How do I subscribe to an event using task expressions?

Question

I have a function doSomething : EventArgs -> Task<Unit>, an event source Source and an event Source.OnChanged. How can I subscribe doSomething to Source.OnChanged? In C# we would use an async event handler but this doesn't seem possible in F#.

Answer 1

I think this does what you want:

let source = Source()
source.OnChanged.Add(
    doSomething
        >> Async.AwaitTask
        >> Async.StartImmediate)

Full example:

type Source() =

    let event = Event<int>()

    [<CLIEvent>]
    member _.OnChanged = event.Publish

    member _.Trigger(arg) = event.Trigger(arg)

let doSomething arg =
    task {
        printfn $"Doing something with {arg}"
    }

let source = Source()
source.OnChanged.Add(
    doSomething
        >> Async.AwaitTask
        >> Async.StartImmediate)

source.Trigger(0)

Output is:

Doing something with 0

See this similar SO question for more details.