Скачать или смотреть Resolving Column Widget Errors in Flutter

Learn how to fix 'Evaluation of this constant expression throws an exception' in Flutter when adding buttons to a Column widget.
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This video is based on the question https://stackoverflow.com/q/76869294/ asked by the user 'LostTexan' ( https://stackoverflow.com/u/14295515/ ) and on the answer https://stackoverflow.com/a/76869335/ provided by the user 'offworldwelcome' ( https://stackoverflow.com/u/11760076/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving Column Widget Errors in Flutter: A Simple Guide

When developing applications with Flutter, you might encounter various obstacles, especially when working with widgets like Column. A common issue arises when trying to add buttons to a Column, resulting in an error: "Evaluation of this constant expression throws an exception." This guide will address the problem and provide a clear solution.

Understanding the Problem

Let's break down the issue:

Scenario: You have a Column widget within a Center widget, containing several TextFields and a button.

Error: When adding a button, specifically OutlinedButton, you encounter the error mentioned above.

Code Example Leading to the Issue

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

Why the Error Occurs

The error stems from a mismatch in the use of const:

The Center widget is marked as const, which signifies that it and all of its children must also be constant.

The onPressed function of the OutlinedButton is not constant because it contains logic (navigator action) that cannot be determined at compile time.

How to Fix It

The Solution: Remove the const Modifier

The simplest fix to resolve the issue is to remove the const keyword from the top-most Center widget. Here’s how you do it:

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

Additional Notes

By removing const, you allow for dynamic construction of the widget tree, which isn’t possible when everything is defined as constants.

This change won't impact the performance negatively unless you have a large widget tree where performance optimizations using const can be beneficial.

Conclusion

Errors in Flutter, particularly with widget constructions, can often be traced back to misinterpretations of constant expressions and widget constraints. By understanding the relationship between const and dynamic components, we can quickly resolve issues and enhance our development experience. If you follow the outlined solution, you can successfully integrate buttons into a Column without experiencing the dreaded evaluation error.

Happy coding in Flutter!