Actor declaration syntax in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of actor method call

What is the output of this Swift code using an actor?

Swift

actor Counter {
    var value = 0
    func increment() async -> Int {
        value += 1
        return value
    }
}

let counter = Counter()
Task {
    let result = await counter.increment()
    print(result)
}

A 0

B Runtime error

C Compilation error

D 1

💡 Hint

Remember that actor methods are asynchronous and state changes inside actors are safe.

The actor Counter starts with value 0. Calling increment() adds 1 and returns the new value, which is 1.

📝 Syntax

intermediate

Identify the correct actor declaration

Which of the following is the correct syntax to declare an actor named Logger in Swift?

A

actor Logger {
    func log(_ message: String) {
        print(message)
    }
}

B

class Logger {
    actor func log(_ message: String) {
        print(message)
    }
}

C

actor Logger() {
    func log(_ message: String) {
        print(message)
    }
}

D

actor Logger {
    actor func log(_ message: String) {
        print(message)
    }
}

💡 Hint

Actors are declared like classes but with the keyword 'actor' and no parentheses.

Option A correctly declares an actor named Logger with a method log. Option A wrongly uses 'actor func' inside a class. Option A uses parentheses after actor name which is invalid. Option A wrongly uses 'actor func' inside actor.

🔧 Debug

advanced

Why does this actor code cause a compile error?

Consider this Swift code: actor BankAccount { var balance: Int = 0 func deposit(amount: Int) { balance += amount } } let account = BankAccount() account.deposit(amount: 100) Why does this code cause a compile error?

Swift

actor BankAccount {
    var balance: Int = 0
    func deposit(amount: Int) async {
        balance += amount
    }
}

let account = BankAccount()
Task {
    await account.deposit(amount: 100)
}

A Because actors cannot have methods that modify state

B Because 'balance' cannot be a variable inside an actor

C Because actor methods must be called with 'await' since they are asynchronous

D Because 'deposit' must be marked as 'async' explicitly

💡 Hint

Think about how actor methods are called from outside the actor.

Actor methods are isolated and asynchronous by default. Calling deposit requires 'await' keyword to handle concurrency.

🧠 Conceptual

advanced

Actor isolation and data race prevention

Which statement best describes how actors prevent data races in Swift?

A Actors use locks internally to block threads when accessing variables

B Actors serialize access to their mutable state by allowing only one task at a time to access it

C Actors copy their state to each thread to avoid sharing data

D Actors require all variables to be immutable to prevent data races

💡 Hint

Think about how actors handle concurrent access to their properties.

Actors ensure only one task can access their mutable state at a time, preventing data races without explicit locks.

❓ Predict Output

expert

Output of concurrent actor calls

What is the output of this Swift code using an actor with concurrent tasks?

Swift

actor Counter {
    var value = 0
    func increment() async -> Int {
        value += 1
        return value
    }
}

let counter = Counter()

Task {
    async let a = counter.increment()
    async let b = counter.increment()
    let results = await [a, b]
    print(results.sorted())
}

A [1, 2]

B [2, 2]

C [1, 1]

D Runtime error due to data race

💡 Hint

Actors serialize access, so increments happen one after another.

The two increments happen sequentially inside the actor, so values returned are 1 and 2 in some order.

Practice

1. What keyword is used to declare an actor in Swift?

easy

A. class

B. enum

C. struct

D. actor

Solution

1. Step 1: Understand actor declaration

   Actors in Swift are declared using a special keyword to indicate their concurrency-safe nature.

2. Step 2: Identify the correct keyword

   The keyword actor is used to declare actors, differentiating them from classes, structs, or enums.

3. Final Answer:

   actor -> Option D

4. Quick Check:

   Actor declaration keyword = actor [OK]

Hint: Actors always start with the keyword 'actor' [OK]

Common Mistakes:

• Using class instead of actor
• Confusing struct with actor
• Trying to declare actor with enum

2. Which of the following is the correct syntax to declare an actor named DataManager?

easy

A. actor DataManager() { }

B. actor DataManager { }

C. struct DataManager { }

D. class DataManager { }

Solution

1. Step 1: Recall actor declaration syntax

   Actors are declared with the keyword actor followed by the name and curly braces without parentheses.

2. Step 2: Check each option

   actor DataManager { } uses correct syntax: actor DataManager { }. actor DataManager() { } incorrectly adds parentheses after the name.

3. Final Answer:

   actor DataManager { } -> Option B

4. Quick Check:

   Correct actor syntax = actor Name { } [OK]

Hint: Actors do not use parentheses after their name [OK]

Common Mistakes:

• Adding parentheses after actor name
• Using class or struct instead of actor
• Omitting curly braces

3. What will be the output of the following Swift code?

actor Counter {
    var value = 0
    func increment() {
        value += 1
    }
}

let counter = Counter()
Task {
    await counter.increment()
    print(await counter.value)
}

medium

A. 1

B. 0

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand actor behavior and method calls

   The actor Counter has a variable value initialized to 0 and a method increment() that adds 1 to value. The method is called with await inside a Task.

2. Step 2: Trace the code execution

   The increment() method increases value from 0 to 1. Then print(await counter.value) prints the updated value, which is 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Incremented value = 1 [OK]

Hint: Remember to await actor method calls and property access [OK]

Common Mistakes:

• Forgetting to use await with actor calls
• Expecting initial value 0 after increment
• Assuming synchronous access to actor properties

4. Identify the error in the following actor declaration and fix it:

actor Logger {
    var logs: [String]
    func addLog(_ message: String) {
        logs.append(message)
    }
}

medium

A. Missing initialization of 'logs' array

B. Actor cannot have functions

C. Use 'class' instead of 'actor'

D. Missing 'await' keyword in function

Solution

1. Step 1: Check property initialization in actor

   The property logs is declared but not initialized. In Swift, stored properties must have initial values or be initialized in an initializer.

2. Step 2: Identify the fix

   Initialize logs as an empty array: var logs: [String] = [] to fix the error.

3. Final Answer:

   Missing initialization of 'logs' array -> Option A

4. Quick Check:

   Stored properties need initial values [OK]

Hint: Always initialize stored properties in actors [OK]

Common Mistakes:

• Assuming default initialization for arrays
• Thinking actors cannot have functions
• Forgetting to add await when calling actor methods

5. You want to create an actor BankAccount that safely manages a balance. Which of the following code snippets correctly declares the actor and provides a method to deposit money asynchronously?

hard

A. actor BankAccount {
var balance: Double = 0.0
func deposit(amount: Double) {
balance += amount
}
}

B. actor BankAccount {
var balance: Double = 0.0
func deposit(amount: Double) {
await balance += amount
}
}

C. actor BankAccount {
var balance: Double = 0.0
func deposit(amount: Double) async {
await Task.sleep(1_000_000_000)
balance += amount
}
}

D. class BankAccount {
var balance: Double = 0.0
func deposit(amount: Double) async {
balance += amount
}
}

Solution

1. Step 1: Understand actor method concurrency

   Actor methods that perform asynchronous work should be marked async. actor BankAccount { var balance: Double = 0.0 func deposit(amount: Double) { balance += amount } } lacks async on the method, so it cannot await inside.

2. Step 2: Check for realistic async behavior

   actor BankAccount { var balance: Double = 0.0 func deposit(amount: Double) async { await Task.sleep(1_000_000_000) balance += amount } } marks deposit as async and simulates asynchronous delay with await Task.sleep(), showing proper async usage inside an actor.

3. Step 3: Verify actor declaration

   class BankAccount { var balance: Double = 0.0 func deposit(amount: Double) async { balance += amount } } uses class instead of actor, so it does not provide concurrency safety.

4. Final Answer:

   correctly declares an actor with an async deposit method including await -> Option C

5. Quick Check:

   Actor + async method + await inside = actor BankAccount { var balance: Double = 0.0 func deposit(amount: Double) async { await Task.sleep(1_000_000_000) balance += amount } } [OK]

Hint: Async actor methods must be marked async and use await inside [OK]

Common Mistakes:

• Omitting async keyword on async actor methods
• Using class instead of actor for concurrency
• Not using await inside async methods when needed