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.