Custom validators in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple validator that checks if the input is empty.

Flutter

String? validateNotEmpty(String? value) {
  if (value == null || value[1] '') {
    return 'Field cannot be empty';
  }
  return null;
}

Drag options to blanks, or click blank then click option'

A!=

B==

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using != instead of == causes the validator to fail when the field is empty.

Using > or < operators on strings causes errors.

The validator returns an error message if the input is null or equals an empty string. The operator == checks for equality.

2fill in blank

medium

Complete the code to create a validator that checks if the input length is at least 6 characters.

Flutter

String? validateMinLength(String? value) {
  if (value == null || value.length [1] 6) {
    return 'Minimum 6 characters required';
  }
  return null;
}

Drag options to blanks, or click blank then click option'

A<

B>

C<=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using > or >= will allow shorter inputs incorrectly.

Using <= includes length 6 as invalid, which is wrong.

The validator returns an error if the input length is less than 6. The operator < checks this condition.

3fill in blank

hard

Fix the error in the validator that checks if the input contains only digits.

Flutter

String? validateDigitsOnly(String? value) {
  final regex = RegExp(r'^[0-9]+$');
  if (value == null || !regex.[1](value)) {
    return 'Only digits allowed';
  }
  return null;
}

Drag options to blanks, or click blank then click option'

Amatch

Bcontains

ChasMatch

DstartsWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using match causes a runtime error because it does not exist.

Using contains or startsWith only checks part of the string.

The hasMatch method checks if the entire string matches the pattern. Using match or others causes errors or wrong checks.

4fill in blank

hard

Fill both blanks to create a validator that checks if the input is a valid email format.

Flutter

String? validateEmail(String? value) {
  final emailRegex = RegExp(r'^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$');
  if (value == null || !emailRegex.[1](value)) {
    return 'Enter a valid email';
  }
  return [2];
}

Drag options to blanks, or click blank then click option'

AhasMatch

Bmatch

Cnull

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the input value instead of null causes the form to think there is an error.

Using match causes errors.

The hasMatch method checks if the input matches the email pattern. The validator returns null when the input is valid, meaning no error.

5fill in blank

hard

Fill all three blanks to create a validator that checks if the input password has at least 8 characters, contains a digit, and returns null if valid.

Flutter

String? validatePassword(String? value) {
  final digitRegex = RegExp(r'\d');
  if (value == null || value.length [1] 8 || !digitRegex.[2](value)) {
    return 'Password must be 8+ chars and include a digit';
  }
  return [3];
}

Drag options to blanks, or click blank then click option'

A<

BhasMatch

Cnull

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using > instead of < for length check allows short passwords.

Using wrong regex method causes errors.

Returning value instead of null causes validation failure.

The validator checks if the password length is less than 8 and if it does not contain a digit using hasMatch. It returns null when valid.

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