WrappedValue and projectedValue in Swift

Introduction

WrappedValue and projectedValue help you control how a property behaves and what extra information it can provide.

When you want to add extra behavior to a property, like validation or logging.

When you want to expose additional info about a property, like whether it changed.

When you want to customize how a property's value is stored or accessed.

When you want to create reusable property wrappers to simplify your code.

Syntax

Swift

@propertyWrapper
struct WrapperName {
    var wrappedValue: Type
    var projectedValue: ExtraType

    init(wrappedValue: Type) {
        self.wrappedValue = wrappedValue
        self.projectedValue = ... // set extra info
    }
}

wrappedValue is the main value the property holds.

projectedValue is extra info you can access with a $ prefix.

Examples

This wrapper capitalizes the string whenever it is set.

Swift

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

This wrapper prints a message every time the value changes.

Swift

@propertyWrapper
struct Logged {
    var wrappedValue: Int {
        didSet { print("Value changed to \(wrappedValue)") }
    }

    init(wrappedValue: Int) {
        self.wrappedValue = wrappedValue
    }
}

This wrapper provides a string description of the boolean value via projectedValue.

Swift

@propertyWrapper
struct Toggle {
    var wrappedValue: Bool
    var projectedValue: String {
        wrappedValue ? "On" : "Off"
    }

    init(wrappedValue: Bool) {
        self.wrappedValue = wrappedValue
    }
}

Sample Program

This program uses a property wrapper Trimmed that removes spaces around a string. The projectedValue gives the length of the trimmed string. We create a user with spaces around the name, then print the trimmed name and its length.

Swift

import Foundation

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

    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

struct User {
    @Trimmed var name: String
}

var user = User(name: "  Alice  ")
print("Name: '\(user.name)'" )
print("Length: \(user.$name)")

Important Notes

You access wrappedValue normally, but projectedValue with a $ before the property name.

Property wrappers make your code cleaner by reusing common property logic.

Not all property wrappers need a projectedValue, but it is useful for extra info.

Summary

wrappedValue is the main value stored or controlled by the property wrapper.

projectedValue is extra information or behavior you can access with $property.

Use these to add helpful features to your properties without repeating code.

Practice

1. What does wrappedValue represent in a Swift property wrapper?

easy

A. The main value stored or controlled by the property wrapper

B. An extra method to modify the property

C. A way to access the property's type information

D. A function to reset the property to default

Solution

1. Step 1: Understand the role of wrappedValue

   The wrappedValue is the core value that the property wrapper manages or stores internally.

2. Step 2: Differentiate from other features

   Unlike projectedValue, wrappedValue is the actual data, not extra info or behavior.

3. Final Answer:

   The main value stored or controlled by the property wrapper -> Option A

4. Quick Check:

   wrappedValue = main stored value [OK]

Hint: wrappedValue is the property's actual stored value [OK]

Common Mistakes:

• Confusing wrappedValue with projectedValue
• Thinking wrappedValue is a method
• Assuming wrappedValue holds extra metadata

2. Which syntax correctly accesses the projectedValue of a property wrapper named score?

easy

A. score.projectedValue

B. $score

C. score.$projectedValue

D. score->projectedValue

Solution

1. Step 1: Recall how to access projectedValue

   In Swift, the projectedValue is accessed using the dollar sign prefix before the property name.

2. Step 2: Match syntax to property name

   For a property named score, the projectedValue is accessed as $score.

3. Final Answer:

   $score -> Option B

4. Quick Check:

   projectedValue = $propertyName [OK]

Hint: Use $ before property name to get projectedValue [OK]

Common Mistakes:

• Trying to access projectedValue as a normal property
• Using dot syntax incorrectly
• Confusing $score with score.$projectedValue

3. Given this code, what will be printed?

 @propertyWrapper
 struct Capitalized {
     private var value: String = ""
     var wrappedValue: String {
         get { value }
         set { value = newValue.capitalized }
     }
     var projectedValue: String {
         "Length: \(value.count)"
     }
 }

 struct Person {
     @Capitalized var name: String
 }

var p = Person()
p.name = "john doe"
print(p.name)
print(p.$name)

medium

A. John Doe
Length: 8

B. john doe
Length: 8

C. John Doe
Length: 7

D. john doe
Length: 7

Solution

1. Step 1: Understand wrappedValue behavior

   Setting p.name to "john doe" triggers the setter, which capitalizes the string to "John Doe".

2. Step 2: Calculate projectedValue output

   The projectedValue returns the length of the capitalized string. "John Doe" has 8 characters including the space.

3. Final Answer:

   John Doe\nLength: 8 -> Option A

4. Quick Check:

   wrappedValue capitalizes, projectedValue shows length [OK]

Hint: wrappedValue changes value, projectedValue gives extra info [OK]

Common Mistakes:

• Counting characters without space
• Ignoring capitalization effect
• Using p.name instead of p.$name for projectedValue

4. Identify the error in this property wrapper usage:

 @propertyWrapper
 struct Trimmed {
     var wrappedValue: String {
         didSet { wrappedValue = wrappedValue.trimmingCharacters(in: .whitespaces) }
     }
 }

 struct User {
     @Trimmed var username: String
 }

var u = User()
u.username = "  alice  "
print(u.username)

medium

A. wrappedValue cannot be mutated inside didSet

B. Cannot use didSet in property wrapper

C. Missing initial value for wrappedValue

D. Property wrapper must be a class, not struct

Solution

1. Step 1: Check wrappedValue declaration

   The wrappedValue property lacks an initial value or backing storage, causing a compile error.

2. Step 2: Understand property wrapper requirements

   Property wrappers must have a stored wrappedValue or initializer to set it; here it's missing.

3. Final Answer:

   Missing initial value for wrappedValue -> Option C

4. Quick Check:

   wrappedValue needs initial value or storage [OK]

Hint: wrappedValue must have storage or initializer [OK]

Common Mistakes:

• Trying to mutate wrappedValue inside didSet
• Assuming didSet works without storage
• Confusing struct vs class requirement

5. You want to create a property wrapper that stores an integer and also tracks how many times the value was changed. Which approach correctly uses wrappedValue and projectedValue to achieve this?

hard

A. Use projectedValue only to store and access the integer

B. Use projectedValue to store the integer and wrappedValue to return the change count

C. Use wrappedValue for both storing integer and change count

D. Use wrappedValue to store the integer and projectedValue to return the change count

Solution

1. Step 1: Define roles of wrappedValue and projectedValue

   wrappedValue should hold the main integer value; projectedValue can provide extra info like change count.

2. Step 2: Match roles to requirements

   Tracking changes is extra info, so projectedValue is ideal to expose the count without mixing with main value.

3. Final Answer:

   Use wrappedValue to store the integer and projectedValue to return the change count -> Option D

4. Quick Check:

   wrappedValue = main data, projectedValue = extra info [OK]

Hint: wrappedValue stores data, projectedValue tracks extra info [OK]

Common Mistakes:

• Swapping roles of wrappedValue and projectedValue
• Trying to store multiple values in wrappedValue
• Using projectedValue as main storage