Global actors (@MainActor) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using @MainActor?

Consider this Swift code snippet using the @MainActor attribute. What will be printed when the program runs?

Swift

import Foundation

@MainActor
class Counter {
    var value = 0

    func increment() {
        value += 1
        print("Value is now \(value)")
    }
}

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

// Wait briefly to allow Task to run
Thread.sleep(forTimeInterval: 0.1)

A Compilation error due to missing await

B Value is now 0

C Value is now 1

D Runtime error due to data race

💡 Hint

Remember that @MainActor ensures code runs on the main thread and that await is needed when calling async methods.

The @MainActor attribute means the increment() method runs on the main thread. The call to increment() is awaited inside a Task, so it runs correctly and prints "Value is now 1".

🧠 Conceptual

intermediate

Which statement about @MainActor is true?

Choose the correct statement about the @MainActor attribute in Swift concurrency.

A @MainActor requires all methods to be synchronous.

B @MainActor makes code run on a background thread automatically.

C @MainActor disables concurrency for the annotated code.

D @MainActor ensures the annotated code runs on the main thread.

💡 Hint

Think about what the 'Main' in @MainActor refers to.

The @MainActor attribute guarantees that the annotated code runs on the main thread, which is important for UI updates and thread safety.

🔧 Debug

advanced

What error does this code produce?

This Swift code uses @MainActor but has a problem. What error will the compiler show?

Swift

import Foundation

@MainActor
class DataManager {
    var data = ""

    func updateData() {
        data = "Updated"
    }
}

let manager = DataManager()
manager.updateData()

A Call to actor-isolated property 'data' from outside the actor requires 'await'

B Syntax error: missing async keyword on updateData()

C No error, code runs fine

D Runtime error: data race detected

💡 Hint

Accessing actor-isolated properties from outside requires special handling.

Because DataManager is isolated to @MainActor, accessing its properties or methods from outside requires 'await' and async context. Calling updateData() directly causes a compiler error.

📝 Syntax

advanced

Which option correctly declares a global function on the main actor?

You want to declare a global function that always runs on the main actor. Which syntax is correct?

A func @MainActor updateUI() { print("UI updated") }

B @MainActor func updateUI() { print("UI updated") }

C func updateUI() @MainActor { print("UI updated") }

D func updateUI() { @MainActor print("UI updated") }

💡 Hint

Remember how attributes are placed before declarations in Swift.

The correct syntax places @MainActor before the function declaration. Other options are invalid syntax.

🚀 Application

expert

How many times is 'print' called in this code?

Analyze this Swift code using @MainActor and async calls. How many times will "Hello from main actor" be printed?

Swift

import Foundation

@MainActor
func greet() {
    print("Hello from main actor")
}

func run() async {
    await greet()
    Task.detached {
        await greet()
    }
    await greet()
}

Task {
    await run()
}

// Wait to allow async tasks to complete
Thread.sleep(forTimeInterval: 0.2)

A 3

B 2

C 1

D 0

💡 Hint

Consider how many times greet() is called and that Task.detached can call @MainActor functions with await.

The greet() function is called three times: twice directly awaited in run(), and once inside Task.detached awaited. All calls print the message.