Why does a @MainActor annotated class initialisation gives the error

Question

I have a class named Person, Which is annotated with @MainActor. But as I try to create an instance of this mainactor class it gives an error.

"Call to main actor-isolated initializer 'init(firstName:lastName:)' in a synchronous nonisolated context".

@MainActor
class Person {
    var firstName: String = ""
    var lastName: String = ""
    
    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }
    
    func tryToPrintNameOnMainThread() {
        print("Is Main Thread: \(Thread.isMainThread)")
        print("\(firstName) \(lastName)")
    }
}

class AsyncAwaitViewModel {
     let personActor = Person(firstName: "", lastName: "") // Error
     let someTestLabel = UILabel() // No Error 
}


     

But if I create UILabel() instance it works. Where UILabel is also a @MainActor annotated class.

My question is why i am seeing this error and what should be ideal way to have @MainActor class instance in this type of scenario.

Creating the Person instance from @MainActor annotated UIViewController subclass works fine.

Answer 1

Your isolated initialiser needs to be called from within an "async context". That is, call it from within a function being async, i.e. func f() async -> { ... }, or from within a Swift task: Task { ... }.

In your scenario, it would be better though to make the model the actor, and use a struct for your Person. If Person is an "Entity", which is used as a "DTO" send to a network API or CoreData, this makes much sense.

The model actor would then ensure thread-safety when mutating the Person value. When done with it, it would send the Person value to some service, for example. Since the Person value would not be mutated along the way, it doesn't need to be an actor.