使用Combine观察Singleton时.sink未被调用的问题

问题描述 #

使用Combine监听单例MREUserService的ownUser变化时，sink闭包始终不触发。预期每隔3秒打印“MODDED”后会打印“CHECK”，但实际仅输出“MODDED”。已尝试将单例设为@ObservableObject并手动调用objectWillChange.send()，但问题仍未解决。

原因分析 #

核心问题在于@Published的触发机制：

• @Published仅当被包装的对象引用发生变更时，才会向数据流发送新事件。
• 当前代码中只是修改了ownUser的userName属性，ownUser本身的引用没有改变，因此MREUserService.shared.$ownUser的数据流不会发出新值，导致sink闭包无法执行。
• 手动调用objectWillChange.send()对@Published的数据流无效，因为@Published有独立的事件发布逻辑。

解决方案 #

提供两种可行的修复方案，可根据实际场景选择：

方案一：让MREUser成为可观察对象 #

将MREUser改为ObservableObject，并把userName标记为@Published，这样属性变化时能主动发出通知，同时调整监听逻辑来监听内部属性变化。

方案二：替换整个ownUser对象 #

如果不需要MREUser本身是可观察对象，可在修改属性时直接替换整个ownUser实例，让@Published检测到引用变化并触发事件。

完整修正代码 #

方案一代码 #

class MREViewModel: ObservableObject {
    
    private var cancellables = Set<AnyCancellable>()
    
    @Published var ownUser: MREUser
    
    init() {
        ownUser = MREUserService.shared.ownUser
        setupBindings()
    }
    
    func setupBindings() {
        // 监听ownUser的userName属性变化
        MREUserService.shared.$ownUser
            .flatMap { $0.$userName }
            .receive(on: DispatchQueue.main)
            .sink { [weak self] _ in
                print("CHECK")
                // 因为ownUser引用未变，这里只需更新本地属性或直接触发视图刷新
                self?.objectWillChange.send()
            }
            .store(in: &cancellables)
    }
}

struct ViewSwiftUI: View {
    @ObservedObject var viewModel: MREViewModel
    
    var body: some View {
        Text(viewModel.ownUser.userName)
    }
}

class MREUserService: ObservableObject {
    static var shared: MREUserService = MREUserService()
    
    @Published var ownUser = MREUser(userName: "initialised")
    
    var timer: Timer?
    
    init() {
        timer = Timer.scheduledTimer(withTimeInterval: 3.0, repeats: true) { timer in
            self.ownUser.userName = "\(Date())"
            print("MODDED: \(self.ownUser.userName)")
        }
    }
}

// 修改MREUser为ObservableObject
class MREUser: ObservableObject {
    var _id = UUID().uuidString
    @Published var userName: String
    
    init(userName: String) {
        self.userName = userName
    }
}

方案二代码 #

class MREViewModel: ObservableObject {
    
    private var cancellables = Set<AnyCancellable>()
    
    @Published var ownUser: MREUser
    
    init() {
        ownUser = MREUserService.shared.ownUser
        setupBindings()
    }
    
    func setupBindings() {
        MREUserService.shared.$ownUser
            .receive(on: DispatchQueue.main)
            .sink { [weak self] user in
                print("CHECK")
                self?.ownUser = user
            }
            .store(in: &cancellables)
    }
}

struct ViewSwiftUI: View {
    @ObservedObject var viewModel: MREViewModel
    
    var body: some View {
        Text(viewModel.ownUser.userName)
    }
}

class MREUserService: ObservableObject {
    static var shared: MREUserService = MREUserService()
    
    @Published var ownUser = MREUser(userName: "initialised")
    
    var timer: Timer?
    
    init() {
        timer = Timer.scheduledTimer(withTimeInterval: 3.0, repeats: true) { timer in
            // 替换整个ownUser对象，让@Published检测到引用变化
            self.ownUser = MREUser(userName: "\(Date())")
            print("MODDED: \(self.ownUser.userName)")
        }
    }
}

class MREUser {
    var _id = UUID().uuidString
    var userName: String
    
    init(userName: String) {
        self.userName = userName
    }
}

内容的提问来源于stack exchange，提问作者ambassador