ActorError.swift

import AppKit

struct Hoge {
    @discardableResult
    static func doSomething(handler block: @escaping () -> Void) -> Any? { nil }
}

actor SomeService {
    func update() {}

    func subscribe1() {
        Hoge.doSomething(handler: { [weak self] in
            self?.update()
        })
    }

    func subscribe2() {
        Hoge.doSomething {
            self.update()
        }
    }
}