Rx framework observable and callbacks

[UnfitElf]

Hi all,

I am having somewhat of a hard time diving into the Rx framework. I have a specific, simple, problem that I would like to solve, but getting a clear example is proving difficult. I can only assume I am going about things wrong and missing something.

I have a class (call is 'A') that processes data based on an event. I want to provide a mechanism whereby class 'A' can inform an observer when it has done some processing. In the following example an observer would implement IMyCallback and be notified.

public class A
{
  private readonly IMyCallback _myCallback;
  public A(IMyCallback callback)
  {
    _myCallback = null;
  }

  private OnSomeExternallyTriggeredEvent()
  {
    DoSomeProcessing();
  }

  private DoSomeProcessing()
  {
    byte[] bytes = new bytes[128];
    //.. 
    _myCallback?.OnDataRecv(bytes);
  }
}

As an alternative class 'A' could have an event and the observer could subscribe to it. If I was to run with an event based approach, from my understanding so far class 'A' could almost directly replace this by using a subject.

public class A
{
  private readonly Subject<byte[]> _subject = new Subject<byte[]>();

  public IObservable<byte[]> Values{ get { return _doValues.AsObservable(); } }

  //... etc
}

Following on from this, there is a lot of talk about not using subjects but what further confused me was this from another question

If "The system raises an event or a call back when a message arrives... we just wrap it with the appropriate FromEvent method and we are done. To the Pub!"

Once again, if I was using events I could see how to use FromEvent. What is confusing me if the "or a call back". In the class 'A' example above I am using a callback, but I cannot determine how I would use FromEvent..

Am I complicating things, should I simply use Subjects?

Note: All code is under my control and can be changed if needed.

Best,
Dan