Serial communication using Reactive Extensions

The main purpose of Reactive Extensions (Rx) is to enable processing event streams. I set out to use Reactive Extensions for receiving data on the serial port and learned some new things.

Here is the code that uses Observable.FromEventPattern<T>() to create an IObservable<T> from the .NET event SerialPort.DataReceivedEvent:

The event does not actually contain any information on the data received, it only indicates that there is data available. Reading the data is done inside the lambda expression. Reading serial data will return a list of bytes. This list may contain a complete message or just a part of a message or even multiple messages. To handle this, I want the observable to be an IObservable<byte>, i.e., it will produce a raw stream of bytes without any indication of where a message begins or ends. This is done through the extension method public static IObservable<TResult> SelectMany<TSource, TResult>(this IObservable<TSource> source, Func<TSource, IEnumerable<TResult>> selector) that is used to flatten the sequence returned by the lambda.

So I now have a stream of bytes. I want these bytes to be chunked into messages. For my particular protocol, messages are separated by a special byte. Separation can be done in two ways:

Here, a new observable is created using Observable.Create(). This observable subscribes to the byte stream, collects the data in a local collection and fires OnNext() whenever a message delimiter is encountered.

This version uses the Scan() operator to achieve the same thing. The output is an IObservable<IEnumerable<byte>> that fires an IEnumerable<byte> for every new message.

This code worked well up until the point I started attaching multiple observers to the message stream, one to process the messages and one to just dump received messages to a debug console. What happened then was that the code in the first code sample was called multiple times: once for each subscriber. This meant that each chunk of serial data was only received by one subscriber, not all subscribers. There are two possible solutions to this: Either introduce a Subject<IEnumerable<byte>> subscribing to serialPortSource and have consumers subscribe to the subject or use the Publish() operator that does the work for you.

Creating a new observable that produces deserialized messages from the observable producing lists of bytes is now trivial using a simple Select().

What remains is the question of how to use the received data in a typical workflow of sending out a message and receiving a response in return. Here is an example:

This example uses the Replay() operator. Replay will capture all events from the observable that are fired after the call to Connect(). After calling Connect() the call is sent to the device at the other end of the serial connection. The second await filters the incoming messages for the desired message (even using a filter criterion that was not known before the request was sent), adds a timeout, uses FirstAsync() to return an observable that only returns the first element followed by OnCompleted(), and waits for that OnCompleted() using await. Since Replay() is capturing all messages, the following await call on the observable should consider all answers from the target, whether they are received before or after the second call to await.

2 thoughts on “Serial communication using Reactive Extensions”

  1. Hi Kerry,

    I’ve only been able to get the fragments you provided above to something around half way working.
    For the communication I’m having with the serial port, I can have the following two main scenarios:
    1. Send a single command – Receive a corresponding response.
    2. Send a single command – Receive n number of asynchronous ‘interim’ messages, then the corresponding response.
    Obviously scenario 2 looked like a good fit for RX, but I don’t think that’s how you’ve done things. Is that correct? Could you give me any pointers.
    How did you fire OnComplete to indicate that you’d received all of the data you were expecting?
    Lastly, SerialPort.DataReceived always fires on a different thread from the one used to send the data – how did you work around this?

    Regards

    1. It seems like a good fit.

      The observable produced by ToMessage() will spit out new messages infinitely. I used FirstAsync() to turn it into an observable that will only produce one message and then call OnCompleted(). You can use the Where() before that to only trigger on the final message you are expecting (as I did in the example). This is assuming you don’t need the information in the interim messages in this place.

      await will return the message before OnCompleted(), which is what you want here. It will also marshal the response received on a different thread into the context of the awaiting method.

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