Скачать или смотреть Resolving the Flutter EqualUnmodifiableListView Error When Handling JSON Data

Encountering the `EqualUnmodifiableListView` error in Flutter? This guide offers simple and effective solutions to properly handle JSON arrays using Freezed and Dart.
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Understanding the EqualUnmodifiableListView Error in Flutter

As Flutter developers, we often work with JSON data received from web APIs. However, there may be instances when we encounter issues due to incompatible data types. One common error developers face is the EqualUnmodifiableListView error when trying to add a JSON array into a list variable.

In this guide, we will discuss the causes of this error and provide a clear solution to help you resolve it effectively.

The Problem: What Is the EqualUnmodifiableListView Error?

When working with Flutter and the Freezed library, developers may receive JSON responses that contain arrays. The challenge arises when you try to process these arrays into specific Flutter data types.

For example, let's say you defined a model in Flutter like this:

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

If you attempt to convert a JSON response directly into your list of ConfigsDto like so:

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

You might run into an error message similar to this:

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

This message indicates that you are trying to cast EqualUnmodifiableListView<Config>, which is an immutable list, to a list of type ConfigsDto.

Analyzing the Cause of the Error

This error arises because:

The EqualUnmodifiableListView is a wrapper around the original list (in this case, List<Config>).

There is a type mismatch between Config and ConfigsDto, which are considered unrelated types.

In Dart, casting cannot convert between types that do not share a parent-child relationship in their class hierarchy.

The Solution: Transforming the Data

To resolve this issue, rather than attempting a type cast, you should perform a transformation. What you need is to create a new instance of ConfigsDto using the data received from the server.

Here’s how you can do that:

Clear the List: Make sure to clear the existing list before adding new data.

Add the ConfigsDto: Instead of adding each item directly, encapsulate your data into a new ConfigsDto.

Updated Code Example

Here’s an example of how to correctly handle the addition:

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

This approach takes the existing data, which is a list of Config types, and wraps it inside the ConfigsDto type. This is the correct way to assemble your data into the desired structure.

Summary

Handling JSON data in Flutter using the Freezed library can sometimes lead to type errors like the EqualUnmodifiableListView issue. The key takeaway here is understanding that when you encounter such an error, it is essential to perform data transformations rather than type casting.

By following the correct data handling practices, you can ensure your Flutter applications work smoothly and efficiently with parsed JSON data from APIs.

Now, the next time you encounter this error, you have a handy solution at your disposal. Happy coding!