How do I conform to a protocol with an actor?

Question

When I try to define an actor that conforms to a protocol , Xcode gives me the error Actor-isolated instance method 'foo()' cannot be used to satisfy nonisolated protocol requirement . I can make the func nonisolated I don't think I want to. Do I need to? What would I be losing if I do so?

Here's the example code that causes the error:

protocol Fooable {
    func foo() -> Int
}

actor Bar: Fooable { // Error: Actor-isolated instance method...
    func foo() -> Int {
        return 42
    }
}

Answer 1

If you tap on the “auto fix” button in front of the error message it will expand to show you two options:

In short, you can either:

• Add nonisolated to foo() to make this instance method not isolated to the actor:

   protocol Fooable { func foo() -> Int } actor Bar: Fooable { nonisolated func foo() -> Int { return 42 } }

• Mark the protocol requirement foo() async to allow actor-isolated conformance:

   protocol Fooable { func foo() async -> Int } actor Bar: Fooable { func foo() -> Int { return 42 } }

In short, the protocol needs to know whether the method can be called synchronously across actor-boundaries or not (ie, whether it is nonisolated or not).

---

You asked whether you could use nonisolated :

What would I be losing if I do so?

If it is nonisolated , the method simply cannot directly access actor-isolated properties nor call actor-isolated methods. As the method currently stands, you can safely make it nonisolated .

In short, if a method does not require actor-isolation, you should generally feel free to mark it as nonisolated .