SwiftUI List在Mac端用ObservableObject选值触发控制台警告求助

问题描述 #

开发带选值功能的SwiftUI List，将selection绑定到标注@ObservableObject的ViewModel对象时，功能正常，但仅在Mac设备运行时控制台会输出警告：

Publishing changes from within view updates is not allowed, this will cause undefined behavior

iOS端运行相同代码无此问题，预览模式下也不会触发警告。因需兼容iOS16，无法迁移至@Observable宏。

复现代码 #

import SwiftUI
import CoreData

final class SelectionModel: NSObject, ObservableObject {
    @Published public var selectedItems = Set<Item.ID>()
}

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults<Item>

    @ObservedObject var viewModel: SelectionModel = SelectionModel()
    //@State var selectedItems = Set<Item.ID>() // 使用此方式无警告
    
    var body: some View {
        NavigationStack {
            List(selection: $viewModel.selectedItems) {
                ForEach(items) { item in
                    Text(item.timestamp!, formatter: itemFormatter)
                }
            }
            .toolbar {
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()

            do {
                try viewContext.save()
            } catch {
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
}

private let itemFormatter: DateFormatter = {
    let formatter = DateFormatter()
    formatter.dateStyle = .short
    formatter.timeStyle = .medium
    return formatter
}()

#Preview {
    ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
}

问题是否需要重视？ #

必须重视。虽然当前功能正常，但警告中的"未定义行为"意味着后续系统版本更新、业务逻辑复杂化时，可能出现UI错乱、崩溃等不可预知的问题。Mac端SwiftUI的List选中逻辑与iOS存在底层差异，这种警告的潜在风险比iOS端更高。

ObservableObject前提下的解决方法 #

方案1：延迟状态更新到视图周期外 #

给ViewModel的selectedItems添加自定义setter，通过DispatchQueue.main.async延迟更新，避开视图更新阶段的发布操作：

final class SelectionModel: NSObject, ObservableObject {
    private var _selectedItems = Set<Item.ID>()
    @Published public var selectedItems: Set<Item.ID> {
        get { _selectedItems }
        set {
            // 延迟到主队列下一个周期执行，避免在视图更新中发布变更
            DispatchQueue.main.async {
                self._selectedItems = newValue
            }
        }
    }
}

方案2：用@State中转并双向同步 #

在视图内使用@State变量作为List的selection绑定源，再通过onChange实现与ViewModel的双向同步：

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults<Item>

    @ObservedObject var viewModel: SelectionModel = SelectionModel()
    @State private var localSelectedItems = Set<Item.ID>()
    
    var body: some View {
        NavigationStack {
            List(selection: $localSelectedItems) {
                ForEach(items) { item in
                    Text(item.timestamp!, formatter: itemFormatter)
                }
            }
            // 本地状态变更同步到ViewModel
            .onChange(of: localSelectedItems) { newValue in
                viewModel.selectedItems = newValue
            }
            // ViewModel状态变更同步到本地
            .onChange(of: viewModel.selectedItems) { newValue in
                localSelectedItems = newValue
            }
            .toolbar {
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()

            do {
                try viewContext.save()
            } catch {
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
}

方案3：手动控制ObservableObject的发布时机 #

不使用@Published，手动调用objectWillChange.send()，并确保发布操作在视图更新周期外执行：

final class SelectionModel: NSObject, ObservableObject {
    public var selectedItems = Set<Item.ID>() {
        didSet {
            DispatchQueue.main.async {
                self.objectWillChange.send()
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Z S