Скачать или смотреть Solving the List dynamic Assignment Error in Flutter's Caching Utility

Learn how to resolve the common Dart error: 'The argument type List dynamic can't be assigned to the parameter type FutureOr List T ?' in your caching utility with Sembast in Flutter.
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Understanding the Dart Error: List<dynamic> Assignment

When working with Flutter and Dart, you may encounter several errors as you attempt to manage generics, types, and asynchronous programming. One particularly annoying error occurs when you try to return a List<dynamic> where a FutureOr<List<T>>? is expected. If you’re developing a caching utility with Sembast and you’ve hit this roadblock, you’re in the right place. In this guide, we’ll break down the problem and provide a clear solution.

The Initial Problem

You might be implementing a caching controller in Flutter using the Sembast package. Your goal is to create a generic caching utility that works with various types of data. However, when you attempt to return a list of objects from your caching controller, you’re faced with the following error message:

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

This error typically arises when the Dart compiler can't guarantee that the types match due to the generic T being used in your utility.

Analyzing the Code

Let’s take a look at the relevant part of your CachingController class and the MapSerializer abstract class to understand what's happening:

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

The getObjects method attempts to return a list of objects after mapping through the records retrieved from the database. However, objectList is inferred as List<dynamic> because of its source being records, which are of type GameRecord (or whatever your actual record type is). The Dart compiler sees a mismatch with the expected return type of FutureOr<List<T>>?.

The Solution

A straightforward way to fix the error is to ensure that the mapSerializer is correctly typed and aligns with T. Here’s how you can resolve this issue:

1. Updating the Method Signature

Ensure the mapSerializer parameter in your getObjects method is generic and correctly typed. This guarantees the returned object is expected to be of type T.

Updated Code Example:

Here's how the getObjects method should look:

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

2. Avoiding Casting Where Possible

While casting the list to FutureOr<List<T>> might solve the problem superficially, it’s not the cleanest way to handle types. If the above solution doesn't work, consider the following:

Make sure that your financial method has the correct return type.

If Dart still shows a type error, it might be useful to investigate the compilation context or how you handle the records.

Conclusion

By ensuring proper generic types and modifying your method signature appropriately, you can effectively resolve the List<dynamic> assignment error in your Caching Utility. Type safety is one of Dart’s biggest strengths, and you should always strive to maintain clarity and correctness in your generics. Happy coding!