How does one code an asynchronous WPF (or Windows Forms) event handler in F#? Specifically, is there any coding pattern that approximates C# 5's async and await?
Here is a complete C# WPF app:
using System;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
class Program
{
static int IncrementSlowly(int previous)
{
Thread.Sleep(3000);
if (previous == 2) throw new Exception("Oops!");
return previous + 1;
}
static async void btn_Click(object sender, RoutedEventArgs e)
{
var btn = sender as Button;
btn.IsEnabled = false;
try
{
var prev = (int)btn.Content;
btn.Content = await Task.Run(() => IncrementSlowly(prev));
}
catch (Exception ex)
{
btn.Content = ex.Message;
}
finally
{
btn.IsEnabled = true;
}
}
[STAThread]
static void Main(string[] args)
{
var btn = new Button() { Content = 0 };
var win = new Window() { Content = btn };
btn.Click += btn_Click;
new Application().Run(win);
}
}
I am having trouble figuring out what the equivalent would be using F#. I have made several attempts using combinations of async workflows and Async methods. It just gets really messy real fast. I'm hoping there is an easy way that I'm just overlooking.
Here is my starting point, which locks up the UI at btn.Content <- incrementSlowly prev
. What do I do next?
open System
open System.Threading
open System.Threading.Tasks
open System.Windows
open System.Windows.Controls
let incrementSlowly previous =
Thread.Sleep(3000)
if previous = 2 then failwith "Oops!"
previous + 1
let btn_Click (sender : obj) e =
let btn = sender :?> Button
btn.IsEnabled <- false
try
try
let prev = btn.Content :?> int
btn.Content <- incrementSlowly prev
with ex -> btn.Content <- ex.Message
finally
btn.IsEnabled <- true
[<EntryPoint>][<STAThread>]
let main _ =
let btn = new Button(Content = 0)
let win = new Window(Content = btn)
btn.Click.AddHandler(RoutedEventHandler(btn_Click))
Application().Run(win)
By the way, assume that incrementSlowly
cannot be modified.
The first step is to make incrementSlowly
asynchronous. This is actually synchronous in your C# code, which is probably not a good idea - in a realistic scenario, this could be communicating with network, so very often this can actually be asynchronous:
let incrementSlowly previous = async {
do! Async.Sleep(3000)
if previous = 2 then failwith "Oops!"
return previous + 1 }
Now, you can make the button click handler also asynchronous. We'll start it using Async.StartImmediate
later to make sure that we can access UI elements, so we do not have to worry about dispatechers or UI threads for now:
let btn_Click (sender : obj) e = async {
let btn = sender :?> Button
btn.IsEnabled <- false
try
try
let prev = btn.Content :?> int
let! next = incrementSlowly prev
btn.Content <- next
with ex -> btn.Content <- ex.Message
finally
btn.IsEnabled <- true }
The final step is to change the event registration. Something like this should do the trick:
btn.Click.Add(RoutedEventHandler(fun sender e ->
btn_Click sender e |> Async.StartImmediate)
The key thing is Async.StartImmediate
which starts the asynchronous workflow. When we call this on the UI thread, it ensures that all the actual work is done on the UI thread (unless you offload it explicitly to background) and so it is safe to access UI elements in your code.
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