Mac监听VoiceOver状态变更：Swift 6迁移问题排查

问题背景 #

通过NSWorkspace的KVO监听VoiceOver状态变化的代码如下：

public final class MyContext: ObservableObject {
    public weak var actionHandler: ActionHandler?
    public weak var dataProvider: DataProvider?

    @Published public private(set) var isVoiceoverEnabled: Bool
    @objc private var voiceOverObserver: NSKeyValueObservation?

    public init(
        actionHandler: ActionHandler,
        dataProvider: DataProvider
    ) {
        self.actionHandler = actionHandler
        self.dataProvider = dataProvider
        self.isVoiceoverEnabled = NSWorkspace.shared.isVoiceOverEnabled
        self.voiceOverObserver = NSWorkspace.shared.observe(\.isVoiceOverEnabled) { [weak self] _, _ in
            self?.updateVoiceOverState()
        }
    }

    @objc
    private func updateVoiceOverState() {
        if isVoiceoverEnabled != NSWorkspace.shared.isVoiceOverEnabled {
            isVoiceoverEnabled = NSWorkspace.shared.isVoiceOverEnabled
        }
    }
}

迁移至Swift 6时遇到两个核心问题：

1. 编译器报错：在@Sendable闭包中捕获了非Sendable类型的'self'，提示类未遵循Sendable协议。
2. 尝试让类遵循Sendable时，因存在weak var属性受阻；尝试将类隔离至MainActor时，又报错：在同步非隔离上下文中调用了MainActor隔离的实例方法'updateVoiceOverState()'。

解决方案 #

方案一：让类遵循Sendable协议 #

weak var本身不影响Sendable合规性，问题出在actionHandler和dataProvider这两个弱引用的类型未遵循Sendable。按以下步骤修改：

1. 让关联协议遵循Sendable：

protocol ActionHandler: AnyObject, Sendable {}
protocol DataProvider: AnyObject, Sendable {}

2. 调整MyContext类，确保所有属性Sendable兼容，同时处理KVO回调的线程问题：

public final class MyContext: ObservableObject, Sendable {
    public weak var actionHandler: ActionHandler?
    public weak var dataProvider: DataProvider?

    @Published public private(set) var isVoiceoverEnabled: Bool
    private var voiceOverObserver: NSKeyValueObservation?

    public init(
        actionHandler: ActionHandler,
        dataProvider: DataProvider
    ) {
        self.actionHandler = actionHandler
        self.dataProvider = dataProvider
        self.isVoiceoverEnabled = NSWorkspace.shared.isVoiceOverEnabled
        self.voiceOverObserver = NSWorkspace.shared.observe(\.isVoiceOverEnabled) { [weak self] _, _ in
            guard let self else { return }
            // KVO回调可能在任意线程触发，需切换到MainActor修改@Published属性
            Task { @MainActor in
                self.updateVoiceOverState()
            }
        }
    }

    private func updateVoiceOverState() {
        if isVoiceoverEnabled != NSWorkspace.shared.isVoiceOverEnabled {
            isVoiceoverEnabled = NSWorkspace.shared.isVoiceOverEnabled
        }
    }
}

方案二：用MainActor隔离类 #

如果选择通过MainActor隔离类，需确保KVO回调在MainActor上下文调用方法，同时关联协议仍需遵循Sendable：

1. 先让协议遵循Sendable（同方案一第一步）。
2. 调整MyContext类：

@MainActor
public final class MyContext: ObservableObject {
    public weak var actionHandler: ActionHandler?
    public weak var dataProvider: DataProvider?

    @Published public private(set) var isVoiceoverEnabled: Bool
    private var voiceOverObserver: NSKeyValueObservation?

    public init(
        actionHandler: ActionHandler,
        dataProvider: DataProvider
    ) {
        self.actionHandler = actionHandler
        self.dataProvider = dataProvider
        self.isVoiceoverEnabled = NSWorkspace.shared.isVoiceOverEnabled
        self.voiceOverObserver = NSWorkspace.shared.observe(\.isVoiceOverEnabled) { [weak self] _, _ in
            guard let self else { return }
            // 显式切换到MainActor上下文调用隔离方法
            Task { @MainActor in
                self.updateVoiceOverState()
            }
        }
    }

    private func updateVoiceOverState() {
        if isVoiceoverEnabled != NSWorkspace.shared.isVoiceOverEnabled {
            isVoiceoverEnabled = NSWorkspace.shared.isVoiceOverEnabled
        }
    }
}

关键知识点 #

• Swift 6中KVO的闭包默认是@Sendable，捕获的对象必须是Sendable类型或在指定Actor隔离。
• @Published属性的修改必须在MainActor线程（ObservableObject的默认行为），而KVO回调可能在后台线程触发，因此必须显式切换线程。
• 弱引用的类型只要遵循Sendable，就不会影响宿主类的Sendable合规性。

内容的提问来源于stack exchange，提问作者rottweiler