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Explore solutions for managing asynchronous processes in your Flutter app using Bloc and StreamSubscription, ensuring your UI waits for the entire process to complete.
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How to Handle UI Async Processes with a Bloc StreamSubscription in Flutter

Flutter's Bloc pattern is a powerful tool for managing state in your applications. However, handling asynchronous processes, particularly in cases like user authentication, can be challenging. If you're trying to ensure your UI properly awaits all necessary async calls—such as API requests following user authentication—you've come to the right place.

In this post, we'll address a common issue: how to manage async processes effectively when using Bloc with Flutter, specifically focusing on synchronizing data with the UI.

The Problem

You're working on a Flutter app that implements the Bloc pattern for state management. During the login flow, after authenticating a user with Firebase, you need to make additional API calls to fetch user-related data. However, the current implementation either doesn't wait for these calls to complete, or it ends up emitting state updates prematurely, leading to null data in your UI.

Key Components

View: User interface containing the button to trigger the login process.

LoginCubit: Manages the state and logic associated with user authentication.

AppBloc: Global Bloc managing the application's state, including user information.

AppEvent: Declares event classes used in the AppBloc.

Proposed Solutions

To effectively handle the asynchronous workflow in your Flutter application, we can explore two methods. Each will help ensure that your UI correctly reflects the state of your application after all async operations are completed.

Ideal Method: Use Repository to Sync Data

First, consider keeping your Bloc completely independent of the UI. Instead of directly using Flutter's BuildContext within your Cubit, leverage the repository pattern for data fetching. Here’s how:

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This approach allows the Cubit to wait for the user document's API call to complete before emitting a successful state. It ensures that all necessary data is available to the UI.

Alternative Method: Wait for Bloc State Change

If you need to use the context within your LoginCubit, an alternative is to wait for the AppBloc's state to change. This method relies on the assumption that the user-related information is updated in the AppBloc. Here’s the sample implementation:

[[See Video to Reveal this Text or Code Snippet]]

This code snippet waits until the AppBloc updates its state, ensuring that information such as userDocument is populated before proceeding to emit the result.

Conclusion

Managing async processes in Flutter apps using Bloc can be tricky, but with the right approach, you can ensure your UI remains responsive and accurate. The methods discussed above should allow you to effectively synchronize async calls with UI updates in your application’s login flow.

By keeping your loading state and querying logic cleanly separated, whether through repositories or state change subscriptions, you can create a more robust and reliable user experience in your Flutter applications.

Implementing these solutions will not only solve your current problem but also lay a solid foundation for managing future asynchronous workflows effectively.