Custom validation property wrappers in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple custom validation property wrapper

What is the output of this Swift code using a custom validation property wrapper?

Swift

import Foundation

@propertyWrapper
struct NonEmpty {
    private var value: String = ""
    var wrappedValue: String {
        get { value }
        set {
            if newValue.isEmpty {
                print("Empty string not allowed")
            } else {
                value = newValue
            }
        }
    }
}

struct User {
    @NonEmpty var name: String
}

var user = User()
user.name = ""
print(user.name)
user.name = "Alice"
print(user.name)

A

Empty string not allowed


Alice

B

Empty string not allowed

C

Alice

D

Empty string not allowed

Alice

💡 Hint

Look at how the setter handles empty strings and what is printed.

The setter prints a message when an empty string is assigned but does not change the stored value. Initially, the value is empty, so printing it shows an empty line. After assigning "Alice", the value changes and prints "Alice".

🧠 Conceptual

intermediate

Purpose of custom validation property wrappers

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

A To improve the performance of property accessors

B To allow properties to be accessed from multiple threads safely

C To automatically validate and control the values assigned to properties

D To convert properties into computed properties without storage

💡 Hint

Think about what validation means in everyday life.

Custom validation property wrappers help ensure that property values meet certain rules before being stored, like checking if a string is not empty.

🔧 Debug

advanced

Identify the error in this custom validation property wrapper

What error will this Swift code produce when compiled?

Swift

@propertyWrapper
struct Positive {
    private var value: Int
    var wrappedValue: Int {
        get { value }
        set {
            if newValue > 0 {
                value = newValue
            } else {
                print("Value must be positive")
            }
        }
    }
}

struct Account {
    @Positive var balance: Int
}

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

A Runtime error: Unexpected nil value

B Error: Property 'value' not initialized before use

C Error: Cannot assign to property 'balance' because it is a 'let' constant

D No error, prints 0

💡 Hint

Check if all stored properties have initial values.

The property wrapper's stored property 'value' is not initialized, causing a compile-time error.

📝 Syntax

advanced

Correct syntax for a property wrapper with projectedValue

Which option correctly defines a property wrapper with a projectedValue that returns a Bool indicating if the value is valid?

Swift

struct Length {
    private var value: String
    var wrappedValue: String {
        get { value }
        set { value = newValue }
    }
    var projectedValue: Bool {
        // returns true if length > 3
    }
}

A var projectedValue: Bool { value.count > 3 }

B var projectedValue: Bool { return value.length > 3 }

C var projectedValue: Bool { value.count > 3 else false }

D var projectedValue: Bool { if value.count > 3 { true } }

💡 Hint

Use the correct property for string length and a simple expression.

Option A correctly uses 'value.count' and returns a Bool expression. Other options have syntax errors or use invalid properties.

🚀 Application

expert

Using a custom validation property wrapper in a struct

Given this property wrapper, what will be the output of the following code?

Swift

@propertyWrapper
struct Clamped {
    private var value: Int
    private let range: ClosedRange<Int>

    var wrappedValue: Int {
        get { value }
        set { value = min(max(newValue, range.lowerBound), range.upperBound) }
    }

    init(wrappedValue: Int, _ range: ClosedRange<Int>) {
        self.range = range
        self.value = min(max(wrappedValue, range.lowerBound), range.upperBound)
    }
}

struct Settings {
    @Clamped(0...10) var volume: Int = 5
}

var s = Settings()
s.volume = 15
print(s.volume)
s.volume = -3
print(s.volume)
s.volume = 7
print(s.volume)

A

10
0
7

B

5
5
7

C

15
-3
7

D

10
-3
7

💡 Hint

The wrapper clamps values to the range 0 to 10.

The property wrapper limits the value to the range 0 to 10. Assigning 15 clamps to 10, -3 clamps to 0, and 7 stays as 7.

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