How to Suspend Async Method Execution Until an Event Occurs in a Metro App?

Pausing Asynchronous Operations Awaiting User Input in UWP Apps

This guide demonstrates how to temporarily halt the execution of an asynchronous method within a Universal Windows Platform (UWP) application until a specific event, such as a button click, is triggered. This is particularly useful when a long-running asynchronous process (GetResults in this example) needs user interaction before proceeding.

Method 1: Using SemaphoreSlim as a Signal

The SemaphoreSlim class offers a straightforward signaling mechanism. Here's how to implement it:

<code class="language-csharp">private SemaphoreSlim signal = new SemaphoreSlim(0, 1); // Initially 0 permits, max 1

// Event handler to release the semaphore
private void buttonContinue_Click(object sender, RoutedEventArgs e)
{
    signal.Release();
}

// Asynchronous method pausing for the signal
private async Task GetResults()
{
    // ... Perform lengthy operations ...
    await signal.WaitAsync(); // Pause until the signal is released
    // ... Continue execution after the event ...
}</code>

Method 2: Leveraging TaskCompletionSource<bool>

TaskCompletionSource<bool> provides a more flexible approach by creating a Task<bool> that represents the completion of the user interaction.

<code class="language-csharp">private TaskCompletionSource<bool> tcs = new TaskCompletionSource<bool>();

// Event handler to set the task's result
private void buttonContinue_Click(object sender, RoutedEventArgs e)
{
    tcs.SetResult(true);
}

// Asynchronous method awaiting task completion
private async Task GetResults()
{
    // ... Perform lengthy operations ...
    await tcs.Task; // Pause until the task completes
    // ... Continue execution after the event ...
}</code>

Both methods effectively pause the GetResults asynchronous operation until the specified event occurs, avoiding inefficient polling techniques. Choose the method that best suits your application's architecture and complexity.

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