Скачать или смотреть Resolving the Expected a value of type List dynamic but got one type _JsonMap Error in Flutter

Discover how to fix the Flutter error "Expected a value of type List dynamic but got one type _JsonMap" when fetching data from Django REST Framework.
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This video is based on the question https://stackoverflow.com/q/78199295/ asked by the user 'Jonathan Kwabena Ewenam Kudiab' ( https://stackoverflow.com/u/23704593/ ) and on the answer https://stackoverflow.com/a/78201815/ provided by the user 'Sreelal TS' ( https://stackoverflow.com/u/10006183/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding and Fixing the Expected a value of type List<dynamic> but got one type _JsonMap Error in Flutter

When developing mobile applications, especially those that rely on data from a server, developers often come across unexpected issues. One common error encountered when using Flutter with Django REST Framework is the message: Expected a value of type List<dynamic> but got one type _JsonMap. This guide will explore the cause of this issue and provide a comprehensive solution.

The Problem

The error arises when attempting to decode a JSON response from a Django REST Framework. Specifically, it typically occurs in the following scenario:

You're trying to convert a JSON response into a List of dynamic type in Flutter.

The error indicates that the JSON being returned is not a List but a Map instead.

For example, let’s consider the following excerpt from your code:

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

Explanation of the Error

The root of the problem lies in the format of the JSON returned from the server. In your case, the response.body contains a JSON object structured more like a Map rather than a simple List. When you try to cast it as a List, Flutter throws the error because it encounters a Map instead.

Solution to Fix the Issue

To resolve this issue, you need to adjust how you're parsing the JSON response. It seems your intention is to extract a "results" list from the response body. Here's how to adjust your code accordingly.

Updated Code

Here's the modified version of the fetchData method that properly accesses the "results" key from the JSON response:

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

Breakdown of the Changes

JSON Decoding: The original line of code attempts to cast the entire response to a List right away. The update first decodes the JSON into a Map:

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

Accessing the Results: The crucial change here is accessing the "results" key of the decoded JSON, allowing you to correctly retrieve the list:

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

Checking for Errors: Ensure that your error handling remains robust. If the server returns an unexpected response, your app should appropriately handle it to prevent crashes.

Conclusion

Errors are a natural part of any development process. Encountering Expected a value of type List<dynamic> but got one type _JsonMap when working between Flutter and Django REST Framework is common, especially if the server's JSON structure doesn’t match your expectations. By correctly decoding the JSON and accessing the appropriate keys, you can handle this problem effectively.

With this solution, you can ensure that your Flutter app retrieves and displays data as intended, enhancing the overall user experience.

If you need further assistance with Flutter or Django Development, feel free to ask!