如何消除Xcode 15 Beta 8中Non-Sendable Notification跨Actor边界警告？

我开发了一款Mac应用，需要在Mac进入睡眠状态时执行操作。为此，我采用以下「现代」方式监听通知：

@MainActor
final class AppController: NSObject, ObservableObject
{
   var sleepTask: Task<Void, Never>? = nil

   override init()
   {
       sleepTask = Task { [weak self] in
          for await _ in NSWorkspace.shared.notificationCenter.notifications(named: NSWorkspace.willSleepNotification) 
          {
              self?.doSomething()
          }
       }
   }
}

Xcode 15 Beta 8在for/await调用处新增了一条警告：

Non-sendable type 'Notification?' returned by implicitly asynchronous call to nonisolated function cannot cross actor boundary

1. Generic enum 'Optional' does not conform to the 'Sendable' protocol (Swift.Optional)

但我根本没有使用该notification对象，它并未跨越Actor边界。

问询 #

如何消除这条警告？（除了强制让Optional遵循@unchecked Sendable之外的方法）

这里提供几种无需修改Optional协议一致性的可行方案：

方案一：将监听逻辑移到非Main Actor上下文 #

NSWorkspace的通知中心属于非隔离（nonisolated）对象，我们可以把异步流的监听逻辑放到全局并发上下文，避免跨Actor的Sendable检查冲突，之后再切回Main Actor执行业务操作：

override init() {
    sleepTask = Task.detached { [weak self] in
        for await _ in NSWorkspace.shared.notificationCenter.notifications(named: NSWorkspace.willSleepNotification) {
            await MainActor.run {
                self?.doSomething()
            }
        }
    }
}

方案二：改用传统的NotificationCenter监听方式 #

如果不需要异步流的语法糖，退回到经典的addObserver写法，可以完全避开这个警告，同时兼容性更好：

@MainActor
final class AppController: NSObject, ObservableObject {
    private var observation: NSObjectProtocol?

    override init() {
        super.init()
        observation = NotificationCenter.default.addObserver(
            forName: NSWorkspace.willSleepNotification,
            object: nil,
            queue: .main
        ) { [weak self] _ in
            self?.doSomething()
        }
    }

    deinit {
        if let observation {
            NotificationCenter.default.removeObserver(observation)
        }
    }
}

方案三：自定义符合Sendable的异步流包装器 #

创建一个简单的扩展，把原API返回的Notification?流转换成只传递Void的符合Sendable的异步流，既保留现代异步写法，又规避原API的类型问题：

extension NotificationCenter {
    func sendableNotifications(named name: NSNotification.Name) -> AsyncStream<Void> {
        AsyncStream { continuation in
            let token = addObserver(forName: name, object: nil, queue: nil) { _ in
                continuation.yield()
            }
            continuation.onTermination = { _ in
                removeObserver(token)
            }
        }
    }
}

之后在代码中使用这个包装后的流：

sleepTask = Task { [weak self] in
    for await _ in NSWorkspace.shared.notificationCenter.sendableNotifications(named: NSWorkspace.willSleepNotification) {
        self?.doSomething()
    }
}

内容的提问来源于stack exchange，提问作者Bryan