Sendable protocol for thread safety in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a struct as conforming to the Sendable protocol.

Swift

struct DataModel: [1] {
    var value: Int
}

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

AIdentifiable

BCodable

CSendable

DEquatable

Attempts:

3 left

💡 Hint

Common Mistakes

Using Codable instead of Sendable

Forgetting to conform to any protocol

To mark a struct as safe to use across threads, it must conform to the Sendable protocol.

2fill in blank

medium

Complete the code to mark a class as conforming to Sendable.

Swift

final class Counter: [1] {
    var count = 0
}

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

AObservableObject

BSendable

CIdentifiable

DCodable

Attempts:

3 left

💡 Hint

Common Mistakes

Using ObservableObject instead of Sendable

Not marking the class as final when conforming to Sendable

Marking a class as Sendable indicates it is safe to use across threads.

3fill in blank

hard

Fix the error by completing the code to make the struct Sendable when it contains a non-Sendable property.

Swift

struct Wrapper: Sendable {
    var data: [1]
}

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

AInt

BUnsafeMutablePointer<Int>

CString

DDispatchQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using pointer types which are not Sendable

Using DispatchQueue which is not Sendable

Only types that conform to Sendable can be used safely. Int is Sendable, but UnsafeMutablePointer and DispatchQueue are not.

4fill in blank

hard

Fill both blanks to create a Sendable struct with a computed property.

Swift

struct Point: [1] {
    var x: Int
    var y: Int
    var description: [2] {
        "(\(x), \(y))"
    }
}

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

ASendable

BString

CInt

DCodable

Attempts:

3 left

💡 Hint

Common Mistakes

Using Codable instead of Sendable for the struct

Using Int instead of String for the description property

The struct conforms to Sendable and the computed property returns a String.

5fill in blank

hard

Fill all three blanks to create a Sendable struct with a closure property marked as @Sendable.

Swift

struct Task: [1] {
    var id: Int
    var action: [2] () -> Void
    init(id: Int, action: [3] () -> Void) {
        self.id = id
        self.action = action
    }
}

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

ASendable

B@Sendable

C() -> Void

DCodable

Attempts:

3 left

💡 Hint

Common Mistakes

Not marking the closure as @Sendable

Using Codable instead of Sendable for the struct

The struct conforms to Sendable. The closure property and initializer parameter are marked @Sendable and have type () -> Void to ensure thread safety.

Practice

1. What does the Sendable protocol in Swift ensure?

easy

A. That a function runs asynchronously.

B. That a class can only be used on the main thread.

C. That a variable is immutable.

D. That data can be safely shared between threads without causing race conditions.

Solution

1. Step 1: Understand the purpose of Sendable

   The Sendable protocol marks types that can be safely passed between threads without causing data races.

2. Step 2: Compare options with the definition

   Only That data can be safely shared between threads without causing race conditions. correctly describes this safety guarantee. Other options describe unrelated concepts.

3. Final Answer:

   That data can be safely shared between threads without causing race conditions. -> Option D

4. Quick Check:

   Sendable = safe cross-thread data sharing [OK]

Hint: Sendable means safe to share data across threads [OK]

Common Mistakes:

• Confusing Sendable with immutability
• Thinking Sendable restricts to main thread only
• Assuming Sendable makes functions async

2. Which of the following is the correct way to declare a struct as Sendable in Swift?

easy

A. struct Data { var value: Int } implements Sendable

B. struct Data: Sendable { var value: Int }

C. struct Data Sendable { var value: Int }

D. struct Data { var value: Int } : Sendable

Solution

1. Step 1: Recall Swift protocol conformance syntax

   In Swift, a type conforms to a protocol by listing it after a colon following the type name.

2. Step 2: Match syntax to options

   struct Data: Sendable { var value: Int } uses correct syntax: struct Data: Sendable { ... }. Others are invalid Swift syntax.

3. Final Answer:

   struct Data: Sendable { var value: Int } -> Option B

4. Quick Check:

   Protocol conformance uses colon after type name [OK]

Hint: Use colon after struct name to conform to Sendable [OK]

Common Mistakes:

• Placing protocol after braces
• Using 'implements' keyword (not Swift)
• Omitting colon between type and protocol

3. What will happen when you try to compile this Swift code?

struct Counter: Sendable {
  var count: Int
}

let counter = Counter(count: 5)
Task {
  print(counter.count)
}

medium

A. It compiles and prints 5.

B. It causes a compile-time error because Counter is not Sendable.

C. It causes a runtime error due to thread safety violation.

D. It compiles but prints 0.

Solution

1. Step 1: Check if Counter conforms to Sendable

   The struct explicitly conforms to Sendable and contains only an Int, which is Sendable.

2. Step 2: Understand Task usage

   The Task runs asynchronously but can safely access counter because it is Sendable.

3. Final Answer:

   It compiles and prints 5. -> Option A

4. Quick Check:

   Sendable struct with Int prints value safely [OK]

Hint: Sendable structs with simple types compile and run safely [OK]

Common Mistakes:

• Assuming compile error without checking conformance
• Confusing runtime errors with compile errors
• Thinking default structs are not Sendable

4. Identify the error in this Swift code using Sendable:

class DataHolder: Sendable {
  var data: [Int]
  init(data: [Int]) {
    self.data = data
  }
}

medium

A. Missing explicit Sendable conformance on Array.

B. Class cannot conform to Sendable at all.

C. Array<Int> is not Sendable by default, causing a compile error.

D. No error; code compiles fine.

Solution

1. Step 1: Check Sendable conformance for class with mutable property

   Classes with mutable stored properties like arrays are not automatically Sendable.

2. Step 2: Understand why Array<Int> is not Sendable by default

   Array is a reference type and mutable, so compiler disallows automatic Sendable conformance without extra guarantees.

3. Final Answer:

   Array<Int> is not Sendable by default, causing a compile error. -> Option C

4. Quick Check:

   Mutable reference types block Sendable conformance [OK]

Hint: Mutable class properties like arrays block Sendable by default [OK]

Common Mistakes:

• Thinking classes cannot conform to Sendable at all
• Assuming arrays are always Sendable
• Ignoring mutability in Sendable checks

5. You want to make this struct safely shareable across threads. What is the best way to fix it?

struct UserData: Sendable {
  var name: String
  var preferences: [String: String]
}

hard

A. Make preferences an immutable dictionary and ensure keys and values are Sendable.

B. Remove Sendable conformance because dictionaries are never Sendable.

C. Change preferences to a class type and mark it as Sendable.

D. Add @Sendable attribute to the struct.

Solution

1. Step 1: Understand Sendable requirements for dictionary properties

   Dictionaries are Sendable only if their keys and values are Sendable and the dictionary is used immutably.

2. Step 2: Apply immutability and ensure key/value types conform

   Making preferences immutable (let) and using Sendable key/value types (String) satisfies Sendable.

3. Final Answer:

   Make preferences an immutable dictionary and ensure keys and values are Sendable. -> Option A

4. Quick Check:

   Immutable dictionary with Sendable types = Sendable struct [OK]

Hint: Use immutable dictionary with Sendable keys/values for Sendable struct [OK]

Common Mistakes:

• Thinking dictionaries are never Sendable
• Confusing @Sendable attribute with Sendable protocol
• Using mutable properties in Sendable structs