Question

I am working with an observable byte stream, coming off of the network, and I would like to take that up one layer of abstraction. The format has two bytes that contain the length of the next message. I’d like to make this fit into the reactive framework pretty well. What I have so far feels not quite right, so I am wondering what tricks I may have missed to eliminate the while loop here.

Here’s the concept I have in mind:

public static IObservable<Stream> GetToplevelStreams(IObservable<byte> byteStreamArg) {
    return Observable.Create((IObserver<Stream>o)=>{
        bool done = false;
        var byteStream = byteStreamArg.Do(
            b => { }, (ex) => { done = true; }, () => { done = true; });
        while (!done)
        {
            var size = byteStream.Take(2).
                           Aggregate(0, (n, b) => (n << 8) + b).Single();
            var buf = byteStream.Skip(2).Take(size);
            var stream = new MemoryStream(buf.ToEnumerable().ToArray());
            if (!done)
            {
                o.OnNext(stream);
            }
        }
        return (() => {});
    });
}

Answer 1

An IObservable is a bit weird here – remember that you’re returning a “Future List of Streams” – I’d actually just return a Stream, or perhaps an IObservable<byte[]>, where each array represents a message. Or do even better, and return an IObservable<ParsedMessage>

Also, your While loop makes this non-async and act strangely. How about something more like this:

public static IObservable<System.IO.Stream> GetToplevelStreams(IObservable<byte> byteStream)
{
    return Observable.Create((IObserver<System.IO.Stream> o) =>
    {
        int? size1=null;
        int? size=null;
        var buf = new MemoryStream();
        var subscription = byteStream.Subscribe(v =>
        {
            if (!size1.HasValue)
            {
                size1 = ((int)v) << 8;
            }
            else if (!size.HasValue)
            {
                size = size1.Value + v;
            }
            else
            {
                buf.WriteByte(v);
            }
            if (size.HasValue && buf.Length == size)
            {
                buf.Position = 0;
                o.OnNext(buf);
                buf.SetLength(0);
                size1 = null;
                size = null;
            }

        }, (ex)=>o.OnError(ex), ()=>o.OnCompleted());
        return () => subscription.Dispose();
    });
}