Solving the Bad state Error in Flutter BLoC Streams: A Guide to Proper Stream Management

Encountering the `Bad state: Cannot add new events after calling close` error in your Flutter BLoC application? Discover how to manage Streams effectively in your application to avoid these pitfalls.
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This video is based on the question https://stackoverflow.com/q/74186898/ asked by the user 'Marcel Dz' ( https://stackoverflow.com/u/12186851/ ) and on the answer https://stackoverflow.com/a/74189279/ provided by the user 'Tomiwa Idowu' ( https://stackoverflow.com/u/16385301/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Encountering a Bad State Error in Flutter BLoC Streams?

If you've been working with Flutter and using the BLoC pattern for state management, you might have run into a tricky error:

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This error typically occurs when you try to add events to a stream after it has already been closed. In this guide, we will uncover the causes of this issue, how to handle stream management properly, and ensure your application runs smoothly.

Understanding the Issue

In the context of your Flutter app, you set up two StreamController instances in your Repository class to manage incoming data:

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The GameBloc class listens to these streams. However, when you call the close() method on a bloc or a stream, it terminates the stream and disallows any further events from being added.

When your application attempts to add events again after the streams have been closed, the error arises, leading to runtime exceptions.

The Close Method

The close method in your bloc looks as follows:

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Here, you are closing both StreamControllers, which releases all listening resources. While this is good practice, it can lead to complications if you need to add events again later.

Best Practices for Stream Management

1. Use StreamSubscription

Instead of directly closing the stream controllers in the close() method, utilize StreamSubscription to manage your streams.

Modify your BLoC as follows:

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2. Cancel Subscriptions Instead of Closing the Stream

In the close() method, cancel the subscriptions rather than closing the stream controllers:

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Using this approach allows you to manage your listeners more effectively. If you want to re-establish the stream later, you won’t face the Bad state error as the stream itself remains open and continues to broadcast events.

Conclusion

It's crucial to manage your streams correctly to avoid common pitfalls in Flutter BLoC applications. By utilizing StreamSubscription and focusing on canceling subscriptions instead of closing StreamControllers, you can enhance your app's performance and reliability.

Next time you encounter the Bad state error, remember to double-check your stream management practices, and you will be well on your way to smooth sailing in your Flutter development journey!