For real-time form validation in Flutter, the ________ callback is used in form fields.

For real-time form validation in Flutter, the 'onChanged' callback is used in form fields. This callback is triggered whenever the value of the form field changes, providing a real-time hook to validate user input. By implementing the 'onChanged' callback, developers can perform validation logic and update the UI dynamically, giving users immediate feedback on their input. Understanding the role of 'onChanged' is crucial for effective form validation in Flutter applications.