Custom validators in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom validator in Flutter forms?

A custom validator is a function you write to check if a form field's input meets specific rules you define, like checking if a password is strong enough.

beginner

How do you attach a custom validator to a TextFormField in Flutter?

You pass your validator function to the TextFormField's 'validator' property. This function returns null if input is valid or an error message string if invalid.

beginner

What should a custom validator function return when the input is valid?

It should return null to indicate the input passes validation.

intermediate

Why use custom validators instead of built-in validators?

Custom validators let you check rules specific to your app, like matching passwords or checking a username format, which built-in validators might not cover.

beginner

What is a simple example of a custom validator that checks if a field is not empty?

A function that returns 'Please enter some text' if the input is empty, otherwise returns null.

What does a Flutter validator function return when the input is invalid?

A An error message string

B null

C true

D false

A validator returns an error message string to show the user what is wrong with their input.

Where do you assign a custom validator in a Flutter form?

A In the TextFormField's controller

B In the Scaffold widget

C In the MaterialApp widget

D In the TextFormField's validator property

The validator property of TextFormField is where you assign your custom validation function.

Which of these is a valid return value from a Flutter validator for valid input?

A null

B "Input is valid"

C "" (empty string)

D false

Returning null means the input passed validation with no errors.

Why might you create a custom validator instead of using built-in ones?

A To disable validation

B To add app-specific rules like password strength

C To avoid using TextFormField

D Because built-in validators are slower

Custom validators let you enforce rules unique to your app's needs.

What happens if a validator returns a non-null string?

A The app crashes

B The form submits successfully

C The form field shows an error message

D The input is cleared

Returning a string shows that string as an error message below the form field.

Explain how to create and use a custom validator in a Flutter form.

Think about what the validator function needs to do and where it goes.

Describe why custom validators are important in mobile app forms.

Consider what built-in validators might miss.