Скачать или смотреть Solving the Formz Validation Error in Flutter: Type Mismatch Explained

Discover how to resolve the `bool` to `FormzSubmissionStatus?` assignment issue in Flutter when using the Formz package for form validation.
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This video is based on the question https://stackoverflow.com/q/77176600/ asked by the user 'Jesse Josiah' ( https://stackoverflow.com/u/19589068/ ) and on the answer https://stackoverflow.com/a/77178980/ 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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Understanding the Formz Validation Error in Flutter

When developing applications in Flutter, specifically while implementing form validation using the Formz package, developers can encounter a frustrating type error. This article aims to unravel the details of the error message: “The argument type 'bool' can't be assigned to the parameter type 'FormzSubmissionStatus?'”. We will not only discuss the cause of this problem but will also provide a solution that aligns with best practices in Flutter and Dart development.

The Problem: Type Mismatch

In our scenario, the core issue arises from a type mismatch within the SignupController class, particularly in how we manage form validation states.

Here is the relevant portion of the code that triggers the error:

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

In the above snippet, the method Formz.validate returns a bool value indicating whether the form is valid or not. However, the status field within the SignUpState class is defined as FormzSubmissionStatus, which is an enumeration type. This mismatch leads to the type assignment error.

Another Issue: isValidated Getter Not Defined

Adding to the confusion, if you have attempted to check if the form submission status is validated using this line:

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

You may have encountered the error “The getter 'isValidated' isn't defined for the type 'FormzSubmissionStatus'.” This is another indication that the structure of our application's state model may need reconsideration.

The Solution: Correcting Type Assignments

Step 1: Adjust the State Management

To resolve these issues, we can modify our SignupController by either changing the type of the status field or implementing an additional field for form validation status.

Option 1: Change the Type of status

Instead of using FormzSubmissionStatus for status, we can opt to use a bool. This is the most straightforward solution but can limit future scalability, as you would need to derive status states (e.g., loading, success, failure) from boolean values alone.

Option 2: Introduce an isValid Field

A more extensible solution is to introduce an isValid field in the SignUpState to track the validation state. This way, you can assign the result of Formz.validate to isValid without changing the existing type of status. Here’s how you would do it:

Edit the SignUpState Class:

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

Modify the onNameChange Method:

Update the onNameChange method in your SignupController to manage the isValid field.

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

Check Validity Before Signup:

Lastly, make sure to use the isValid field to verify form validation before proceeding with the signup process:

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

Conclusion

By addressing the type mismatch and implementing an effective structure for handling validation states in your Flutter application, you can avoid the frustrating errors encountered during form validation. Utilizing an isValid boolean field alongside the FormzSubmissionStatus enumeration allows your code to remain clean, readable, and future-proof.

If you're building forms in Flutter, these strategies will help you efficiently manage validation and enhance overall user experience. Happy coding!