Resolving the TextFormField Validator Argument Type Error in Flutter

Learn how to fix the `dynamic Function()` error that can occur with the `validator` property in Flutter's `TextFormField`. This guide offers clear explanations and structured solutions.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: TextFormField validator The argument type 'dynamic Function()' can't be assigned to the

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the TextFormField Validator Issue in Flutter

If you have ever encountered the message “The argument type 'dynamic Function()' can't be assigned to the parameter type 'String? Function(String?)?'” while working with the TextFormField widget in Flutter, you are not alone. This error typically signifies that the validator function you have provided does not match the expected type required by TextFormField. Understanding this mismatch is critical for creating effective forms in your Flutter applications.

The Problem

In a Flutter application, when you create a custom widget that includes a TextFormField, you might want to include validation logic. However, due to the strict type requirements in Dart, if the function signature of your validator doesn’t match what Flutter expects, you'll run into a type error. Here’s the immediate code snippet that demonstrates the error:

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

This line is expecting a function with a specific signature but instead receives one that doesn't conform, leading to the type error. This can be frustrating, especially when you are trying to implement custom functionality within your app.

Solution

The key to resolving this issue lies in defining the validator function correctly. The validator parameter of TextFormField expects a function that receives a String? (the input from the user), checks for validation, and returns a String? for error messages—or null if there are no errors.

1. Adjusting the Validator Function Signature

To comply with the expected signature, you need to make sure your validator function takes a String? argument and returns a String?, as shown in the corrected example below:

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

2. Implementing it in Your Custom Widget

Now let’s apply this correctly structured validator function in your custom widget. Here is the proper implementation of MyTextFormField:

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

3. Using the Custom Widget

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

Conclusion

By ensuring that the validator function has the required signature, you can effectively resolve the type error and allow TextFormField to properly validate user input. This adjustment not only fixes the error message but also enhances the usability and functionality of your Flutter application. Testing different input scenarios will help assure that your validation logic works as intended. Happy coding!