How to properly observe non-standard events?

Question

I am new to Reactive Extensions, and dealing with a COM Library that has events defined like this:

public delegate void MyDelegate(int requestId, double price, int amount);
public event MyDelegate MyEvent;

How do I properly observe this? I tried using Observable.FromEvent() but as the event's parameters are not of type EventArgs I don't see how FromEvent() or FromEventPattern() is going to work.

My current workaround is to attach a custom delegate to the event then invoke a Subject.OnNext() but I am guessing that's not how I should do it.

Here's an example of my current workaround:

        MyEvent += new MyDelegate((int requestId, double price, int amount) =>
        {
            Task.Run(() =>
            {
                var args = new MyArgs()
                {
                    requestId = requestId,
                    price = price,
                    amount = amount,
                };
                this.mySubject.OnNext(args);
            });
        });

score 4 · Accepted Answer · answered Aug 08 '15 at 20:20

There is a special overload of FromEvent for it. It is a little goofy to get your head around but the function signature looks like:

IObservable<TEventArgs> FromEvent<TDelegate, TEventArgs>(Func<Action<TEventArgs>, TDelegate> conversion, 
                                                         Action<TDelegate> addHandler, 
                                                         Action<TDelegate> removeHandler);

The conversion function is the important part here, basically you are telling Rx how your delegate maps to a concrete type.

In your scenario it ends up looking something like this:

Observable.FromEvent<MyDelegate, MyArgs>(
  converter => new MyDelegate(
                  (id, price, amount) => converter(new MyArgs { 
                                                        RequestId = id, 
                                                        Price = price, 
                                                        Amount = amount
                                                       })
               ),
  handler => MyEvent += handler,
  handler => MyEvent -= handler);

So what is all this doing? Internally, it is similar to what you are doing (I'll paraphrase what it does conceptually, since the implementation is slightly more complicated). When a new subscription is made, the conversion function will be invoked with observer.OnNext passed in as the converter argument. This lambda will return a new MyDelegate instance that wraps the conversion function that we provided ((id, price, amount) => ...). This is what is then passed to the handler => MyEvent += handler method.

After that each time the event is fired it will call our lambda method and convert the passed arguments into an instance of MyArgs which is then delivered to converter/observer.OnNext.

In addition, to all that magic it will also take care of cleaning up the event handlers when you are done with it, gracefully hand exceptions down stream and will manage the memory over head by sharing a single event handler across multiple observers.

Source code

How to properly observe non-standard events?

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