Do Not Use BuildContext in Async Gaps: Why and How to Handle Flutter Context Correctly

In this post, we’ll look at a common yet often overlooked problem in Flutter app development: using BuildContext across async gaps. We will investigate the why and how of this key part of Flutter, highlighting the need of proper context handling in order to avoid potential errors and memory leaks. Understanding the fundamentals and best practices will help you traverse the difficulties of asynchronous programming in Flutter, ensuring that your app runs smoothly and quickly.

So, what does this mean?

The warning “Do not use BuildContext in async gaps" is an important reminder for Flutter developers to avoid using BuildContext in asynchronous operations. In Flutter, BuildContext is a critical parameter used to access information about a widget's location in the widget tree and to perform tasks like navigating to other screens, showing dialogs, accessing theme data, and so on.

However, when developers pass BuildContext across async boundaries, such as within Future Methods, StreamBuilder, or isolates, it can lead to problems. This warning advises against doing so because it can result in unexpected and erroneous behaviors in your app.

When BuildContext is used in asynchronous gaps, it might refer to a widget that no longer exists, leading to issues like:

1. Stale Data: If a widget is rebuilt or disposed of while an asynchronous operation is ongoing, the BuildContext reference may point to an outdated or non-existent widget. This can lead to displaying incorrect or stale data in your app.
2. Memory Leaks: Holding a reference to a BuildContext that should be disposed of can result in memory leaks, as the framework won't be able to garbage-collect resources associated with that widget.
3. App Crashes: In some cases, using BuildContext in an asynchronous gap might even lead to app crashes if the referenced widget is disposed before the operation is completed.

In essence, this warning encourages developers to think carefully about how they handle BuildContext in asynchronous operations, emphasizing the importance of understanding widget lifecycle management and avoiding common pitfalls that can impact the reliability and performance of their Flutter apps.

But, what should I do now?

Solution 1: Using a GlobalKey and Keyed Subtrees

To address the issue of not using BuildContext across async gaps, we can utilize GlobalKey and keyed subtrees. This approach ensures that the correct BuildContext is associated with asynchronous operations, even if the widget is disposed and rebuilt.

Here’s how you can implement this solution with an example:

Step 1: Create a GlobalKey

Start by creating a GlobalKey in your Widget State and attaching it to the widget that is the parent of your asynchronous operation. This ensures that the BuildContext retrieved from this GlobalKey is valid.

final GlobalKey<_MyWidgetState> myWidgetKey = GlobalKey();

Step 2: Retrieve the BuildContext

Within your asynchronous operation, you can retrieve the BuildContext using the GlobalKey.

Future<void> fetchData() async {
  /// Getting the current context of the widget in the widget tree
  final context = myWidgetKey.currentContext;

  var result = await navigator.of(context).pushNamed('/user_selection_page');

  if (context != null && context.mounted) {
    /// statements after async gap without warning
    Scaffold.of(context).showSnackBar(SnackBar(
      content: Text(result.name),
    ));
  }
}

By following this approach, you ensure that the BuildContext remains valid during async operations, preventing issues related to stale or invalid contexts.

Benefits:

• Reliable Context: Using GlobalKey guarantees that the associated BuildContext is always up-to-date and accurate.
• Predictable Behavior: The widget subtree remains correctly associated with its respective BuildContext even during asynchronous operations.
• Less Prone to Errors: This approach reduces the chances of errors and crashes due to stale BuildContext references.

However, there is another way that does the same thing…

Solution 2: Using the “then” Method with Future

The “then” method is a straightforward approach to handling asynchronous operations that require the use of a valid BuildContext. It ensures that your code executes only after the asynchronous operation is completed successfully, thus providing access to the correct BuildContext. Here's how you can implement this solution with an example:

Future<void> fetchData() async {
  await navigator.of(context).pushNamed('/user_selection_page')
  .then((result) {
    /// statements after async gap without warning
    Scaffold.of(context).showSnackBar(SnackBar(
      content: Text(result.name),
    ));
  });
}

Benefits:

• Consistent Context: Using “then” ensures that the code is executed within the same execution context as the asynchronous operation, providing access to a reliable BuildContext.
• Clear Flow: The code stays organized and intuitive, with the logic following a sequential pattern, making it easier to understand and maintain.

In case if you ever wanted to use BuildContext in async gaps but the warning is annoying you,

then add use_build_context_synchronously rule under linter > rules in your analysis_options.yaml file:

linter:
  rules:
    - use_build_context_synchronously

Conclusion

It is critical in Flutter development to have a clear and robust method to handle asynchronous actions to avoid issues caused by utilizing the incorrect BuildContext. The warning “Do not use BuildContext in async gaps” serves as a helpful reminder of the dangers.

Remember, adopting best practices like these will lead to more robust and organized code, ultimately resulting in better user experiences. So, take this valuable lesson and continue building Flutter apps that are both efficient and user-friendly.

That’s it for this article! I hope you enjoyed it and leave a few claps if you did. Follow me for more Flutter articles and comment for any feedback you might have about this article.

I‘m Ashish Sharma aka theFlutterist. You can find me on LinkedIn or maybe follow me on GitHub as well.

Checkout my other articles:

Parallax Scroll Effect in Flutter: Elevate Your App’s User Experience

Debouncing in Flutter: Enhancing User Experience