Question

So I’m attempting to use reactives to recompose chunked messages identified by ID and am having a problem terminating the final observable. I have a Message class which consists of Id, Total Size, Payload, Chunk Number and Type and have the following client-side code:

I need to calculate the number of messages to Take at runtime

(from messages in
   (from messageArgs in Receive select Serializer.Deserialize<Message>(new MemoryStream(Encoding.UTF8.GetBytes(messageArgs.Message))))
 group messages by messages.Id into grouped select grouped)
.Subscribe(g =>
{
    var cache = new List<Message>();
    g.TakeWhile((int) Math.Ceiling(MaxPayload/g.First().Size) < cache.Count)
      .Subscribe(cache.Add, 
    _ => { /* Rebuild Message Parts From Cache */ });
});

First I create a grouped observable filtering messages by their unique ID and then I am trying to cache all messages in each group until I have collected them all, then I sort them and put them together. The above seems to block on g.First().

I need a way to calculate the number to take from the first (or any) of the messages that come through however am having difficulty doing so. Any help?

Answer 1

First is a blocking operator (how else can it return T and not IObservable<T>?)

I think using Scan (which builds an aggregate over time) could be what you need. Using Scan, you can hide the “state” of your message re-construction in a “builder” object.

MessageBuilder.IsComplete returns true when all the size of messages it has received reaches MaxPayload (or whatever your requirements are). MessageBuilder.Build() then returns the reconstructed message.

I’ve also moved your “message building” code into a SelectMany, which keeps the built messages within the monad.

(Apologies for reformatting the code into extension methods, I find it difficult to read/write mixed LINQ syntax)

Receive
    .Select(messageArgs => Serializer.Deserialize<Message>(
        new MemoryStream(Encoding.UTF8.GetBytes(messageArgs.Message))))
    .GroupBy(message => message.Id)
    .SelectMany(group =>
    {
        // Use the builder to "add" message parts to
        return group.Scan(new MessageBuilder(), (builder, messagePart) =>
        {
            builder.AddPart(messagePart);

            return builder;
        })
        .SkipWhile(builder => !builder.IsComplete)
        .Select(builder => builder.Build());
    })
    .Subscribe(OnMessageReceived);