How can I see what my reactive extensions query is doing?

Question

I\'m writing a complex Reactive Extensions query with lots of operators. How can I see what\'s going on?

I\'m asking and answering this as it comes up a fair bit and

Answer 1

Another three years have passed, and I'm still using your idea. My version has now evolved as follows:

• Overload for choice of logging destination
• Log number of subscriptions
• Log 'downstream' exceptions from bad subscribers.

The code:

public static IObservable Spy(this IObservable source, string opName = null)
{
    return Spy(source, opName, Console.WriteLine);
}

public static IObservable Spy(this IObservable source, string opName, 
                                                              Action logger)
{
    opName = opName ?? "IObservable";
    logger($"{opName}: Observable obtained on Thread: {Thread.CurrentThread.ManagedThreadId}");

    var count = 0;
    return Observable.Create(obs =>
    {
        logger($"{opName}: Subscribed to on Thread: {Thread.CurrentThread.ManagedThreadId}");
        try
        {
            var subscription = source
                .Do(x => logger($"{opName}: OnNext({x}) on Thread: {Thread.CurrentThread.ManagedThreadId}"),
                    ex => logger($"{opName}: OnError({ex}) on Thread: {Thread.CurrentThread.ManagedThreadId}"),
                    () => logger($"{opName}: OnCompleted() on Thread: {Thread.CurrentThread.ManagedThreadId}")
                )
                .Subscribe(t =>
                {
                    try
                    {
                        obs.OnNext(t);
                    }
                    catch(Exception ex)
                    {
                        logger($"{opName}: Downstream exception ({ex}) on Thread: {Thread.CurrentThread.ManagedThreadId}");
                        throw;
                    }
                }, obs.OnError, obs.OnCompleted);

            return new CompositeDisposable(
                    Disposable.Create(() => logger($"{opName}: Dispose (Unsubscribe or Observable finished) on Thread: {Thread.CurrentThread.ManagedThreadId}")),
                    subscription,
                    Disposable.Create(() => Interlocked.Decrement(ref count)),
                    Disposable.Create(() => logger($"{opName}: Dispose (Unsubscribe or Observable finished) completed, {count} subscriptions"))
                );
        }
        finally
        {
            Interlocked.Increment(ref count);
            logger($"{opName}: Subscription completed, {count} subscriptions.");
        }
    });
}