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.

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