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
    }
}

CodePudding user response：

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 (i.e., 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.