Скачать или смотреть Resolving DropdownButton Value Errors in Flutter: A Clear Guide

Struggling with `DropdownButton` errors in Flutter? Learn how to resolve the issue of value mismatches with our easy-to-follow guide.
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This video is based on the question https://stackoverflow.com/q/71283298/ asked by the user 'Mahi' ( https://stackoverflow.com/u/14519278/ ) and on the answer https://stackoverflow.com/a/71283677/ provided by the user 'Eimantas G' ( https://stackoverflow.com/u/13166430/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving DropdownButton Value Errors in Flutter: A Clear Guide

When developing Flutter applications, many developers encounter a common error message related to the DropdownButton. If you've stumbled upon the issue stating: “There should be exactly one item with [DropdownButton]'s value: Select district. Either zero or 2 or more [DropdownMenuItem]s were detected.”, you're not alone. This error can be frustrating, especially when everything else in your application seems to be functioning well.

Understanding the Issue

The error typically arises when the DropdownButton has no valid item that corresponds to its current value. To be specific, there are some rules concerning the DropdownMenuItem list. Let’s break it down:

There must be exactly one selected item that corresponds to the value of the DropdownButton.

If there is no selected item or if there are multiple items with the same value, you will encounter this error.

How to Solve the Error

Fixing this error requires a proper setup of your DropdownMenuItem instances. Here’s how you can ensure that your DropdownButton works correctly:

Step 1: Set Unique Values for DropdownMenuItem

For each DropdownMenuItem, make sure to assign a unique value property. This is crucial because the DropdownButton uses these values to determine which item is currently selected.

Step 2: Corresponding Values

The value set in DropdownButtonFormField should match one of the DropdownMenuItem values to avoid error messages.

Example Code

Here’s a simple code example to illustrate these principles:

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

Explanation of the Code

Creating the List: The ll variable holds your dropdown options. Each DropdownMenuItem has a unique value associated with it.

Setting the Initial Value: The value parameter in the DropdownButtonFormField should be one of the values defined in your DropdownMenuItem list, ensuring that it matches correctly.

Handling Changes: The onChanged callback will handle user selections. Here, it simply prints the value, but you can implement more complex logic as needed for your application.

Conclusion

By following these steps, you should be able to resolve the DropdownButton value error in Flutter. Always ensure that each item in your dropdown list has a unique value and that the DropdownButton corresponds to a valid value. With this knowledge, you can build responsive and error-free dropdown menus in your Flutter applications. Happy coding!