How to Fix NoSuchMethodError When Parsing JSON into Objects in Flutter

Learn how to resolve the `NoSuchMethodError` issue when mapping JSON data to Dart objects in Flutter. Follow our detailed guide to correctly parse JSON from Storyblok CMS.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Flutter: Parse json into object in Flutter, NoSuchMethodError: Class '_Map String, dynamic ' has no instance method 'cast' with matching arguments

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the NoSuchMethodError in Flutter

If you are developing a Flutter application and trying to handle JSON data coming from a CMS like Storyblok, you might encounter an error that can halt your progress. One such common issue is the NoSuchMethodError, specifically this scenario: Class '_Map<String, dynamic>' has no instance method 'cast' with matching arguments. This error can be frustrating, especially when you're trying to map JSON data to Dart objects.

In this post, we'll take a look at the cause of this error and provide you with a solution. We'll guide you through the process of correctly parsing JSON into your Dart objects, allowing your application to run smoothly without any disruptions.

The Problem

When parsing JSON data, specifically in a scenario like this:

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

You might see the error mentioned above. This happens because jsonDecode returns a Map<String, dynamic> and not a List as you are attempting to handle it.

This mismatch is the primary cause of the error you're experiencing. Let's dive into how you can fix it.

The Solution

To resolve the NoSuchMethodError, you'll need to adjust the way you parse the JSON data returned from the API. Most importantly, you'll need to extract the stories list from the decoded JSON map before trying to process it further.

Step-by-Step Solution

Modify the Parsing Function:

Update your existing parseExercises function to properly extract the stories list and cast it to the appropriate type:

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

Adjust the Factory Constructor:

Make sure your Exercise model has a correct fromMap constructor. In this case, you are on the right track:

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

Why This Works

Understanding Structure: You need to recognize the structure of the JSON response correctly. jsonDecode provides a map containing the stories key, which is a list of objects that we want to read.

Casting Correctly: By explicitly stating that we are casting jsonDecode(responseBody)['stories'] as a List, we align the data type with what we expect, avoiding method call errors.

Conclusion

Parsing JSON correctly is crucial when working with Flutter applications, especially when integrating with CMS services like Storyblok. By understanding the structure of your JSON data and properly managing it with the defined Dart models, you can avoid frustrating errors like NoSuchMethodError.

Feel free to test these adjustments in your Flutter app, and you should find that the error is resolved, allowing you to proceed with your development seamlessly. Happy coding!