How to 'await' raising an EventHandler eve

Question

Sometimes the event pattern is used to raise events in MVVM applications by or a child viewmodel to send a message to its parent viewmodel in a loosely coupled way like this.

Parent ViewModel

searchWidgetViewModel.SearchRequest += (s,e) => 
{
    SearchOrders(searchWidgitViewModel.SearchCriteria);
};

SearchWidget ViewModel

public event EventHandler SearchRequest;

SearchCommand = new RelayCommand(() => {

    IsSearching = true;
    if (SearchRequest != null) 
    {
        SearchRequest(this, EventArgs.Empty);
    }
    IsSearching = false;
});

---

In refactoring my application for .NET4.5 I am making as much as code possible to use async and await. However the following doesn't work (well I really wasn't expecting it to)

 await SearchRequest(this, EventArgs.Empty);

The framework definitely does this to call event handlers such as this, but I'm not sure how it does it?

private async void button1_Click(object sender, RoutedEventArgs e)
{
   textBlock1.Text = "Click Started";
   await DoWork();
   textBlock2.Text = "Click Finished";
}

Anything I've found on the subject of raising events asynchrously is ancient but I can't find something in the framework to support this.

How can I await the calling of an event but remain on the UI thread.

Answer 1

Edit: This doesn't work well for multiple subscribers, so unless you only have one I wouldn't recommend using this.

---

Feels slightly hacky - but I have never found anything better:

Declare a delegate. This is identical to EventHandler but returns a task instead of void

public delegate Task AsyncEventHandler(object sender, EventArgs e);

You can then run the following and as long as the handler declared in the parent uses async and await properly then this will run asynchronously:

if (SearchRequest != null) 
{
    Debug.WriteLine("Starting...");
    await SearchRequest(this, EventArgs.Empty);
    Debug.WriteLine("Completed");
}

Sample handler :

 // declare handler for search request
 myViewModel.SearchRequest += async (s, e) =>
 {                    
     await SearchOrders();
 };

Note: I've never tested this with multiple subscribers and not sure how this will work - so if you need multiple subscribers then make sure to test it carefully.

Answer 2

To answer the direct question: I do not think EventHandler allows implementations to communicate sufficiently back to the invoker to allow proper awaiting. You might be able to perform tricks with a custom synchronization context, but if you care about waiting for the handlers, it is better that the handlers are able to return their Tasks back to the invoker. By making this part of the delegate’s signature, it is clearer that the delegate will be awaited.

I suggest using the Delgate.GetInvocationList() approach described in Ariel’s answer mixed with ideas from tzachs’s answer. Define your own AsyncEventHandler<TEventArgs> delegate which returns a Task. Then use an extension method to hide the complexity of invoking it correctly. I think this pattern makes sense if you want to execute a bunch of asynchronous event handlers and wait for their results.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

public delegate Task AsyncEventHandler<TEventArgs>(
    object sender,
    TEventArgs e)
    where TEventArgs : EventArgs;

public static class AsyncEventHandlerExtensions
{
    public static IEnumerable<AsyncEventHandler<TEventArgs>> GetHandlers<TEventArgs>(
        this AsyncEventHandler<TEventArgs> handler)
        where TEventArgs : EventArgs
        => handler.GetInvocationList().Cast<AsyncEventHandler<TEventArgs>>();

    public static Task InvokeAllAsync<TEventArgs>(
        this AsyncEventHandler<TEventArgs> handler,
        object sender,
        TEventArgs e)
        where TEventArgs : EventArgs
        => Task.WhenAll(
            handler.GetHandlers()
            .Select(handleAsync => handleAsync(sender, e)));
}

This allows you to create a normal .net-style event. Just subscribe to it as you normally would.

public event AsyncEventHandler<EventArgs> SomethingHappened;

public void SubscribeToMyOwnEventsForNoReason()
{
    SomethingHappened += async (sender, e) =>
    {
        SomethingSynchronous();
        // Safe to touch e here.
        await SomethingAsynchronousAsync();
        // No longer safe to touch e here (please understand
        // SynchronizationContext well before trying fancy things).
        SomeContinuation();
    };
}

Then simply remember to use the extension methods to invoke the event rather than invoking them directly. If you want more control in your invocation, you may use the GetHandlers() extension. For the more common case of waiting for all the handlers to complete, just use the convenience wrapper InvokeAllAsync(). In many patterns, events either don’t produce anything the caller is interested in or they communicate back to the caller by modifying the passed in EventArgs. (Note, if you can assume a synchronization context with dispatcher-style serialization, your event handlers may mutate the EventArgs safely within their synchronous blocks because the continuations will be marshaled onto the dispatcher thread. This will magically happen for you if, for example, you invoke and await the event from a UI thread in winforms or WPF. Otherwise, you may have to use locking when mutating EventArgs in case if any of your mutations happen in a continuation which gets run on the threadpool).

public async Task Run(string[] args)
{
    if (SomethingHappened != null)
        await SomethingHappened.InvokeAllAsync(this, EventArgs.Empty);
}

This gets you closer to something that looks like a normal event invocation, except that you have to use .InvokeAllAsync(). And, of course, you still have the normal issues that come with events such as needing to guard invocations for events with no subscribers to avoid a NullArgumentException.

Note that I am not using await SomethingHappened?.InvokeAllAsync(this, EventArgs.Empty) because await explodes on null. You could use the following call pattern if you want, but it can be argued that the parens are ugly and the if style is generally better for various reasons:

await (SomethingHappened?.InvokeAllAsync(this, EventArgs.Empty) ?? Task.CompletedTask);

Answer 3

Since delegates (and events are delegates) implement the Asynchronous Programming Model (APM), you could use the TaskFactory.FromAsync method. (See also Tasks and the Asynchronous Programming Model (APM).)

public event EventHandler SearchRequest;

public async Task SearchCommandAsync()
{
    IsSearching = true;
    if (SearchRequest != null)
    {
        await Task.Factory.FromAsync(SearchRequest.BeginInvoke, SearchRequest.EndInvoke, this, EventArgs.Empty, null);
    }
    IsSearching = false;
}

The above code, however, will invoke the event on a thread pool thread, i.e. it will not capture the current synchronization context. If this is a problem, you could modify it as follows:

public event EventHandler SearchRequest;

private delegate void OnSearchRequestDelegate(SynchronizationContext context);

private void OnSearchRequest(SynchronizationContext context)
{
    context.Send(state => SearchRequest(this, EventArgs.Empty), null);
}

public async Task SearchCommandAsync()
{
    IsSearching = true;
    if (SearchRequest != null)
    {
        var search = new OnSearchRequestDelegate(OnSearchRequest);
        await Task.Factory.FromAsync(search.BeginInvoke, search.EndInvoke, SynchronizationContext.Current, null);
    }
    IsSearching = false;
}

Answer 4

Based on Simon_Weaver's answer, I created a helper class that can handle multiple subscribers, and has a similar syntax to c# events.

public class AsyncEvent<TEventArgs> where TEventArgs : EventArgs
{
    private readonly List<Func<object, TEventArgs, Task>> invocationList;
    private readonly object locker;

    private AsyncEvent()
    {
        invocationList = new List<Func<object, TEventArgs, Task>>();
        locker = new object();
    }

    public static AsyncEvent<TEventArgs> operator +(
        AsyncEvent<TEventArgs> e, Func<object, TEventArgs, Task> callback)
    {
        if (callback == null) throw new NullReferenceException("callback is null");

        //Note: Thread safety issue- if two threads register to the same event (on the first time, i.e when it is null)
        //they could get a different instance, so whoever was first will be overridden.
        //A solution for that would be to switch to a public constructor and use it, but then we'll 'lose' the similar syntax to c# events             
        if (e == null) e = new AsyncEvent<TEventArgs>();

        lock (e.locker)
        {
            e.invocationList.Add(callback);
        }
        return e;
    }

    public static AsyncEvent<TEventArgs> operator -(
        AsyncEvent<TEventArgs> e, Func<object, TEventArgs, Task> callback)
    {
        if (callback == null) throw new NullReferenceException("callback is null");
        if (e == null) return null;

        lock (e.locker)
        {
            e.invocationList.Remove(callback);
        }
        return e;
    }

    public async Task InvokeAsync(object sender, TEventArgs eventArgs)
    {
        List<Func<object, TEventArgs, Task>> tmpInvocationList;
        lock (locker)
        {
            tmpInvocationList = new List<Func<object, TEventArgs, Task>>(invocationList);
        }

        foreach (var callback in tmpInvocationList)
        {
            //Assuming we want a serial invocation, for a parallel invocation we can use Task.WhenAll instead
            await callback(sender, eventArgs);
        }
    }
}

To use it, you declare it in your class, for example:

public AsyncEvent<EventArgs> SearchRequest;

To subscribe an event handler, you'll use the familiar syntax (the same as in Simon_Weaver's answer):

myViewModel.SearchRequest += async (s, e) =>
{                    
   await SearchOrders();
};

To invoke the event, use the same pattern we use for c# events (only with InvokeAsync):

var eventTmp = SearchRequest;
if (eventTmp != null)
{
   await eventTmp.InvokeAsync(sender, eventArgs);
}

If using c# 6, one should be able to use the null conditional operator and write this instead:

await (SearchRequest?.InvokeAsync(sender, eventArgs) ?? Task.CompletedTask);

Answer 5

I'm not clear on what you mean by "How can I await the calling of an event but remain on the UI thread". Do you want the event handler to be executed on the UI thread? If that's the case then you can do something like this:

var h = SomeEvent;
if (h != null)
{
    await Task.Factory.StartNew(() => h(this, EventArgs.Empty),
        Task.Factory.CancellationToken,
        Task.Factory.CreationOptions,
        TaskScheduler.FromCurrentSynchronizationContext());
}

Which wraps the invocation of the handler in a Task object so that you can use await, since you can't use await with a void method--which is where your compile error stems from.

But, I'm not sure what benefit you expect to get out of that.

I think there's a fundamental design issue there. It's fine to kick of some background work on a click event and you can implement something that supports await. But, what's the effect on how the UI can be used? e.g. if you have a Click handler that kicks off an operation that takes 2 seconds, do you want the user to be able to click that button while the operation is pending? Cancellation and timeout are additional complexities. I think much more understanding of the usability aspects needs to be done here.

Answer 6

public static class FileProcessEventHandlerExtensions
{
    public static Task InvokeAsync(this FileProcessEventHandler handler, object sender, FileProcessStatusEventArgs args)
     => Task.WhenAll(handler.GetInvocationList()
                            .Cast<FileProcessEventHandler>()
                            .Select(h => h(sender, args))
                            .ToArray());
}