@MainActor

@MainActor is an attribute that isolates a function or type to the main actor, so it only runs on the main thread. This can be used to ensure that UI updates are only done on the main thread, and not on a background thread.

Since Swift 6.2, projects can opt in to main actor by default isolation (on by default in new Xcode app projects), which isolates all code in a module to the main actor without explicit @MainActor annotations.

• @MainActor functions
• @MainActor structs and classes
• Limitations
• See also
• Further reading

@MainActor functions

You can ensure that a function only runs on the main thread by marking it with @MainActor:

@MainActor
func someFunction() {
  print("This function is running on the main thread.")
}

Task {
  await someFunction()
}

// Output: This function is running on the main thread.

@MainActor structs and classes

You can also ensure an entire struct or class runs on the main thread with @MainActor:

@MainActor
struct SomeStruct {
  func someFunction() {
    print("This function is running on the main thread.")
  }
}

Task {
  let someStruct = await SomeStruct()
  await someStruct.someFunction()
}

Limitations

A piece of code marked with @MainActor might still contain additional logic that runs on a different thread if it is sent to a different dispatch queue:

import Foundation

@MainActor
func someFunction() {
  print("1️⃣ On main thread: \(Thread.isMainThread)")

  DispatchQueue.global(qos: .background).async {
    print("2️⃣ On main thread: \(Thread.isMainThread)")
  }
}

Task {
  await someFunction()
}

// Output:
// 1️⃣ On main thread: true
// 2️⃣ On main thread: false

See also

• Dispatch
• async/await
• isMainThread

Further reading

• MainActor documentation