Why actors prevent data races in Swift - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an actor in Swift.

Swift

actor [1] {
    var count = 0
}

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

Aclass

BCounter

Cstruct

Denum

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'class' or 'struct' instead of 'actor' to declare an actor.

The keyword actor declares an actor type named Counter.

2fill in blank

medium

Complete the code to safely increment the count inside the actor.

Swift

actor Counter {
    var count = 0
    func increment() {
        [1]
    }
}

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

Aself.count = count + 1

Bcount += 1

Ccount = count + 1

Dself.count += 1

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting 'self.' before 'count' inside the actor method.

Inside an actor, use self.count += 1 to modify the property safely.

3fill in blank

hard

Fix the error in calling the actor's method asynchronously.

Swift

let counter = Counter()
Task {
    await [1].increment()
}

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

Acounter

BCounter

Cself

Dincrement

Attempts:

3 left

💡 Hint

Common Mistakes

Using the actor type name instead of the instance to call the method.

You must call the method on the actor instance counter using await because actor methods are asynchronous.

4fill in blank

hard

Fill both blanks to define an actor with a method that returns the count.

Swift

actor [1] {
    var count = 0
    func getCount() async -> [2] {
        return count
    }
}

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

ASafeCounter

BInt

CString

DDouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong return type like String or Double for the count.

The actor is named SafeCounter and the method returns an Int representing the count.

5fill in blank

hard

Fill all three blanks to create an actor that prevents data races by isolating state and providing safe access.

Swift

actor [1] {
    private var value: [2] = 0
    func updateValue(to newValue: [3]) {
        value = newValue
    }
}

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

ADataGuard

BInt

CString

DDouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types for the variable and method parameter.

The actor DataGuard isolates the Int state value and safely updates it through a method, preventing data races.

Practice

1. Why do Swift actors help prevent data races?

easy

A. They duplicate data for each task automatically.

B. They allow only one task to access their data at a time.

C. They run all tasks in parallel without waiting.

D. They disable concurrency features in Swift.

Solution

1. Step 1: Understand what a data race is

   A data race happens when multiple tasks try to change the same data at the same time.

2. Step 2: How actors manage access

   Actors allow only one task to access or change their data at once, preventing simultaneous changes.

3. Final Answer:

   They allow only one task to access their data at a time. -> Option B

4. Quick Check:

   Actors serialize access = prevent data races [OK]

Hint: Actors serialize access to data, so no simultaneous changes happen [OK]

Common Mistakes:

• Thinking actors duplicate data for each task
• Believing actors run tasks in parallel without control
• Assuming actors disable concurrency features

2. Which is the correct way to call an actor's method in Swift?

easy

A. actorInstance.method()

B. actorInstance.method.await()

C. await actorInstance.method()

D. async actorInstance.method()

Solution

1. Step 1: Recall actor method call syntax

   Actor methods are asynchronous and require the await keyword to call.

2. Step 2: Identify correct usage

   The correct syntax is to write await actorInstance.method() to wait for the method to complete.

3. Final Answer:

   await actorInstance.method() -> Option C

4. Quick Check:

   Use await before actor calls [OK]

Hint: Always use await before calling actor methods [OK]

Common Mistakes:

• Calling actor methods without await
• Using async keyword incorrectly in call
• Placing await after method call

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

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

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

medium

A. 0

B. Runtime crash

C. 1

D. Compilation error

Solution

1. Step 1: Understand actor property access

   Accessing actor properties from outside requires await because it is asynchronous.

2. Step 2: Trace the code execution

   The increment() method increases value by 1, but since increment() is not marked as async, calling it with await causes a compilation error.

3. Final Answer:

   Compilation error -> Option D

4. Quick Check:

   increment() must be async to be awaited [OK]

Hint: increment() must be async to be awaited [OK]

Common Mistakes:

• Forgetting await when accessing actor property
• Expecting 0 because of missing synchronization
• Assuming increment is synchronous

4. Identify the error in this Swift actor code:

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

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

medium

A. Missing await before deposit call

B. balance should be a constant

C. deposit method cannot modify balance

D. print statement should not use await

Solution

1. Step 1: Check actor method call rules

   Calling an actor's method from outside must use await because it's asynchronous.

2. Step 2: Identify missing await

   The code calls account.deposit(amount: 100) without await, causing a compile error.

3. Final Answer:

   Missing await before deposit call -> Option A

4. Quick Check:

   Actor method calls require await [OK]

Hint: Always put await before actor method calls outside actor [OK]

Common Mistakes:

• Ignoring await on actor method calls
• Thinking balance must be constant
• Assuming print can't await actor properties

5. Given this actor code, why does using an actor prevent a data race when multiple tasks call increment()?

actor SafeCounter {
    var count = 0
    func increment() {
        count += 1
    }
}

hard

A. Because actor methods run one at a time, queuing calls safely.

B. Because the count variable is copied for each task.

C. Because Swift disables concurrency inside actors.

D. Because increment() is automatically atomic without queuing.

Solution

1. Step 1: Understand actor method execution

   Actor methods run serially, meaning one finishes before the next starts.

2. Step 2: How this prevents data races

   Since calls to increment() are queued and run one by one, simultaneous changes to count cannot happen.

3. Final Answer:

   Because actor methods run one at a time, queuing calls safely. -> Option A

4. Quick Check:

   Actors serialize method calls = no data races [OK]

Hint: Actors queue method calls, so only one runs at a time [OK]

Common Mistakes:

• Thinking variables are copied per task
• Believing concurrency is disabled inside actors
• Assuming increment is atomic without queuing