Скачать или смотреть How to Parse JSON into Flutter List dynamic and Avoid 'List is not a subtype' Errors

Learn how to effectively parse JSON data into a List and Map in Flutter while resolving common errors. Discover step-by-step solutions and best practices for handling JSON in your Flutter apps.
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This video is based on the question https://stackoverflow.com/q/68010047/ asked by the user 'Mohamed Mazen' ( https://stackoverflow.com/u/6600501/ ) and on the answer https://stackoverflow.com/a/68010222/ provided by the user 'Rohan Thacker' ( https://stackoverflow.com/u/11385546/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving JSON Parsing Issues in Flutter: Avoiding 'List is not a subtype of type Map String, dynamic ' Error

When developing applications with Flutter, you may encounter various challenges related to JSON data handling. One common issue arises when trying to parse JSON responses into a structure that your application can work with. If you've ever faced the error saying 'List' is not a subtype of type 'Map<String, dynamic>', this guide is for you! We'll explore this issue and provide a clear solution step-by-step.

Understanding the Problem

The error message you're encountering occurs when your code expects a Map<String, dynamic> return type but receives a List<dynamic> instead. This can happen often when working with APIs that return arrays of items rather than a single object. Let's delve into a specific example:

Sample Model Class

Here's a simple model class representing a product:

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

In this model, the fromJson factory method expects a single JSON object, returning a CategoriesModel instance. However, when you perform the API request, you may receive a list of items instead.

The API Request: What's Happening Behind the Scenes

In your getCategories method, you are attempting to parse the API response using the fromJson method, but the method is not compatible with the data format you are receiving. Here’s an excerpt of your current method:

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

In the example above, value.data is a list, not a single object meant for the fromJson method. Thus arises the error.

The Solution: Parse the List Correctly

The solution to this problem requires updating the way you handle the incoming JSON data. Instead of assuming you have a single object, you should treat the data as a list and convert each item appropriately.

Updated getCategories Method

Here’s the corrected version of your getCategories method:

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

Key Changes:

I explicitly cast value.data to a List, allowing Dart to properly understand the data structure.

Instead of directly assigning a single CategoriesModel instance, I created a List<CategoriesModel> to store all parsed items.

Conclusion

Now that you've updated the code to handle the JSON list correctly, you should be able to parse your API data without encountering the 'List' is not a subtype of type 'Map<String, dynamic>' error. Remember, it is critical to understand the structure of the JSON data being returned by your API and adjust your parsing logic accordingly.

By following these guidelines, you can enhance your Flutter application’s robustness and ensure smoother data handling across its various components. Happy coding!