Is it possible to await an event instead of another async method?

Question

In my C#/XAML metro app, there's a button which kicks off a long-running process. So, as recommended, I'm using async/await to make sure the UI thread doesn't get blocked:

private async void Button_Click_1(object sender, RoutedEventArgs e)  {      await GetResults(); }  private async Task GetResults() {       // Do lot of complex stuff that takes a long time      // (e.g. contact some web services)   ... } 

Occasionally, the stuff happening within GetResults would require additional user input before it can continue. For simplicity, let's say the user just has to click a "continue" button.

My question is: how can I suspend the execution of GetResults in such a way that it awaits an event such as the click of another button?

Here's an ugly way to achieve what I'm looking for: the event handler for the continue" button sets a flag...

private bool _continue = false; private void buttonContinue_Click(object sender, RoutedEventArgs e) {     _continue = true; } 

... and GetResults periodically polls it:

 buttonContinue.Visibility = Visibility.Visible;  while (!_continue) await Task.Delay(100);  // poll _continue every 100ms  buttonContinue.Visibility = Visibility.Collapsed; 

The polling is clearly terrible (busy waiting / waste of cycles) and I'm looking for something event-based.

Any ideas?

Btw in this simplified example, one solution would be of course to split up GetResults() into two parts, invoke the first part from the start button and the second part from the continue button. In reality, the stuff happening in GetResults is more complex and different types of user input can be required at different points within the execution. So breaking up the logic into multiple methods would be non-trivial.

Answer 1

You can use an instance of the SemaphoreSlim Class as a signal:

private SemaphoreSlim signal = new SemaphoreSlim(0, 1);  // set signal in event signal.Release();  // wait for signal somewhere else await signal.WaitAsync(); 

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Alternatively, you can use an instance of the TaskCompletionSource<T> Class to create a Task<T> that represents the result of the button click:

private TaskCompletionSource<bool> tcs = new TaskCompletionSource<bool>();  // complete task in event tcs.SetResult(true);  // wait for task somewhere else await tcs.Task; 

Answer 2

When you have an unusual thing you need to await on, the easiest answer is often TaskCompletionSource (or some async-enabled primitive based on TaskCompletionSource).

In this case, your need is quite simple, so you can just use TaskCompletionSource directly:

private TaskCompletionSource<object> continueClicked;  private async void Button_Click_1(object sender, RoutedEventArgs e)  {   // Note: You probably want to disable this button while "in progress" so the   //  user can't click it twice.   await GetResults();   // And re-enable the button here, possibly in a finally block. }  private async Task GetResults() {    // Do lot of complex stuff that takes a long time   // (e.g. contact some web services)    // Wait for the user to click Continue.   continueClicked = new TaskCompletionSource<object>();   buttonContinue.Visibility = Visibility.Visible;   await continueClicked.Task;   buttonContinue.Visibility = Visibility.Collapsed;    // More work... }  private void buttonContinue_Click(object sender, RoutedEventArgs e) {   if (continueClicked != null)     continueClicked.TrySetResult(null); } 

Logically, TaskCompletionSource is like an async ManualResetEvent, except that you can only "set" the event once and the event can have a "result" (in this case, we're not using it, so we just set the result to null).