Скачать или смотреть Resolving Concurrent Modification Exception in Flutter/Dart

Learn how to effectively handle `Concurrent Modification Exception` errors in Flutter/Dart when dealing with list iterations. This guide provides clear solutions and strategies to prevent these errors in your application development.
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This video is based on the question https://stackoverflow.com/q/64628171/ asked by the user 'Mário Gamelas' ( https://stackoverflow.com/u/12520854/ ) and on the answer https://stackoverflow.com/a/64630308/ provided by the user 'First_Strike' ( https://stackoverflow.com/u/10657409/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Concurrent Modification Exception in Flutter/Dart

As developers, we often encounter unexpected issues during the testing phase of our applications. One such issue is the dreaded Concurrent Modification Exception, particularly when iterating over lists. If you’re working with Flutter and Dart and have seen the following error message during testing:

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You are not alone. In this guide, we will explore the problem, understand why it occurs, and walk through effective solutions to prevent this exception in your code.

The Problem: Why Does This Exception Occur?

The exception typically arises when the list being iterated over is modified in some way while the iteration is still in progress. In your scenario, the error arose inside a loop that processes a list of tags. Here's a quick look at how your loop currently works:

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Key Observations:

The exception only occurs sometimes, indicating that the list tagsList becomes unexpectedly empty after the first iteration.

It seems that during the await, the Dart runtime may be executing other code, potentially altering the state of tagsList due to various async calls or processes.

The Solution: Best Practices for Asynchronous Code

Avoid Using await Inside Loops

One of the primary causes of the Concurrent Modification Exception in this context is the use of await inside a for-loop. When an await is encountered, the function's execution is suspended, allowing other code execution (potentially altering the list) to occur before resuming the loop.

Example of Where the Issue Lies

Consider the following example that also triggers this exception:

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Recommended Strategy: Use Future.wait

To avoid modifying a shared state during iteration, a better practice is to process your asynchronous tasks in bulk using Future.wait. This approach prevents concurrent modifications by gathering all asynchronous operations first and waiting for them collectively. Here’s how to adjust your code:

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Conclusion: Simplifying Asynchronous Workflows

By understanding the implications of asynchronous code and following best practices, you can prevent Concurrent Modification Exception errors in your Flutter/Dart applications. Always be mindful when using await inside loops, and consider utilizing Future.wait to improve your code's robustness.

Adopting these strategies will not only enhance your application’s stability but will also streamline your development process, making for a much smoother experience. Happy coding!