Custom validation property wrappers in Swift - Mini Project: Build & Apply

Custom Validation Property Wrappers

📖 Scenario: You are building a simple user registration form in Swift. You want to make sure the user's input is valid before saving it.

🎯 Goal: Create a custom property wrapper to validate a username so it is at least 5 characters long. Use this property wrapper in a User struct and print the username if it is valid.

📋 What You'll Learn

Create a custom property wrapper called ValidatedUsername

The property wrapper should check that the username is at least 5 characters long

Create a User struct with a username property using the ValidatedUsername wrapper

Create an instance of User with the username "SwiftDev"

Print the username from the User instance

💡 Why This Matters

🌍 Real World

Property wrappers help keep your code clean by centralizing validation logic for user input or data models.

💼 Career

Understanding property wrappers and validation is useful for Swift developers building safe and maintainable apps.

1

Create the ValidatedUsername property wrapper

Create a property wrapper called ValidatedUsername with a wrappedValue property of type String. Initialize wrappedValue with an empty string.

Swift

# Create the ValidatedUsername property wrapper
# Your code here

Hint

Use @propertyWrapper before the struct. Define wrappedValue as a String property with default empty string.

2

Add validation logic to ValidatedUsername

Modify the ValidatedUsername property wrapper to accept an initial wrappedValue in its initializer. Add validation so that if the username is shorter than 5 characters, it sets wrappedValue to an empty string.

Swift

@propertyWrapper
struct ValidatedUsername {
    var wrappedValue: String

    // Add initializer with validation
    // Your code here
}

Hint

Use an initializer init(wrappedValue: String). Check the length with wrappedValue.count. Assign accordingly.

3

Create the User struct using ValidatedUsername

Create a struct called User with a property username that uses the @ValidatedUsername property wrapper.

Swift

@propertyWrapper
struct ValidatedUsername {
    var wrappedValue: String

    init(wrappedValue: String) {
        if wrappedValue.count >= 5 {
            self.wrappedValue = wrappedValue
        } else {
            self.wrappedValue = ""
        }
    }
}

// Create User struct with username property
// Your code here

Hint

Define struct User. Add @ValidatedUsername before var username: String.

4

Create a User instance and print the username

Create a constant user of type User with the username "SwiftDev". Then print user.username.

Swift

@propertyWrapper
struct ValidatedUsername {
    var wrappedValue: String

    init(wrappedValue: String) {
        if wrappedValue.count >= 5 {
            self.wrappedValue = wrappedValue
        } else {
            self.wrappedValue = ""
        }
    }
}

struct User {
    @ValidatedUsername var username: String
}

// Create user instance and print username
// Your code here

Hint

Create let user = User(username: "SwiftDev"). Then print(user.username).

Practice

1. What is the main purpose of a custom validation property wrapper in Swift?

easy

A. To automatically check and enforce rules on property values

B. To change the type of a property at runtime

C. To make properties private and inaccessible

D. To store multiple values in a single property

Solution

1. Step 1: Understand property wrappers

   Property wrappers in Swift add extra behavior to properties, such as validation.

2. Step 2: Identify validation role

   Custom validation wrappers check if values meet certain rules before storing them.

3. Final Answer:

   To automatically check and enforce rules on property values -> Option A

4. Quick Check:

   Validation = enforce rules [OK]

Hint: Validation wrappers enforce rules on data automatically [OK]

Common Mistakes:

• Thinking wrappers change property type
• Confusing validation with access control
• Assuming wrappers store multiple values

2. Which of the following is the correct syntax to declare a custom validation property wrapper named NonEmpty for a String property in Swift?

easy

A. var name: @NonEmpty String

B. var @NonEmpty name: String

C. @NonEmpty var name: String

D. NonEmpty var name: String

Solution

1. Step 1: Recall property wrapper syntax

   In Swift, property wrappers are applied with @ before the wrapper name, placed before the variable declaration.

2. Step 2: Match correct syntax

   Correct syntax is @WrapperName var propertyName: Type, so @NonEmpty var name: String is correct.

3. Final Answer:

   @NonEmpty var name: String -> Option C

4. Quick Check:

   @WrapperName before var = correct syntax [OK]

Hint: Use @ before wrapper name, then var property [OK]

Common Mistakes:

• Placing @ after var
• Putting wrapper inside type
• Omitting @ symbol

3. Consider this Swift code using a custom validation property wrapper Positive that only allows positive integers:

 @propertyWrapper
 struct Positive {
     private var value: Int = 1
     var wrappedValue: Int {
         get { value }
         set { value = newValue > 0 ? newValue : 1 }
     }
 }

 struct Account {
     @Positive var balance: Int
 }

 let acc = Account()
 print(acc.balance)

What will be printed when this code runs?

medium

A. 1

B. 0

C. nil

D. Compilation error

Solution

1. Step 1: Understand default value in wrapper

   The wrapper's private value starts at 1, and sets new values only if positive; else resets to 1.

2. Step 2: Check initial property value

   Account's balance uses @Positive but no initial value is set, so it uses wrapper's default 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Default wrapper value = 1 [OK]

Hint: Default wrapper value used if no initial value given [OK]

Common Mistakes:

• Expecting 0 as default
• Assuming nil is possible
• Thinking code won't compile

4. This Swift code attempts to create a custom validation property wrapper NonNegative but has an error:

 @propertyWrapper
 struct NonNegative {
     var value: Int
     var wrappedValue: Int {
         get { value }
         set { if newValue >= 0 { value = newValue } }
     }
 }

 struct Score {
     @NonNegative var points: Int = -5
 }

What is the main problem causing this code to fail?

medium

A. wrappedValue setter does not handle negative values correctly

B. Property wrapper lacks an initializer to set initial value

C. Cannot assign default value to property with wrapper

D. Missing @ symbol before property wrapper

Solution

1. Step 1: Check property wrapper requirements

   Property wrappers must have an init(wrappedValue:) to accept initial values.

2. Step 2: Identify missing initializer

   NonNegative lacks init(wrappedValue:) so compiler cannot assign -5 to points.

3. Final Answer:

   Property wrapper lacks an initializer to set initial value -> Option B

4. Quick Check:

   Missing init(wrappedValue:) = error [OK]

Hint: Add init(wrappedValue:) to accept initial values [OK]

Common Mistakes:

• Ignoring missing initializer
• Blaming setter logic
• Thinking default value assignment is invalid

5. You want to create a custom validation property wrapper LimitedLength that limits a String property's length to a maximum number of characters. Which of the following implementations correctly enforces this limit and allows setting the max length when declaring the property?

 @propertyWrapper
 struct LimitedLength {
     private var value: String = ""
     private let maxLength: Int

     var wrappedValue: String {
         get { value }
         set {
             if newValue.count <= maxLength {
                 value = newValue
             } else {
                 value = String(newValue.prefix(maxLength))
             }
         }
     }

     // Which initializer is correct?
     ???
 }

 struct User {
     @LimitedLength(maxLength: 5) var username: String
 }

hard

A. init(maxLength: Int) { self.maxLength = maxLength }

B. init(wrappedValue: String) { self.value = wrappedValue; maxLength = 5 }

C. init() { maxLength = 5 }

D. init(wrappedValue: String, maxLength: Int) { self.maxLength = maxLength; self.value = wrappedValue }

Solution

1. Step 1: Understand property wrapper initialization with parameters

   When a wrapper has parameters and initial value, it needs init(wrappedValue:maxLength:) to accept both.

2. Step 2: Match initializer to usage

   Usage passes maxLength: 5 and no initial value, so init(wrappedValue:maxLength:) fits best to set both.

3. Final Answer:

   init(wrappedValue: String, maxLength: Int) { self.maxLength = maxLength; self.value = wrappedValue } -> Option D

4. Quick Check:

   Use init(wrappedValue:maxLength:) for parameters [OK]

Hint: Use init(wrappedValue:maxLength:) to accept value and limit [OK]

Common Mistakes:

• Using init(maxLength:) only misses initial value
• Using init() without parameters
• Using init(wrappedValue:) without maxLength