vb.net - 将 Async CTP Await 与事件一起使用？

Question

与 Silverlight 5/Async CTP 相关

我想创建一个异步函数来启动布局更新，然后等待布局更新完成。就像是:

    Private Async Function UpdateLayoutRoot() As Task
       LayoutRoot.UpdateLayout()
       Await LayoutRoot.LayoutUpdated  <--- (NOT valid but shows desired outcome)           
    End Function

如何才能做到这一点？更一般地说，您如何使用 Await 来等待现有事件？

Answer 1

实现此目的的一种方法是等待 TaskCompletionSource这是在事件内设置的。我不知道 VB.NET，希望你能从 C# 理解它:

// 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;
}