WrappedValue and projectedValue in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the wrapped value of the property wrapper.

Swift

struct User {
    @Logged var name: String

    func printName() {
        print(name[1])
    }
}

Drag options to blanks, or click blank then click option'

A.wrappedValue

B.projectedValue

C()

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using .projectedValue instead of .wrappedValue

Trying to call the property as a function

Using array indexing syntax

Use .wrappedValue to access the actual value stored by the property wrapper.

2fill in blank

medium

Complete the code to access the projected value of the property wrapper.

Swift

struct User {
    @Logged var name: String

    func printLog() {
        print($name[1])
    }
}

Drag options to blanks, or click blank then click option'

A.projectedValue

B.wrappedValue

C()

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using .wrappedValue instead of .projectedValue

Forgetting the $ prefix

Trying to call the property as a function

Use .projectedValue to access the extra value or functionality the wrapper provides, accessed via the $ prefix.

3fill in blank

hard

Fix the error in accessing the wrapped value inside the property wrapper struct.

Swift

@propertyWrapper
struct Logged {
    private var value: String

    var [1]: String {
        get { value }
        set { value = newValue }
    }

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

Drag options to blanks, or click blank then click option'

AprojectedValue

BwrappedValue

Cvalue

Dwrapped

Attempts:

3 left

💡 Hint

Common Mistakes

Using projectedValue instead of wrappedValue

Using a different property name like value or wrapped

The property wrapper must define a property named wrappedValue to work correctly.

4fill in blank

hard

Fill both blanks to define the projectedValue property that returns a log string.

Swift

@propertyWrapper
struct Logged {
    private var value: String
    private(set) var log: String = ""

    var wrappedValue: String {
        get { value }
        set {
            value = newValue
            log += "Value changed to \(newValue)\n"
        }
    }

    var [1]: String {
        return log
    }
}

Drag options to blanks, or click blank then click option'

Alog

BwrappedValue

CprojectedValue

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Naming the property wrappedValue instead of projectedValue

Returning value instead of log

The projectedValue property is named projectedValue and returns the log string.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps property names to their wrapped values if the value is not empty.

Swift

struct User {
    @Logged var firstName: String
    @Logged var lastName: String

    func nonEmptyProperties() -> [String: String] {
        [
            "firstName": $firstName[1],
            "lastName": $lastName[2]
        ].filter { $0.value [3] "" }
    }
}

Drag options to blanks, or click blank then click option'

A.wrappedValue

B!=

C==

D.projectedValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using .projectedValue instead of .wrappedValue

Using == instead of != in the filter

Forgetting to access wrappedValue

Use .wrappedValue to get the actual value from the projected property, and filter where value is not equal to empty string.

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