Troubleshooting json.encode Issue in Flutter: Fixing Instance of 'FabricType' Errors

Описание: Learn how to fix `json.encode` issues in Flutter by implementing the `toJson` method for custom objects. This guide addresses common problems and provides a clear solution.
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This video is based on the question https://stackoverflow.com/q/63955773/ asked by the user 'fucukur' ( https://stackoverflow.com/u/6070956/ ) and on the answer https://stackoverflow.com/a/63955954/ provided by the user 'Autocrab' ( https://stackoverflow.com/u/2771717/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting json.encode Issue in Flutter

If you're working with Flutter, you might encounter a frustrating error when trying to send a list of custom objects as a JSON body in a POST request. Specifically, you may find yourself seeing an output like [Instance of 'FabricType', Instance of 'FabricType'] instead of the expected JSON structure. This common issue usually occurs due to a missing or improperly implemented method for converting your custom class into a JSON-compatible format. In this guide, we’ll walk through the problem and provide a straightforward solution.

Understanding the Problem

In Flutter, when you want to send a complex data structure, like a list of custom objects, to a server, you need to convert this data into JSON. The error message you're encountering arises because the default behavior of the list of objects is to use the default toString() method, resulting in an unhelpful representation of the objects.

Given Code Snippet

You might have a model class like this:

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

And your list of objects looks like this:

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

When you call:

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

You run into the issue where the output is simply instances of the FabricType class, not the meaningful JSON data you need.

The Solution

The key to resolving this issue is to modify your FabricType class to include a toJson method instead of using toMap. The toJson method should represent your class’s properties in a format that can be easily converted to JSON.

Implementing toJson

Replace your existing toMap method with the following toJson method:

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

Updated Code Example

Here is how your updated FabricType class should look:

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

Sending the POST Request

With the toJson method in place, when you call json.encode(fabricList), each FabricType object will now be converted into a JSON-compatible format, and your POST request will send the correct data.

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

Conclusion

By implementing the toJson method in your Flutter model classes, you can easily convert lists of objects into JSON format, avoiding common pitfalls and errors. This change not only fixes the immediate issue but also sets a strong foundation for handling JSON data in your Flutter applications.

If you run into issues in your development journey, remember this solution — and happy coding!