Custom validation property wrappers in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a property wrapper in Swift?

A property wrapper is a Swift feature that lets you add extra behavior to properties, like validation or formatting, by wrapping the property’s value with custom code.

intermediate

How do you define a custom validation property wrapper?

You create a struct or class with the @propertyWrapper attribute, include a wrappedValue property, and add validation logic inside the setter of wrappedValue.

intermediate

What happens if a value fails validation in a custom property wrapper?

You can handle it by rejecting the value, setting a default, throwing an error, or logging a message, depending on how you design the wrapper.

beginner

Show a simple example of a custom validation property wrapper that ensures a string is not empty.

@propertyWrapper struct NonEmpty { private var value: String init(wrappedValue: String) { self.value = wrappedValue.isEmpty ? "Default" : wrappedValue } var wrappedValue: String { get { value } set { value = newValue.isEmpty ? "Default" : newValue } } } struct User { @NonEmpty var name: String }

beginner

Why use custom validation property wrappers instead of validating manually?

They keep your code clean by centralizing validation logic, reduce repeated code, and make properties safer and easier to maintain.

What keyword do you use to mark a custom property wrapper in Swift?

A @validate

B @wrapper

C @custom

D @propertyWrapper

The correct keyword to define a property wrapper is @propertyWrapper.

Where do you put the validation logic inside a custom property wrapper?

A Inside the init method only

B Inside the wrappedValue setter

C Outside the property wrapper

D Inside the wrappedValue getter

Validation logic is usually placed inside the setter of wrappedValue to check or modify new values.

What does the wrappedValue property represent in a property wrapper?

A The actual value being wrapped

B The name of the property

C The type of the property

D The validation function

wrappedValue holds the actual value that the property wrapper manages.

If you want to provide a default value when validation fails, where do you do it?

A In the wrappedValue setter

B In the property declaration only

C In the getter of wrappedValue

D In the main struct using the property wrapper

You provide default values or fallback logic inside the setter of wrappedValue when validation fails.

Which of these is NOT a benefit of using custom validation property wrappers?

A Centralizing validation logic

B Reducing repeated code

C Automatically fixing all bugs

D Making properties safer

Property wrappers help with validation but do not automatically fix all bugs.

Explain how to create a custom validation property wrapper in Swift and why it is useful.

Think about how you can check or change values when they are set.

Describe a simple example of a custom validation property wrapper that ensures a string is never empty.

Imagine you want to avoid empty names in a user profile.

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