WrappedValue and projectedValue in Swift - Mini Project: Build & Apply

Using WrappedValue and projectedValue in Swift Property Wrappers

📖 Scenario: Imagine you are building a simple app that tracks the progress of tasks. You want to create a custom property wrapper that stores a task's progress as a percentage and also provides a message about the progress.

🎯 Goal: Build a Swift program that uses a property wrapper with wrappedValue to store the progress percentage and projectedValue to provide a progress message.

📋 What You'll Learn

Create a property wrapper named ProgressTracker with wrappedValue and projectedValue

Use wrappedValue to store an integer progress percentage

Use projectedValue to return a string message about the progress

Create a struct Task with a property progress using the ProgressTracker wrapper

Print the progress percentage and the progress message

💡 Why This Matters

🌍 Real World

Property wrappers help you add reusable behavior to properties, such as validation, formatting, or extra info, which is common in app development.

💼 Career

Understanding property wrappers and their wrapped and projected values is important for Swift developers working on iOS or macOS apps to write clean and reusable code.

1

Create the ProgressTracker property wrapper

Create a property wrapper named ProgressTracker with a wrappedValue property of type Int initialized to 0.

Swift

# Create the ProgressTracker property wrapper with wrappedValue
// Your code here

Hint

Use @propertyWrapper before the struct. Define wrappedValue as an Int with default 0.

2

Add projectedValue to provide a progress message

Inside ProgressTracker, add a computed property projectedValue of type String that returns "Progress is X%" where X is the current wrappedValue.

Swift

@propertyWrapper
struct ProgressTracker {
    var wrappedValue: Int = 0
    
    // Add projectedValue computed property here
    // Your code here
}

Hint

Define projectedValue as a computed property returning a string with the current wrappedValue.

3

Create Task struct using ProgressTracker

Create a struct named Task with a property progress that uses the @ProgressTracker property wrapper.

Swift

@propertyWrapper
struct ProgressTracker {
    var wrappedValue: Int = 0
    
    var projectedValue: String {
        "Progress is \(wrappedValue)%"
    }
}

// Create Task struct with progress property using @ProgressTracker
// Your code here

Hint

Define struct Task with a property progress using @ProgressTracker.

4

Set progress and print wrappedValue and projectedValue

Create an instance of Task named myTask. Set myTask.progress to 75. Then print myTask.progress and myTask.$progress.

Swift

@propertyWrapper
struct ProgressTracker {
    var wrappedValue: Int = 0
    
    var projectedValue: String {
        "Progress is \(wrappedValue)%"
    }
}

struct Task {
    @ProgressTracker var progress: Int
}

// Create myTask, set progress to 75, and print progress and projectedValue
// Your code here

Hint

Create myTask, assign 75 to progress, then print myTask.progress and myTask.$progress.

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