Скачать или смотреть Resolving the Method "xxx" is not defined for the type "xxx" Error in Flutter and Dart

Encountering the error "Method 'xxx' is not defined" in Dart while developing for Flutter? Discover how to resolve it by addressing private method declarations.
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This video is based on the question https://stackoverflow.com/q/68495875/ asked by the user 'Mark Ardman' ( https://stackoverflow.com/u/10182496/ ) and on the answer https://stackoverflow.com/a/68495921/ provided by the user 'CommonMind' ( https://stackoverflow.com/u/13794223/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the "Method Not Defined" Error in Flutter and Dart

Have you ever been deep into coding with Flutter and Dart only to be greeted by the frustrating error message: "Method 'xxx' is not defined for the type 'xxx'"? This issue can arise even when you believe you've defined the method correctly. Understanding the root cause of this error is paramount to finding a solution.

In this guide, we'll dissect a common scenario that triggers this error and provide you with a step-by-step explanation of how to resolve it effectively.

The Problem Explained

Imagine you are working on a project and you have defined a class Variables where you intend to manage some contact information. Your code looks something like this:

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

You attempt to call the method _setContactName like so:

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However, instead of success, you are bombarded with the error:
"The method '_setContactName' isn't defined for the type 'Variables'."

This can feel particularly frustrating, especially when you've clearly defined the method within the class.

Identifying the Cause of the Error

The source of this error lies in the use of the underscore (_) prefix in your method names. In Dart, prefixing a method with an underscore signifies that the method is private to its library. This means that it can only be accessed within the same file where it is defined. If you attempt to call this method from outside that file, the Dart compiler will not recognize it, leading to the error you encountered.

Key Points to Remember:

Private Methods: In Dart, methods prefixed with _ are private and can't be accessed externally.

Error Messaging: The error message will refer to the method being "not defined," which is misleading in the context of private method access.

The Solution: Removing the Underscore

To resolve this issue, you can make your methods public by simply removing the underscore prefix. Here’s how your updated class definition will look:

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

Now, you can call the method as intended without any errors:

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

Benefits of This Approach:

Access to Methods: Removing the underscore allows you to access methods from anywhere within your application.

Cleaner Code: This makes your code easier to understand and manage, particularly in larger projects.

Conclusion

If you find yourself facing the "Method 'xxx' is not defined for the type 'xxx'" error in Dart, remember to check if you're mistakenly using private methods when publicly accessible methods are required. By simply removing the underscore, you can efficiently resolve the issue and keep your Flutter application running smoothly.

Stay tuned for more coding tips and solutions to common errors encountered while using Flutter and Dart!