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Discover how to set default values in Flutter models using json_serializable, ensuring that null values are replaced effectively.
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How to Use defaultValue Effectively in Flutter's json_serializable?

In the world of Flutter development, utilizing json_serializable to manage JSON data can save you a lot of time and streamline your code. However, a common question arises when working with nullable fields: How do you ensure that default values are applied when a model is constructed with null values? In this post, we will explore the solution to this problem, focusing on a specific case involving default values in models.

The Problem

When creating a class model in Flutter to handle JSON data, you might encounter a situation where the values being passed in are null. For example, consider the following class TestModelA, which has nullable fields for language and location:

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

In this setup, despite specifying a default value via the @ JsonKey annotation, if the incoming JSON has null values, the properties are still set to null instead of the default. This can lead to unexpected behavior in your application, which is clearly not ideal.

The Solution

To address this issue, we need to ensure that when we create our model using fromJson, it checks for null JSON fields and assigns the default values accordingly. Here’s an effective solution you can implement:

Modified fromJson Factory Method

You will need to customize the fromJson factory method to handle null values appropriately. Here’s a revised version of the fromJson factory method that achieves this:

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

Explanation of the Code

Null Check: The code checks if json is not null. If it is null, the factory method constructs a new TestModelA object using default values ("jp" for both language and location).

Swapping JSON Deserialization: If valid JSON is provided (i.e. not null), it falls back to the autogenerated deserialization logic, which will use whatever values are in the JSON.

Benefits of This Approach

Reliable Defaults: This solution ensures that TestModelA always has valid initialization values, even if the input JSON does not provide them.

Flexibility: You can still take advantage of other features provided by json_serializable without sacrificing safety and reliability.

Maintainability: By customizing the fromJson function, you maintain clean and understandable code that adheres to best practices.

Conclusion

Setting up default values in a Flutter model using json_serializable can be tricky when dealing with nullable fields. However, customizing the fromJson method as shown above allows you to effectively assign default values when working with JSON data that may contain nulls. This method leads to safer and cleaner code, making your application robust against unexpected behavior.

By following the steps outlined in this guide, you can ensure your models are set up correctly to handle all scenarios that may arise during JSON deserialization. Happy coding!