How to 'await' raising an EventHandler event

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.