Using Async CTP Await with events? - vb.net

Question

Relevant to Silverlight 5 / Async CTP
I want to create an asynchronous function that initiates a layout update and then Awaits for the layout update to complete. Something like:
Private Async Function UpdateLayoutRoot() As Task
LayoutRoot.UpdateLayout()
Await LayoutRoot.LayoutUpdated <--- (NOT valid but shows desired outcome)
End Function
How can this be done? More generally, how can you use Await to wait for existing events?

Answer 1

One way to accomplish this is to await on a TaskCompletionSource that is set inside the event. I don't know VB.NET, hopefully you can understand it from C#:
// The type and value returned by the TaskCompletionSource
// doesn't matter, so I just picked int.
TaskCompletionSource<int> tcs = new TaskCompletionSource<int>();
// The delegate sets the TaskCompletionSource -- the result value
// doesn't matter, we only care about setting it. Keep hold of
// the delegate so it can be removed later.
EventHandler d = (o, e) => { tcs.TrySetResult(1); };
LayoutRoot.LayoutUpdate += d;
try
{
LayoutRoot.UpdateLayout();
await tcs.Task;
}
finally
{
// Don't leak the delegate!
LayoutRoot.LayoutUpdate -= d;
}

Answer 2

Thanks Cory! Your suggestion to use TaskCompletionSource is just what I needed. I've combined the use of a TaskCompletionSource with the Lucian Wischik's Async CTP specification to develop a pair of generic Silverlight 5 classes that can be used to Await any CLR, or routed event. Only the Async CTP (AsyncCtpLibrary_Silverlight) is required (The formidable Rx library is not needed). Here are the two classes:
Public Class AwaitableEvent(Of TResult)
Private eta As EventTaskAwaiter(Of TResult) = Nothing
Sub New(ByVal Sender As Object, ByVal EventName As String)
eta = New EventTaskAwaiter(Of TResult)
Dim ei as EventInfo = Sender.GetType.GetEvent(EventName)
Dim d = [Delegate].CreateDelegate(ei.EventHandlerType,
Me, "EventCompletedHandler", True, True)
ei.AddEventHandler(Sender, d)
End Sub
Public Function GetAwaiter() As EventTaskAwaiter(Of TResult)
Return eta
End Function
Private Sub EventCompletedHandler(ByVal sender As Object, ByVal e As TResult)
eta.tcs.TrySetResult(e)
End Sub
End Class
Public Class EventTaskAwaiter(Of TResult)
Friend tcs As New TaskCompletionSource(Of TResult)
Public ReadOnly Property IsCompleted As Boolean
Get
Return tcs.Task.IsCompleted
End Get
End Property
Sub OnCompleted(r As Action)
Dim sc = SynchronizationContext.Current
If sc Is Nothing Then
tcs.Task.ContinueWith(Sub() r())
Else
tcs.Task.ContinueWith(Sub() sc.Post(Sub() r(), Nothing))
End If
End Sub
Function GetResult() As TResult
If tcs.Task.IsCanceled Then Throw New TaskCanceledException(tcs.Task)
If tcs.Task.IsFaulted Then Throw tcs.Task.Exception.InnerException
Return tcs.Task.Result
End Function
End Class
Here's an example of using the AwaitableEvent class to Await mouse, keyboard and timer events.
Private Sub AECaller()
GetMouseButtonAsync()
MessageBox.Show("After Await mouse button event")
GetKeyAsync()
MessageBox.Show("After Await key event")
GetTimerAsync()
MessageBox.Show("After Await timer")
End Sub
Private Async Sub GetMouseButtonAsync()
Dim ae As New AwaitableEvent(Of MouseButtonEventArgs)(LayoutRoot, "MouseLeftButtonDown")
Dim e = Await ae
MessageBox.Show(String.Format("Clicked {0} at {1},{2}",
e.OriginalSource.ToString,
e.GetPosition(LayoutRoot).X,
e.GetPosition(LayoutRoot).Y))
End Sub
Private Async Sub GetKeyAsync()
Dim ae As New AwaitableEvent(Of KeyEventArgs)(LayoutRoot, "KeyDown")
Dim e = Await ae
MessageBox.Show(String.Format("Key {0} was pressed", e.Key.ToString))
End Sub
Private Async Sub GetTimerAsync()
Dim StopWatch As New DispatcherTimer
StopWatch.Interval = New TimeSpan(TimeSpan.TicksPerSecond * 6)
Dim ae As New AwaitableEvent(Of EventArgs)(StopWatch, "Tick")
StopWatch.Start()
Await ae
MessageBox.Show(String.Format("It's {0}seconds later!", StopWatch.Interval.TotalSeconds))
StopWatch.Stop()
End Sub
As expected the Await statement returns control to the calling function immediately. When the events are subsequently completed, Await assigns the result (the event args appropriate for the event being monitored) and the remaining code in the asynchronous method is then run.