Скачать или смотреть Resolving the List dynamic ? to List dynamic Error in Flutter

Learn how to fix the Flutter error regarding type assignment with dynamic lists by following simple steps and understanding nullability in Dart.
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This video is based on the question https://stackoverflow.com/q/68976304/ asked by the user 'Irwin' ( https://stackoverflow.com/u/12308995/ ) and on the answer https://stackoverflow.com/a/68976326/ provided by the user 'esentis' ( https://stackoverflow.com/u/12413404/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Fixing the List<dynamic>? to List<dynamic> Error in Flutter

If you've been working with Flutter and Dart, you've probably encountered a type mismatch issue at some point. One common error that developers often come across is:

The argument type List<dynamic>? can't be assigned to the parameter type List<dynamic>.

This error can be particularly confusing, especially for those who understand that both data types appear to be similar. Let's break down the problem and understand how to fix it effectively.

Understanding the Problem

In Dart, there are two types of lists to consider regarding this error:

List<dynamic>?: This represents a list that can either be a List<dynamic> or null. The question mark (?) indicates that the list is optional and may not contain any data.

List<dynamic>: This denotes a non-nullable list that must contain a value before being used.

When you try to pass an optional list (List<dynamic>?) to a function or widget that requires a non-null list (List<dynamic>), Dart raises the error because it cannot guarantee that the optional list will have data (i.e., it might be null).

Where the Error Occurs

Let’s look at your code to identify where this error might be coming from. The relevant pieces of code are:

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

In this snippet, the _listaItems(snapshot.data) line attempts to pass snapshot.data, which is of type List<dynamic>?, to _listaItems, which accepts List<dynamic>. Hence, the type error arises here.

Solution: Using the Bang Operator

To address this error, we can use the bang operator (!) in Dart, which explicitly tells Dart that we are confident snapshot.data will not be null at that moment. Here’s the revised code snippet:

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

By adding ! to snapshot.data, you're indicating that you expect it to be non-null, thus solving the type mismatch issue.

Complete Function Example

Here’s how the corrected _lista() function looks like in its entirety:

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

Further Considerations

Handling Null Values: You may want to consider implementing null handling or default values for safer code. Ensure that snapshot.data is indeed populated before trying to use it; otherwise, the app will throw an error at runtime.

Error Handling in FutureBuilder: It's also a good practice to handle errors in FutureBuilder to avoid displaying an empty or broken UI to users.

Conclusion

Encountering type assignment errors in Flutter can be frustrating, especially when working with dynamic data types. By understanding the difference between optional and non-nullable lists and using the bang operator, you can resolve these issues efficiently. With practice and vigilance, you will become more adept at avoiding and fixing such errors in your Flutter applications.

Now you can continue coding with confidence and make the most out of your Flutter projects!