Custom validators in Flutter

Introduction

Custom validators help check if user input is correct in ways built-in checks can't. They make sure your app gets good data.

When you want to check if a password has special rules like a number or symbol.

When you need to verify a username is not already taken or follows a pattern.

When you want to confirm an email address is in a specific format.

When you want to check a phone number matches your country's style.

When you want to give users clear messages about what is wrong with their input.

Syntax

Flutter

String? customValidator(String? value) {
  if (value == null || value.isEmpty) {
    return 'This field cannot be empty';
  }
  if (!value.contains('@')) {
    return 'Must contain @ symbol';
  }
  return null; // null means input is valid
}

The validator function returns a String?. If it returns null, the input is valid.

If it returns a String, that string is shown as an error message.

Examples

Checks if password is at least 6 characters long.

Flutter

String? passwordValidator(String? value) {
  if (value == null || value.length < 6) {
    return 'Password must be at least 6 characters';
  }
  return null;
}

Ensures username is not empty and has no spaces.

Flutter

String? usernameValidator(String? value) {
  if (value == null || value.isEmpty) {
    return 'Username cannot be empty';
  }
  if (value.contains(' ')) {
    return 'Username cannot contain spaces';
  }
  return null;
}

Sample App

This app shows a form with an email field. The custom validator checks if the email is empty or missing '@'. If invalid, it shows an error below the field. If valid, it shows a success message.

Flutter

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Custom Validator Example')),
        body: const Padding(
          padding: EdgeInsets.all(16),
          child: MyForm(),
        ),
      ),
    );
  }
}

class MyForm extends StatefulWidget {
  const MyForm({super.key});

  @override
  State<MyForm> createState() => _MyFormState();
}

class _MyFormState extends State<MyForm> {
  final _formKey = GlobalKey<FormState>();
  final _emailController = TextEditingController();

  String? emailValidator(String? value) {
    if (value == null || value.isEmpty) {
      return 'Email cannot be empty';
    }
    if (!value.contains('@')) {
      return 'Email must contain @';
    }
    return null;
  }

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: [
          TextFormField(
            controller: _emailController,
            decoration: const InputDecoration(labelText: 'Email'),
            validator: emailValidator,
          ),
          const SizedBox(height: 20),
          ElevatedButton(
            onPressed: () {
              if (_formKey.currentState!.validate()) {
                ScaffoldMessenger.of(context).showSnackBar(
                  const SnackBar(content: Text('Email is valid!')),
                );
              }
            },
            child: const Text('Submit'),
          ),
        ],
      ),
    );
  }
}

Important Notes

Always return null from your validator when the input is valid.

Use TextFormField with a Form widget to use validators easily.

Call _formKey.currentState!.validate() to run all validators and check if form is valid.

Summary

Custom validators check user input with your own rules.

Return a string error message if input is wrong, or null if input is good.

Use validators inside TextFormField and call validate() to check.

Practice

1. What does a custom validator function in Flutter return when the input is valid?

easy

A. false

B. null

C. true

D. An empty string

Solution

1. Step 1: Understand validator return type

   A custom validator returns a String error message if input is invalid, otherwise null.

2. Step 2: Identify valid input return

   When input is valid, the validator returns null to indicate no error.

3. Final Answer:

   null -> Option B

4. Quick Check:

   Valid input returns null [OK]

Hint: Valid input returns null, errors return strings [OK]

Common Mistakes:

• Returning empty string instead of null
• Returning boolean true/false
• Returning error message on valid input

2. Which of the following is the correct syntax for a simple custom validator in Flutter that checks if input is empty?

easy

A. validator: (value) => value.isEmpty ? null : 'Enter text'

B. validator: (value) { if (value.isEmpty) return 'Enter text'; }

C. validator: (value) => if (value.isEmpty) 'Enter text' else null

D. validator: (value) => value!.isEmpty ? 'Enter text' : null

Solution

1. Step 1: Check correct Dart syntax for validator

   The validator must return a String or null. validator: (value) => value!.isEmpty ? 'Enter text' : null uses a concise lambda with a ternary operator correctly.

2. Step 2: Identify null safety and return values

   validator: (value) => value!.isEmpty ? 'Enter text' : null uses value! to assert non-null and returns error string if empty, else null.

3. Final Answer:

   validator: (value) => value!.isEmpty ? 'Enter text' : null -> Option D

4. Quick Check:

   Correct syntax uses ternary and returns null on valid input [OK]

Hint: Use ternary operator and return null for valid input [OK]

Common Mistakes:

• Missing return statement in block syntax
• Using if without else in lambda
• Returning null on empty input instead of error

3. Given this validator code inside a TextFormField:

validator: (value) {
  if (value == null || value.length < 5) {
    return 'Too short';
  }
  return null;
}

What happens if the user enters "abc"?

medium

A. The app crashes due to null error

B. The form accepts input with no error

C. The form shows error message 'Too short'

D. The validator returns null but error shows anyway

Solution

1. Step 1: Analyze input length check

   The validator checks if value is null or length less than 5, then returns 'Too short'.

2. Step 2: Apply input "abc" to condition

   "abc" length is 3, which is less than 5, so validator returns 'Too short'.

3. Final Answer:

   The form shows error message 'Too short' -> Option C

4. Quick Check:

   Input length < 5 triggers error message [OK]

Hint: Check input length against condition to predict error [OK]

Common Mistakes:

• Ignoring null check and assuming no error
• Confusing return null with error message
• Assuming validator crashes on short input

4. Identify the error in this custom validator code:

validator: (value) {
  if (value.isEmpty) {
    return 'Cannot be empty';
  }
}

medium

A. Using isEmpty on nullable value without null check

B. Missing return null for valid input

C. Returning a string instead of boolean

D. Validator should not use if statements

Solution

1. Step 1: Check null safety on value

   value is nullable String?, so calling value.isEmpty without null check causes error.

2. Step 2: Identify missing null check

   Must check if value is null before calling isEmpty to avoid runtime error.

3. Final Answer:

   Using isEmpty on nullable value without null check -> Option A

4. Quick Check:

   Always check for null before isEmpty on nullable String [OK]

Hint: Check for null before calling isEmpty on value [OK]

Common Mistakes:

• Assuming value is non-null without checking
• Not returning null for valid input (though important)
• Confusing return types of validator

5. You want to create a custom validator that checks if input is a valid email and also ends with ".com". Which validator implementation correctly combines these checks?

hard

A. validator: (value) {
if (value == null || !value.contains('@')) return 'Invalid email';
if (!value.endsWith('.com')) return 'Must end with .com';
return null;
}

B. validator: (value) => value!.contains('@') && value.endsWith('.com') ? null : 'Invalid email or domain';

C. validator: (value) {
if (value == null) return null;
if (!value.contains('@') || !value.endsWith('.com')) return 'Invalid email';
return null;
}

D. validator: (value) {
if (value == null || value.contains('@') && value.endsWith('.com')) return null;
return 'Invalid email';
}

Solution

1. Step 1: Check null and '@' presence

   validator: (value) { if (value == null || !value.contains('@')) return 'Invalid email'; if (!value.endsWith('.com')) return 'Must end with .com'; return null; } first checks if value is null or missing '@', returning 'Invalid email'.

2. Step 2: Check if ends with '.com'

   Then it checks if value ends with '.com', returning specific error if not.

3. Step 3: Return null if all checks pass

   If both checks pass, it returns null indicating valid input.

4. Final Answer:

   validator: (value) { if (value == null || !value.contains('@')) return 'Invalid email'; if (!value.endsWith('.com')) return 'Must end with .com'; return null; } -> Option A

5. Quick Check:

   Separate checks with clear error messages [OK]

Hint: Check conditions stepwise, return null only if all pass [OK]

Common Mistakes:

• Combining conditions incorrectly with && or ||
• Returning null on null input without error
• Not checking null before calling methods