SwiftUI：如何监听单例对象中字段的变化？

一、视图不更新的原因及解决办法 #

核心原因 #

SharedData中的@Published var sharedState = SharedState()，由于SharedState是值类型（结构体），直接修改它的内部属性（比如typeTrees.append(ttMaker)）并不会触发@Published的变更通知——因为sharedState本身的实例没有被重新赋值，SwiftUI无法感知到内部数据的变化。

两种修复方案 #

方案1：修改后重新赋值sharedState

每次修改SharedState内部属性时，先创建副本修改，再重新赋值给sharedState，强制触发@Published的通知：

// 替换原有的直接append代码
var updatedState = SharedData.shared.sharedState
updatedState.typeTrees.append(ttMaker)
SharedData.shared.sharedState = updatedState

方案2：将SharedState改为ObservableObject类

把结构体改成遵循ObservableObject的类，并给需要监听的内部属性加上@Published，这样内部属性的变化会自动发送通知：

// 修改SharedState为类
class SharedState: ObservableObject {
    @Published var allMMS: [MMS] = []
    @Published var typeTrees: [TTMaker] = []
    @Published var sampleInputs: [String: String] = [:]
    @Published var selectedTypeTreeName: String?
    @Published var selectedMMSPathName: String?
    @Published var maps: [String: FunctionalMap] = [:]
    @Published var mapId: String?
    @Published var selectedMenuItem: String? = nil
}

// SharedData保持原有结构
class SharedData: ObservableObject {
    static let shared = SharedData()
    @Published var sharedState = SharedState()
}

同时，视图中建议用@StateObject（iOS14+）替代@ObservedObject来持有单例，确保生命周期正确：

struct ContentView: View {
    @StateObject var sharedData = SharedData.shared
    
    var body: some View {
        List {
            ForEach(sharedData.sharedState.typeTrees, id: \.self) { tree in
                Button(action: {
                    sharedData.sharedState.selectedTypeTreeName = tree.newTree.filename
                }) {
                    HStack {
                        Text(tree.newTree.filename)
                        Spacer()
                        if tree.newTree.filename == sharedData.sharedState.selectedTypeTreeName {
                            Image(systemName: "checkmark")
                        }
                    }
                }
            }
        }
    }
}

二、类似React Context的解决方案 #

SwiftUI中对应的是EnvironmentObject，可以在整个视图层级中共享状态，无需手动传递，和React Context的作用完全一致。

使用步骤 #

1. 在App入口注入环境对象：

@main
struct YourApp: App {
    @StateObject private var sharedData = SharedData.shared
    
    var body: some Scene {
        WindowGroup {
            ContentView()
                .environmentObject(sharedData)
        }
    }
}

2. 在任意子视图中通过@EnvironmentObject获取共享数据：

struct ContentView: View {
    @EnvironmentObject var sharedData: SharedData // 无需手动初始化，直接从环境获取
    
    var body: some View {
        List {
            ForEach(sharedData.sharedState.typeTrees, id: \.self) { tree in
                Button(action: {
                    sharedData.sharedState.selectedTypeTreeName = tree.newTree.filename
                }) {
                    HStack {
                        Text(tree.newTree.filename)
                        Spacer()
                        if tree.newTree.filename == sharedData.sharedState.selectedTypeTreeName {
                            Image(systemName: "checkmark")
                        }
                    }
                }
            }
        }
    }
}

这样无论视图层级多深，只要在环境中注入了sharedData，所有子视图都能直接访问，和React中useContext的用法逻辑一致。

内容的提问来源于stack exchange，提问作者János