Global actors (@MainActor) in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a @MainActor in Swift?

@MainActor is a global actor that ensures code runs on the main thread, which is important for updating the user interface safely.

beginner

Why should UI updates be done on the @MainActor?

Because UI frameworks require changes to happen on the main thread to avoid crashes or unexpected behavior.

beginner

How do you mark a Swift function to run on the main actor?

You add @MainActor before the function declaration, like @MainActor func updateUI() { ... }.

intermediate

What happens if you call a @MainActor function from a background thread?

Swift automatically switches execution to the main thread before running the function, ensuring thread safety.

intermediate

Can you mark an entire class as <code>@MainActor</code>? What does it mean?

<p>Yes, marking a class with <code>@MainActor</code> means all its methods and properties run on the main thread.</p>

What does @MainActor guarantee in Swift concurrency?

A Code runs on the main thread

B Code runs on a background thread

C Code runs in parallel on multiple threads

D Code runs only once

@MainActor ensures the code runs on the main thread, which is essential for UI updates.

How do you declare a function to run on the main actor?

A @MainActor func myFunction() {}

B func mainActor() {}

C func @MainActor myFunction() {}

D mainActor func myFunction() {}

You place @MainActor before the function declaration.

If you call a @MainActor method from a background thread, what happens?

A It runs immediately on the background thread

B It causes a crash

C Swift switches to the main thread before running it

D It runs on a random thread

Swift automatically switches to the main thread to run @MainActor code safely.

Which of these can be marked with @MainActor?

A Only functions

B Only structs

C Only variables

D Functions, classes, and properties

You can mark functions, classes, and properties with @MainActor to run them on the main thread.

Why is it important to use @MainActor for UI code?

A To improve performance

B To avoid UI glitches and crashes

C To run code faster

D To use less memory

UI frameworks require updates on the main thread to avoid glitches and crashes.

Explain what @MainActor does and why it is important in Swift concurrency.

Think about where UI updates must happen.

Describe how you would use @MainActor to protect a class that updates the user interface.

Consider marking the whole class instead of individual methods.