Composing property wrappers in Swift - Mini Project: Build & Apply

Composing property wrappers

📖 Scenario: Imagine you are building a simple app that tracks user settings. You want to create reusable property wrappers to add behaviors like capitalizing strings and trimming whitespace. Then, you want to combine these wrappers to apply both behaviors to a single property.

🎯 Goal: Build two property wrappers: Capitalized and Trimmed. Then compose them to create a property that is both trimmed and capitalized automatically.

📋 What You'll Learn

Create a property wrapper called Capitalized that capitalizes the wrapped string value.

Create a property wrapper called Trimmed that trims whitespace from the wrapped string value.

Compose the two wrappers so a property is first trimmed then capitalized.

Print the final value of the composed property to verify the behavior.

💡 Why This Matters

🌍 Real World

Property wrappers are used in Swift apps to add reusable behaviors like validation, formatting, or storage management to properties without repeating code.

💼 Career

Understanding property wrappers and how to compose them is valuable for Swift developers working on clean, maintainable codebases, especially in iOS app development.

1

Create the Capitalized property wrapper

Create a property wrapper called Capitalized with a wrappedValue of type String. In the wrappedValue setter, update the value to be capitalized using capitalized property.

Swift

# Create the Capitalized property wrapper
# Your code here

Hint

Use @propertyWrapper before the struct. Store the value privately. Use capitalized to change the string.

2

Create the Trimmed property wrapper

Create a property wrapper called Trimmed with a wrappedValue of type String. In the wrappedValue setter, update the value by trimming whitespace using trimmingCharacters(in: .whitespacesAndNewlines).

Swift

@propertyWrapper
struct Capitalized {
    private var value: String = ""
    var wrappedValue: String {
        get { value }
        set { value = newValue.capitalized }
    }
    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

# Create the Trimmed property wrapper
# Your code here

Hint

Similar to Capitalized, but use trimmingCharacters(in: .whitespacesAndNewlines) to remove spaces.

3

Compose the property wrappers on a property

Create a struct called UserSettings with a property username of type String. Apply the @Capitalized and @Trimmed wrappers to username so that trimming happens first, then capitalization.

Swift

@propertyWrapper
struct Capitalized {
    private var value: String = ""
    var wrappedValue: String {
        get { value }
        set { value = newValue.capitalized }
    }
    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

@propertyWrapper
struct Trimmed {
    private var value: String = ""
    var wrappedValue: String {
        get { value }
        set { value = newValue.trimmingCharacters(in: .whitespacesAndNewlines) }
    }
    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

// Create struct UserSettings with a property username
// Apply @Capitalized and @Trimmed wrappers to username
# Your code here

Hint

Apply @Capitalized before @Trimmed on the username property.

4

Print the composed property value

Create an instance of UserSettings with username initialized to " hello world ". Then print the username property to see the trimmed and capitalized result.

Swift

@propertyWrapper
struct Capitalized {
    private var value: String = ""
    var wrappedValue: String {
        get { value }
        set { value = newValue.capitalized }
    }
    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

@propertyWrapper
struct Trimmed {
    private var value: String = ""
    var wrappedValue: String {
        get { value }
        set { value = newValue.trimmingCharacters(in: .whitespacesAndNewlines) }
    }
    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

struct UserSettings {
    @Capitalized @Trimmed var username: String
}

// Create an instance of UserSettings with username "  hello world  "
// Print the username property
# Your code here

Hint

Initialize UserSettings with the string containing spaces. Printing username should show trimmed and capitalized text.

Practice

1. What does it mean to compose property wrappers in Swift?

easy

A. Apply property wrappers only to functions

B. Use only one property wrapper per property

C. Create a new property wrapper by subclassing another

D. Stack multiple property wrappers on a single property to combine behaviors

Solution

1. Step 1: Understand property wrappers

   Property wrappers add behavior to properties in Swift.

2. Step 2: Define composition meaning

   Composing means stacking multiple wrappers on one property to combine their effects.

3. Final Answer:

   Stack multiple property wrappers on a single property to combine behaviors -> Option D

4. Quick Check:

   Composing = stacking wrappers [OK]

Hint: Remember: composing means stacking wrappers on one property [OK]

Common Mistakes:

• Thinking only one wrapper can be used per property
• Confusing subclassing with composing
• Applying wrappers to functions instead of properties

2. Which of the following is the correct syntax to compose two property wrappers @WrapperA and @WrapperB on a property var value: Int such that @WrapperA is applied first (innermost)?

easy

A. @WrapperB @WrapperA var value: Int

B. @WrapperA @WrapperB var value: Int

C. var @WrapperA @WrapperB value: Int

D. var value: Int @WrapperA @WrapperB

Solution

1. Step 1: Recall property wrapper order

   Property wrappers apply from right to left, so the rightmost is applied first.

2. Step 2: Check syntax order

   Correct syntax places wrappers before the property, so @WrapperB @WrapperA var value: Int applies @WrapperA first, then @WrapperB.

3. Final Answer:

   @WrapperB @WrapperA var value: Int -> Option A

4. Quick Check:

   Right to left wrapper order = @WrapperB @WrapperA var value: Int [OK]

Hint: Remember wrappers apply right to left, so last wrapper is closest to var [OK]

Common Mistakes:

• Placing wrappers after the variable name
• Reversing the order of wrappers
• Using wrappers inside the variable declaration

3. Given these property wrappers:

 @propertyWrapper
 struct WrapperA {
   var wrappedValue: Int
   init(wrappedValue: Int) { self.wrappedValue = wrappedValue + 1 }
 }

 @propertyWrapper
 struct WrapperB {
   var wrappedValue: Int
   init(wrappedValue: Int) { self.wrappedValue = wrappedValue * 2 }
 }

 struct Example {
   @WrapperA @WrapperB var value: Int = 3
 }

What is the value of Example().value?

medium

A. 7

B. 8

C. 6

D. 10

Solution

1. Step 1: Apply wrappers right to left

   First, @WrapperB applies: 3 * 2 = 6.

2. Step 2: Apply @WrapperA next

   Then @WrapperA adds 1: 6 + 1 = 7.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   (3 * 2) + 1 = 7 [OK]

Hint: Apply wrappers from right to left to find final value [OK]

Common Mistakes:

• Applying wrappers left to right
• Forgetting to add 1 in WrapperA
• Multiplying after adding instead of before

4. Consider this code snippet:

 @propertyWrapper
 struct WrapperX {
   var wrappedValue: Int
   init(wrappedValue: Int) { self.wrappedValue = wrappedValue + 5 }
 }

 struct Test {
   @WrapperX @WrapperX var number: Int = 10
 }

What is the error in this code?

medium

A. Cannot apply the same property wrapper twice on one property

B. The property wrapper's init is called twice, causing unexpected behavior

C. No error; code compiles and runs correctly

D. The property wrapper must have a default initializer

Solution

1. Step 1: Check if multiple same wrappers allowed

   Swift allows stacking the same property wrapper multiple times.

2. Step 2: Analyze wrapper initialization

   Each wrapper adds 5, so first adds 5 to 10 = 15, second adds 5 to 15 = 20.

3. Final Answer:

   No error; code compiles and runs correctly -> Option C

4. Quick Check:

   Same wrapper stacking allowed = No error; code compiles and runs correctly [OK]

Hint: Same wrapper can be stacked; no syntax error [OK]

Common Mistakes:

• Assuming duplicate wrappers cause errors
• Expecting default initializer requirement
• Confusing runtime behavior with compile errors

5. You want to create a property that is both clamped between 0 and 100 and logged when changed. You have two property wrappers:

 @propertyWrapper
 struct Clamped {
   private var value: Int
   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)
   }
 }

 @propertyWrapper
 struct Logged {
   private var value: Int
   var wrappedValue: Int {
     get { value }
     set {
       print("Value changed to \(newValue)")
       value = newValue
     }
   }
   init(wrappedValue: Int) {
     self.value = wrappedValue
   }
 }

 struct Settings {
   @Logged @Clamped(0...100) var score: Int = 150
 }

What will be printed and what is the final value of Settings().score?

hard

A. Prints 'Value changed to 150', final value 150

B. Prints nothing, final value 100

C. Prints 'Value changed to 100', final value 100

D. Prints 'Value changed to 0', final value 0

Solution

1. Step 1: Apply wrappers right to left

   First, @Clamped(0...100) clamps 150 to 100 in init.

2. Step 2: Then @Logged receives 100

   @Logged init sets value=100 directly (no setter called, no print).

3. Final Answer:

   Prints nothing, final value 100 -> Option B

4. Quick Check:

   Clamp then direct init set = no print, value 100 [OK]

Hint: Initial values chain through inits right to left; no setter/print on init [OK]

Common Mistakes:

• Thinking log prints during initialization
• Thinking clamp applies after log
• Assuming propagation calls inner setters