In Xamarin Forms, as with any cross-platform mobile application framework, managing asynchronous methods and understanding when and how to call them can be crucial, especially during the app's lifecycle events. The startup of the application is significantly important because it's the time to prepare and initialize resources the app will need. This article provides guidance on how to properly call async methods when your Xamarin Forms app starts.

Understanding Application Lifecycle in Xamarin Forms

In Xamarin Forms, the application's lifecycle is defined and can be managed with these primary methods in App.xaml.cs:

• OnStart: Called when the application starts.
• OnSleep: Called each time the application is pushed to the background.
• OnResume: Called when the application is resumed, after being sent to the background.

The Challenge with Async in OnStart

The OnStart method is the entry point when the app is launched. However, it's important to note that OnStart is a synchronous method. This characteristic can lead to potential pitfalls when trying to perform asynchronous operations, as calling asynchronous methods in the synchronous startup method can result in unhandled exceptions or blocking the UI thread.

Approaches to Call Async Methods in OnStart

1. Using async void in OnStart

One common approach is to use async void for your startup code. Although generally not recommended at other points in your application due to exception handling problems, async void is appropriate here as OnStart does not have a return type that can be awaited.

• Avoid Blocking the UI Thread: Operations like database initialization or fetching data should not be executed on the main thread as it will block the UI, leading to poor user experience.
• Use Task.Run Wisely: Although Task.Run can be a simple way to run async operations in OnStart, it does introduce concurrency. Be cautious of shared resources.
• Error Handling: As async void does not allow for error checking in the same way as Task, consider how to handle potential exceptions (e.g., using try-catch blocks within the async task).
• Deferred Initialization: Another strategy is deferring some initialization until after the launch screen is displayed. This allows for displaying the main app page faster.
• Use of Dependency Injection: Utilizing a service locator pattern or dependency injection can help organize and manage your app's initialization processes.