How Can I Await an Event in an Async C# Metro App Method?

Async Event Handling in C# Metro Style Apps: A Clean Solution

Asynchronous programming is essential for responsive C#/XAML Metro style apps, preventing UI freezes during lengthy operations. However, situations arise where user interaction is needed mid-process. This article details how to efficiently await events within asynchronous methods, pausing execution until the event triggers.

The Challenge: Awaiting User Input During Async Operations

Imagine a scenario: a button initiates an asynchronous GetResults method. This method might require user input (e.g., clicking a "Continue" button) before proceeding. The key question: how do we pause GetResults until the user interacts?

Inefficient Polling: A Method to Avoid

A simplistic (but flawed) approach involves a flag updated by the "Continue" button's event handler, checked repeatedly within GetResults using Task.Delay. This constant polling wastes resources and is inefficient.

Elegant Event-Based Solutions

C# offers superior event-driven solutions:

1. Utilizing SemaphoreSlim

• Instantiate a SemaphoreSlim object as a signal.
• Release the semaphore when the "Continue" button is pressed.
• In GetResults, await SemaphoreSlim.WaitAsync. This blocks execution until the semaphore is released.

2. Leveraging TaskCompletionSource

• Create a TaskCompletionSource<bool> to represent the "Continue" button click outcome.
• Complete the task upon the button click.
• Inside GetResults, await tcs.Task. Execution halts until the task completes.

These event-driven methods provide a clean and efficient way to integrate user interaction into asynchronous operations, ensuring smooth user experiences in complex scenarios.

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