Resolving the type is not a subtype of type Map String, dynamic Error in Flutter Serialization

Learn how to fix the common serialization error in Flutter applications when using the `json_serializable` package, ensuring smooth handling of complex data types.
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Resolving the type is not a subtype of type Map<String, dynamic> Error in Flutter Serialization

When developing Flutter applications, one common pitfall developers encounter is serialization issues while converting objects to JSON and vice versa. One specific error that can be particularly perplexing is: type is not a subtype of type Map<String, dynamic>. If you've run into this problem while working with the json_serializable package, you're in the right place! In this guide, we will explore how to diagnose and resolve this serialization error.

Understanding the Problem

In your Flutter app, you may have a class structure that utilizes JSON serialization with the json_serializable package. Let's consider the following structure you might have implemented:

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

The error you are encountering is likely stemming from the way the Shift object is being serialized and deserialized within the ShiftState. Specifically, the generated code is expecting a Map<String, dynamic> for the shift property when constructing a ShiftState from JSON. However, it's receiving an instance of Shift instead.

The Error Message

The specific error you're getting is:

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

This error indicates that somewhere in your code, there is an attempt to treat a Shift object as a Map<String, dynamic>, which is not correct.

Analyzing the Generated Code

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

The Solution

To resolve this error, you need to instruct the json_serializable package to explicitly call the toJson method on nested objects such as Shift. You can achieve this by setting explicitToJson: true in the @ JsonSerializable annotation for your ShiftState class:

Step-by-Step Fix

Update the ShiftState Class:

Change the class declaration to include explicitToJson: true:

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

Rerun Code Generation:

After making this change, ensure you regenerate your serialization code by running:

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

Test Your Changes:

With the updated class and generated code, you can now test your serialization again. The error should no longer appear, and your Shift object will be correctly processed as a JSON map.

Why This Change Works

What does the explicitToJson property do? When set to true, it ensures that the toJson() method is invoked on any nested objects. This is important in our case because it allows us to produce the correct map representation expected by the fromJson method when deserializing an object.

Conclusion

Serialization errors such as type is not a subtype of type Map<String, dynamic> are common, especially when dealing with nested objects in Flutter applications. However, by utilizing the explicitToJson option correctly, we can seamlessly navigate through JSON serialization and ensure that our objects are accurately transformed.

By following the steps outlined above, you can eliminate this error and ensure a smooth experience when working with JSON serialization in your Flutter apps. Remember, clear serialization logic is key to preventing these common pitfalls!