All 7 Forms & Input Validation in Flutter questions
More in Forms & Input Validation in Flutter
What is the Form widget in Flutter, and how does it let you properly group together several separate individual input fields to be validated and submitted all together as one single unit? How do you properly implement real time validation feedback in Flutter, showing a genuine validation error message as a user actively types rather than only once they actually submit the entire form? How does properly using TextInputFormatter genuinely help restrict or automatically format exactly what a user can actually type into a specific given text field, such as automatically formatting a phone number or restricting input to only numeric digits? How should a form properly and clearly handle and display a genuine server side validation error, such as discovering that a specific chosen username is already actually taken, that genuinely cannot actually be determined purely through simple client side validation logic alone? How do you properly build a genuinely complex, multi step form, sometimes called a wizard, that guides a user progressively through several separate sequential distinct steps, while genuinely correctly preserving each individual step's own entered data as they move forward and back between the different steps? How can you properly implement debounced asynchronous validation for a specific field, such as checking username availability against a server, without triggering a genuinely brand new separate network request on essentially every single individual keystroke a user actually types?
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Flutter › Forms & Input Validation in Flutter

How does the validator property on a TextFormField let you define a specific custom validation rule, such as properly checking that an entered email address actually contains an genuine at symbol?

Beginner

Answer

The validator property accepts a function that receives the field's own current entered text value and should return either null, indicating that value is genuinely valid, or a specific descriptive error message string if that value is genuinely not actually valid, and Flutter automatically calls this exact same validator function for every single field whenever the surrounding Form's own validate method is actually called, automatically displaying any returned error message directly underneath its own corresponding specific field.
TextFormField(
  validator: (value) {
    if (value == null || !value.contains('@')) {
      return 'Please enter a genuinely valid email address';
    }
    return null;
  },
)
Real-world example A login form's email field displays a clear error message directly underneath it the moment a user attempts to submit the form having entered text that genuinely does not contain the required at symbol, guiding them to properly correct their specific mistake.

Common follow-ups: Can a single field genuinely have more than one separate distinct validation rule applied together?;How do you properly trigger validation to actually happen as a user types, rather than only when they genuinely submit the entire form?

Dart Language Basics & Syntax;Custom Widgets & Reusable Components
All 7 Forms & Input Validation in Flutter questions