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

Events don’t mesh perfectly with async and await, as you’ve discovered.

The way UIs handle async events is different than what you’re trying to do. The UI provides a SynchronizationContext to its async events, enabling them to resume on the UI thread. It does not ever "await" them.

Best Solution (IMO)

I think the best option is to build your own async-friendly pub/sub system, using AsyncCountdownEvent to know when all handlers have completed.

Lesser Solution #1

async void methods do notify their SynchronizationContext when they start and finish (by incrementing/decrementing the count of asynchronous operations). All UI SynchronizationContexts ignore these notifications, but you could build a wrapper that tracks it and returns when the count is zero.

Here’s an example, using AsyncContext from my AsyncEx library:

SearchCommand = new RelayCommand(() => {
  IsSearching = true;
  if (SearchRequest != null) 
  {
    AsyncContext.Run(() => SearchRequest(this, EventArgs.Empty));
  }
  IsSearching = false;
});

However, in this example the UI thread is not pumping messages while it’s in Run.

Lesser Solution #2

You could also make your own SynchronizationContext based on a nested Dispatcher frame that pops itself when the count of asynchronous operations reaches zero. However, you then introduce re-entrancy problems; DoEvents was left out of WPF on purpose.