How to Suspend an Async Method Until an Event Occurs?

Efficiently Pausing Async Operations Awaiting User Input

Asynchronous programming often requires pausing long-running tasks until user interaction, such as a button click. This article explores efficient alternatives to busy-waiting when suspending an async method like GetResults until a specific event triggers.

Avoiding Inefficient Polling

The traditional approach of continuously checking for the event (busy-polling) is highly inefficient. Event-driven solutions provide a more elegant and performant approach.

Utilizing SemaphoreSlim for Synchronization

The SemaphoreSlim class provides a robust semaphore-based solution. Initialize a semaphore with a starting count of 0 and a maximum count of 1. The event handler for the "continue" button releases the semaphore using Release(). Within GetResults, await signal.WaitAsync() blocks execution until the semaphore is released.

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

private void ButtonContinue_Click(object sender, RoutedEventArgs e)
{
    signal.Release();
}

private async Task GetResults()
{
    await signal.WaitAsync();
    // Continue processing
}</code>

Leveraging TaskCompletionSource for Event Signaling

Alternatively, TaskCompletionSource<bool> creates a task representing the button click result. The event handler completes this task using SetResult(). GetResults awaits this task, suspending execution until completion.

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

private void ButtonContinue_Click(object sender, RoutedEventArgs e)
{
    tcs.SetResult(true);
}

private async Task GetResults()
{
    await tcs.Task;
    // Continue processing
}</code>

These event-driven methods offer a clean and efficient way to pause and resume async methods based on user events, eliminating the performance overhead of busy-waiting.

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