Preference keys in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to save a string value to UserDefaults using the key "username".

iOS Swift

UserDefaults.standard.set("John", forKey: [1])

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

A"username"

B"user_name"

C"user"

D"name"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a key with underscores instead of the exact key.

Forgetting to put the key in quotes.

The key must exactly match "username" to store the value correctly.

2fill in blank

medium

Complete the code to retrieve a boolean preference with key "isDarkMode" from UserDefaults.

iOS Swift

let darkModeEnabled = UserDefaults.standard.[1]("isDarkMode")

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

Ainteger(forKey:)

Bbool(forKey:)

Cstring(forKey:)

Ddouble(forKey:)

Attempts:

3 left

💡 Hint

Common Mistakes

Using string(forKey:) when expecting a boolean.

Using integer(forKey:) or double(forKey:) incorrectly.

Use bool(forKey:) to get a boolean value from UserDefaults.

3fill in blank

hard

Fix the error in the code to remove a preference with key "userAge" from UserDefaults.

iOS Swift

UserDefaults.standard.[1](forKey: "userAge")

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

AremoveObject

BdeleteObject

CclearObject

DunsetObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using deleteObject or clearObject which do not exist.

Using set(nil, forKey:) which is not recommended.

The correct method to remove a value is removeObject(forKey:).

4fill in blank

hard

Fill both blanks to save an integer preference "launchCount" incremented by 1.

iOS Swift

let count = UserDefaults.standard.[1]("launchCount")
UserDefaults.standard.set(count [2] 1, forKey: "launchCount")

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

Ainteger(forKey:)

B+

C-

Ddouble(forKey:)

Attempts:

3 left

💡 Hint

Common Mistakes

Using double(forKey:) instead of integer(forKey:).

Using - instead of + to increment.

Retrieve the integer value with integer(forKey:) and add 1 to increment.

5fill in blank

hard

Fill all three blanks to create a dictionary preference with keys "theme", "fontSize", and "notifications".

iOS Swift

let settings: [String: Any] = ["theme": [1], "fontSize": [2], "notifications": [3]]
UserDefaults.standard.set(settings, forKey: "userSettings")

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

A"dark"

B14

Ctrue

D"light"

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes incorrectly around numbers or booleans.

Mixing up the order of values.

The theme is a string "dark", fontSize is integer 14, notifications is boolean true.

Practice

1. What is the main purpose of PreferenceKey in SwiftUI?

easy

A. To store user preferences permanently

B. To share simple data up the view hierarchy

C. To create custom buttons

D. To manage network requests

Solution

1. Step 1: Understand PreferenceKey role

   PreferenceKey is designed to pass data from child views up to parent views in SwiftUI.

2. Step 2: Differentiate from other uses

   It is not for permanent storage or UI elements but for sharing layout or state info upwards.

3. Final Answer:

   To share simple data up the view hierarchy -> Option B

4. Quick Check:

   PreferenceKey = share data up [OK]

Hint: PreferenceKey passes data upward, not stores or creates UI [OK]

Common Mistakes:

• Confusing PreferenceKey with UserDefaults
• Thinking it creates UI components
• Assuming it manages network tasks

2. Which of the following is the correct way to declare a PreferenceKey with a default value of 0 and a reduce function that sums values?

easy

A. struct MyKey: PreferenceKey { static var defaultValue = 0; static func reduce(value: Int, nextValue: () -> Int) { value += nextValue() } }

B. struct MyKey: PreferenceKey { static let defaultValue = 0; static func reduce(value: Int, nextValue: () -> Int) { value += nextValue() } }

C. struct MyKey: PreferenceKey { var defaultValue: Int = 0; func reduce(value: inout Int, nextValue: () -> Int) { value += nextValue() } }

D. struct MyKey: PreferenceKey { static var defaultValue: Int = 0; static func reduce(value: inout Int, nextValue: () -> Int) { value += nextValue() } }

Solution

1. Step 1: Check static defaultValue declaration

   It must be a static property with explicit type: static var defaultValue: Int = 0.

2. Step 2: Verify reduce function signature

   Reduce must be static, take inout value, and add nextValue: static func reduce(value: inout Int, nextValue: () -> Int).

3. Final Answer:

   struct MyKey: PreferenceKey { static var defaultValue: Int = 0; static func reduce(value: inout Int, nextValue: () -> Int) { value += nextValue() } } -> Option D

4. Quick Check:

   Static defaultValue and reduce with inout = struct MyKey: PreferenceKey { static var defaultValue: Int = 0; static func reduce(value: inout Int, nextValue: () -> Int) { value += nextValue() } } [OK]

Hint: defaultValue and reduce must be static with correct signatures [OK]

Common Mistakes:

• Missing static keyword on defaultValue or reduce
• Using var instead of static var for defaultValue
• Incorrect reduce function parameters

3. Given this code snippet, what will be printed?

struct CountKey: PreferenceKey {
  static var defaultValue = 0
  static func reduce(value: inout Int, nextValue: () -> Int) {
    value += nextValue()
  }
}

struct ContentView: View {
  var body: some View {
    VStack {
      Text("Hello").preference(key: CountKey.self, value: 1)
      Text("World").preference(key: CountKey.self, value: 2)
    }
    .onPreferenceChange(CountKey.self) { print($0) }
  }
}

medium

A. 3

B. 2

C. 1

D. 0

Solution

1. Step 1: Understand preference values set

   Two Text views set preferences with values 1 and 2 respectively.

2. Step 2: Apply reduce function logic

   Reduce sums values: 0 (default) + 1 + 2 = 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Sum of preferences = 3 [OK]

Hint: Reduce sums all values, so 1 + 2 = 3 [OK]

Common Mistakes:

• Assuming only last value is used
• Ignoring reduce function behavior
• Confusing defaultValue with output

4. Identify the error in this PreferenceKey implementation:

struct SizeKey: PreferenceKey {
  static var defaultValue: CGSize = .zero
  static func reduce(value: CGSize, nextValue: () -> CGSize) {
    value = nextValue()
  }
}

medium

A. reduce function must have inout parameter for value

B. defaultValue must be non-static

C. nextValue must return Int, not CGSize

D. PreferenceKey cannot use CGSize type

Solution

1. Step 1: Check defaultValue declaration

   defaultValue is correctly static with CGSize type.

2. Step 2: Inspect reduce function signature

   Reduce must take inout for value to modify it, but here it is a value parameter.

3. Final Answer:

   reduce function must have inout parameter for value -> Option A

4. Quick Check:

   Reduce needs inout value parameter [OK]

Hint: Reduce must use inout for value to update it [OK]

Common Mistakes:

• Forgetting inout keyword in reduce
• Changing defaultValue to non-static
• Wrong return type for nextValue

5. You want to pass the maximum width of child views up to a parent using PreferenceKey. Which implementation correctly combines widths to find the maximum?

hard

A. struct MaxWidthKey: PreferenceKey {
static var defaultValue: CGFloat = 0
static func reduce(value: CGFloat, nextValue: () -> CGFloat) {
value = max(value, nextValue())
}
}

B. struct MaxWidthKey: PreferenceKey {
static var defaultValue: CGFloat = 0
static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
value += nextValue()
}
}

C. struct MaxWidthKey: PreferenceKey {
static var defaultValue: CGFloat = 0
static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
value = max(value, nextValue())
}
}

D. struct MaxWidthKey: PreferenceKey {
static var defaultValue: CGFloat = 0
static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
value = min(value, nextValue())
}
}

Solution

1. Step 1: Identify correct reduce logic for max

   To find maximum width, reduce must assign the max of current and next values.

2. Step 2: Verify function signature and logic

   Reduce must be static, take inout value, and use value = max(value, nextValue()).

3. Final Answer:

   struct MaxWidthKey: PreferenceKey { static var defaultValue: CGFloat = 0; static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) { value = max(value, nextValue()) } } -> Option C

4. Quick Check:

   Use max in reduce with inout value [OK]

Hint: Use max function and inout value in reduce [OK]

Common Mistakes:

• Using sum (+) instead of max
• Missing inout in reduce parameter
• Using min instead of max