Observable from callback method

Question

Let say I have a class which iherits legacy API and overrides a virtual method which is called when something happens

type MyClass() as this =
    let somethingObservable: IObservable<Something> = ...

    override _.OnSomething(s: Something) = ...

How can I translate each invokation of OnSomething to a notification of somethingObservable?

That's probably a simple question, but I could not find a way to do it properly (should I use not advised ISubject?). Appreciate your help.

Answer 1

Using a subject like this is fine, and ensures correctness in the implementation. Here's an example using FSharp.Control.Reactive, which gets you the idiomatic way of writing this.

type MyClass() =
    inherit Legacy()
    let somethingObservable = 
        Subject.broadcast

    override _.OnSomething s = 
        somethingObservable |> Subject.onNext s |> ignore

    member _.AsObservable = 
        somethingObservable |> Observable.asObservable

You can also use new Subject<_> and its methods, same thing. In some assembly if you'd prefer not to take on the System.Reactive dependency, F# also natively supports IObservable through events.

type MyClassEvt() =
    inherit Legacy()
    let event = new Event<_>()

    override _.OnSomething s = 
        event.Trigger s

    member _.AsObservable = 
        event.Publish :> IObservable<_>