Add Async.executeOnMainThread

[TimLariviere]

Fixes #946

By default, Cmds in Fabulous 2.0 are not executed on the main UI thread for performance reasons. If you need to execute some logic on the UI thread, you can use the new Async.executeOnMainThread.

let showAlertCmd =
    async {
        do! Application.Current.MainPage.DisplayAlert("An alert!", "OK")
        return AlertShown
    }
    |> Async.executeOnMainThread
    |> Cmd.ofAsyncMsg

[TimLariviere]

@kevkov I think the issue you found is related to the fact Task is executed as soon as it is created while Async waits for Fabulous to execute it.

So doing this was not working:

let task = (...) // the task is executed immediately on the current thread

task
|> Async.AwaitTask // convert the running task into Async
|> Async.executeOnMainThread // ask to execute the return of the task into the UI Thread

Instead you need to put the task inside an async computation

async {
    let! result = (...) // the task. Will only be executed when the parent async comp. is executed
}
|> Async.executeOnMainThread

[edgarfgp]

Maybe a little overkill but . What do you think about a ce ?

  type MainThreadBuilder() =
        member this.Zero() =
            printf "Zero"
            ()

        member this.Delay(f) = 
            DispatchQueue.MainQueue.DispatchAsync(fun _ ->  f())

    let mainThread = MainThreadBuilder()

   mainThread {
   // Code goes here 
  } 

[twop]

@edgarfgp I think it might be an overkill, CE are awesome when they provide a unique ergonomic value (e.g. hard to express intent otherwise) and should be use sparingly (IMHO). I think the main concern I have is when you have multiple different CE within the same code block, that does introduce significant overhead to parse the intent. Again, "in my humble opinion".

[TimLariviere]

I agree with @twop. Having a CE for this wouldn't make much sense and would confuse users. An helper function is enough here.