteLine);