Fixing build_runner to Generate Correct toJson Methods in Flutter Serialization

Discover the solution to common issues with `build_runner` and `json_serializable` in Flutter. Learn how to ensure your `toJson` methods generate correct serialization for your models.
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Fixing build_runner to Generate Correct toJson Methods in Flutter Serialization

When developing Flutter applications, one common challenge developers face is ensuring proper serialization and deserialization of their models. One of the tools used for automating this process is build_runner. However, there might be instances where the toJson() methods generated by build_runner do not include custom class serialization, leading to errors when trying to reconstruct objects from JSON.

The Core Problem

In your scenario, you are experiencing an issue where the generated toJson methods are not outputting the desired structure. For instance, the toJson() implementation for a class (let's call it Foo) is missing the necessary calls to convert the nested objects:

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

Instead, the generated code is simply returning the instance of Foo like this:

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

This results in a incorrectly structured map since it lacks the necessary conversions for nested models. The problem becomes complex when you want to convert objects back from JSON, requiring a proper tree structure of maps.

Understanding the Generated Code

Here's a quick look at your generated code for the Test class:

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

This code snippet shows that the foo field is not being serialized to a map, thus making deserialization (and further serialization) fail.

The Solution

Step-by-Step Fix

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

This configuration explicitly tells the json_serializable package to include the calls to toJson() for any fields that are instances of custom classes.

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

or, for continuous watching during development:

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

Expected Result

Once you implement the above steps, the regenerated toJson() for the Test class should now correctly serialize instances of Foo as well, looking something like this:

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

Conclusion

Make sure to keep your configuration files well-organized and document any custom settings you employ, as they are pivotal for your app's serialization logic.

Happy coding!