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Understanding the 'The class 'PreferredSizeWidget' can't be used as a mixin' Error in Flutter and Dart

Flutter, Dart, Mixins, Error Handling2 min read

Flutter is a popular framework for building cross-platform mobile applications, and Dart is the programming language that powers it. As with any software development environment, upgrades and changes can introduce new challenges and errors for developers to address. In this article, we will discuss a common issue that arises after upgrading to newer versions of Flutter and Dart, specifically the error message: "The class 'PreferredSizeWidget' can't be used as a mixin because it's neither a mixin class nor a mixin." We'll explore what this error means and provide a solution to resolve it.

The Error Message

Here's the error message developers often encounter:

1The class 'PreferredSizeWidget' can't be used as a mixin because it's neither a mixin class nor a mixin.

This error message typically occurs when using the with keyword to include the PreferredSizeWidget as a mixin class in your code.

What Is a Mixin?

Before we dive into the problem and its solution, let's briefly understand what a mixin is. A mixin is a way to reuse a class's code in multiple class hierarchies. It allows classes to inherit methods and properties from multiple sources, which is a powerful feature for code reuse.

In Dart, prior to version 3.0, you could use any class as a mixin as long as it had no declared constructors and no superclass other than Object. However, Dart 3.0 introduced stricter rules for mixins.

The Problem

The error message "The class 'PreferredSizeWidget' can't be used as a mixin" suggests that PreferredSizeWidget doesn't meet the new criteria for mixins introduced in Dart 3. This is because PreferredSizeWidget is not explicitly marked as a mixin using the mixin keyword in its definition. Consequently, trying to use it as a mixin with the with keyword results in this error.

The Solution

The solution to this problem is to change the way you include PreferredSizeWidget in your class. Instead of using the with keyword, you should use the implements keyword. Here's how to do it:

1class TabBarContainer extends StatelessWidget implements PreferredSizeWidget {
2 // Your implementation stuff goes here

By implementing PreferredSizeWidget instead of using it as a mixin, you're explicitly stating that you will provide your implementation of the methods defined by PreferredSizeWidget. This aligns with the new rules of Dart 3 regarding mixins.

Summing Up

In Flutter and Dart, keeping your code up-to-date with the latest language and framework versions is essential. However, it can introduce new challenges, as seen with the stricter mixin rules in Dart 3. The "The class 'PreferredSizeWidget' can't be used as a mixin" error is a common issue that can be resolved by changing the with keyword to implements. This change aligns with the new Dart 3 rules and ensures that your code functions correctly.

As you navigate your Flutter development journey, understanding the nuances of the language and framework is crucial to solving problems and building high-quality, efficient mobile applications.