Actors vs locks decision in Swift - Interactive Practice

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'

Aenum

Bclass

Cstruct

DCounter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'class' or 'struct' instead of an actor name.

In Swift, actor is followed by the actor's name, like Counter.

2fill in blank

medium

Complete the code to call an async method on an actor instance.

Swift

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

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

Aincrement

BincrementSync

CincrementNow

DincrementLater

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names that imply synchronous calls which are not allowed on actors.

Actor methods that modify state are usually async, so calling increment() with await is correct.

3fill in blank

hard

Fix the error in the code to safely access shared data using a lock.

Swift

class SafeCounter {
    private var count = 0
    private let lock = NSLock()

    func increment() {
        [1].lock()
        count += 1
        lock.unlock()
    }
}

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

Acounter

Block

Cself

DNSLock

Attempts:

3 left

💡 Hint

Common Mistakes

Calling lock() on 'self' or class name instead of the lock instance.

The lock instance must be used to call lock() and unlock() to protect the critical section.

4fill in blank

hard

Fill in the blank to create a dictionary filter that selects keys with values greater than 10.

Swift

let filtered = dictionary.filter { $0.value [1] 10 }.map { ($0.key, $0.value) }
let result = Dictionary(uniqueKeysWithValues: filtered) // Result dictionary

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

A!=

B<

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' or '==' which filter wrong values.

To filter values greater than 10, use the '>' operator in the filter closure.

5fill in blank

hard

Fill all three blanks to create an actor method that safely increments and returns the count.

Swift

actor Counter {
    private var count = 0

    func incrementAndGet() async -> Int {
        [1]
        count += 1
        let current = [2]
        [3]
        return current
    }
}

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

Aawait Task.sleep(nanoseconds: 0)

Bcount

Cawait Task.yield()

Dcount + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Returning wrong values or missing async suspension points.

Using await Task.sleep(nanoseconds: 0) and await Task.yield() simulates async suspension points. The count variable holds the current value after increment.

Practice

1. Which of the following best describes the main advantage of using actors in Swift concurrency?

easy

A. They automatically protect data from concurrent access without manual locking.

B. They provide manual control over thread synchronization using locks.

C. They are used only for UI updates in Swift applications.

D. They require explicit lock management to avoid race conditions.

Solution

1. Step 1: Understand what actors do in Swift concurrency

   Actors automatically serialize access to their mutable state, preventing data races without manual locks.

2. Step 2: Compare actors with locks

   Locks require manual management to protect data, while actors handle this automatically.

3. Final Answer:

   They automatically protect data from concurrent access without manual locking. -> Option A

4. Quick Check:

   Actors = automatic data protection [OK]

Hint: Actors handle concurrency safety automatically, unlike manual locks. [OK]

Common Mistakes:

• Thinking actors require manual lock management.
• Confusing actors with UI-only components.
• Assuming locks are always safer than actors.

2. Which of the following is the correct way to declare an actor in Swift?

easy

A. class MyActor {}

B. actor MyActor {}

C. struct MyActor {}

D. lock MyActor {}

Solution

1. Step 1: Recall Swift syntax for actors

   Actors are declared using the keyword actor followed by the name and braces.

2. Step 2: Check each option

   actor MyActor {} uses actor MyActor {}, which is the correct syntax. Others use class, struct, or invalid keyword.

3. Final Answer:

   actor MyActor {} -> Option B

4. Quick Check:

   Actor declaration uses 'actor' keyword [OK]

Hint: Actors start with 'actor' keyword, not class or struct. [OK]

Common Mistakes:

• Using 'class' or 'struct' instead of 'actor'.
• Trying to use 'lock' keyword which doesn't exist.
• Forgetting braces after actor name.

3. Consider this Swift code snippet:

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

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

What will be printed when this code runs?

medium

A. Compilation error due to missing locks

B. 0

C. 1

D. Runtime crash due to data race

Solution

1. Step 1: Understand actor's behavior with state

   The actor serializes access to its state, so increment() safely updates value.

2. Step 2: Analyze the code execution

   The increment() method increases value from 0 to 1, then print outputs the updated value.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Actor state updated safely = 1 [OK]

Hint: Actors serialize access, so state updates are safe and visible. [OK]

Common Mistakes:

• Expecting 0 because of async call.
• Thinking a lock is needed causing error.
• Assuming data race causes crash.

4. This Swift code uses a lock to protect shared data:

class Counter {
    private var value = 0
    private let lock = NSLock()
    func increment() {
        lock.lock()
        value += 1
        lock.unlock()
    }
    func getValue() -> Int {
        lock.lock()
        let val = value
        lock.unlock()
        return val
    }
}

What is a potential problem with this code?

medium

A. It can cause a deadlock if increment() is called recursively.

B. It will not compile because NSLock is not imported.

C. It does not protect the value variable from concurrent access.

D. It uses actors incorrectly instead of locks.

Solution

1. Step 1: Analyze lock usage in increment()

   The method locks before incrementing and unlocks after, but if increment() calls itself, it will try to lock again without unlocking first.

2. Step 2: Understand deadlock risk

   NSLock is not reentrant, so recursive locking causes deadlock, freezing the program.

3. Final Answer:

   It can cause a deadlock if increment() is called recursively. -> Option A

4. Quick Check:

   NSLock + recursion = deadlock risk [OK]

Hint: NSLock is not reentrant; recursive locks cause deadlock. [OK]

Common Mistakes:

• Assuming NSLock is reentrant.
• Thinking code won't compile without import (usually imported).
• Confusing actors with locks in this context.

5. You have a Swift app that needs to update a shared cache from multiple concurrent tasks. You want safety and simplicity without manual lock management. Which approach should you choose?

hard

A. Use a global variable without any synchronization.

B. Use NSLock to manually lock and unlock around cache updates.

C. Use DispatchQueue.sync to serialize access without actors.

D. Use an actor to manage the cache state and provide async methods.

Solution

1. Step 1: Identify requirements for safety and simplicity

   Actors provide automatic data protection and simplify concurrency without manual locks.

2. Step 2: Compare options

   NSLock requires manual lock management, global variable is unsafe, DispatchQueue.sync can block threads and is more complex.

3. Final Answer:

   Use an actor to manage the cache state and provide async methods. -> Option D

4. Quick Check:

   Actors = safe + simple concurrency [OK]

Hint: Actors simplify concurrency by managing safety automatically. [OK]

Common Mistakes:

• Choosing manual locks for simplicity.
• Ignoring thread safety with global variables.
• Using DispatchQueue.sync causing potential deadlocks.