SwiftUI中@Published状态变量为何无法更新UI？

我将AppState类作为环境变量在整个App中共享，用于在所有视图间共享核心数据：

class AppState: ObservableObject {
    @Published var user: User?
    @Published var matches = Matches(
        status: .NOT_LOADED,
        data: [],
        performedFullLoad: false,
        canFetchMore: false
    )
}

AppState在ContentView中实例化：

struct ContentView: View {
    @StateObject private var appState = AppState()
    
    var body: some View {
        Group {
            if appState.user == nil { // 未登录时展示AuthView
                AuthView()
            }
            else { // 登录后展示MatchesView
                MatchesView()
            }
        }
        .environmentObject(appState)
    }
}

用户登录后，调用Matches.loadMatchesList()方法更新AppState.matches中的数据：

final class Matches: ObservableObject {
    @Published var status: LoadingStatus
    @Published var data: [Match]
    @Published var performedFullLoad: Bool
    @Published var canFetchMore: Bool
    
    init(status: LoadingStatus, data: [Match], performedFullLoad: Bool, canFetchMore: Bool) {
        self.status = status
        self.data = data
        self.performedFullLoad = performedFullLoad
        self.canFetchMore = canFetchMore
    }
    
    func loadMatchesList(userId: String) async {
        do {
            // 获取匹配数据并解析为parsedMatches变量
            ...
            
            // 存储匹配数据
            DispatchQueue.main.async {
                self.status = .LOADED
                self.data = parsedMatches
                self.performedFullLoad = true
                self.canFetchMore = decodedData.count > numOfMatchesToLoad
                
                print("\(parsedMatches.count) matches loaded into state variable") // 运行时打印"2"
            }
        } catch let error {
            ...
        }
    }
}

最后在MatchesView中展示加载的匹配列表，问题出现：print语句确认已有2条匹配数据存入数组，但渲染的List却显示0条数据。

struct MatchesView: View {
    @EnvironmentObject var appState: AppState
    
    var body: some View {
            NavigationView {
                List(appState.matches.data) { match in
                    NavigationLink {
                        MatchDetail(match: match)
                    } label: {
                        MatchRow(match: match)       
                    }
                }
        }
    }
}

已确认MatchesView访问的是唯一正确的AppState实例，点击按钮打印appState.matches.data.count会输出"2"，但UI始终不更新显示这2条匹配数据。为何环境对象符合ObservableObject协议且data属性已用@Published修饰，UI却不更新？

• AppState中的matches属性用@Published修饰，但@Published只会在整个Matches对象被替换时才会向视图发送变更通知。
• 当调用loadMatchesList()更新matches内部的data、status等属性时，AppState的matches对象本身并没有被替换，因此AppState不会发出任何变更通知，视图也就不会重新渲染。
• 虽然Matches自身是ObservableObject且内部属性用@Published修饰，但视图监听的是AppState这个环境对象，而非直接监听Matches，所以Matches内部的变更无法触发视图更新。

提供两种可行的解决方式：

方案一：将Matches的属性直接整合到AppState中 #

移除独立的Matches类，把相关属性直接放到AppState里，这样所有@Published属性的变更都会直接触发视图更新：

class AppState: ObservableObject {
    @Published var user: User?
    @Published var matchesStatus: LoadingStatus = .NOT_LOADED
    @Published var matchesData: [Match] = []
    @Published var matchesPerformedFullLoad: Bool = false
    @Published var matchesCanFetchMore: Bool = false
    
    func loadMatchesList(userId: String) async {
        do {
            // 获取匹配数据并解析为parsedMatches变量
            ...
            
            DispatchQueue.main.async {
                self.matchesStatus = .LOADED
                self.matchesData = parsedMatches
                self.matchesPerformedFullLoad = true
                self.matchesCanFetchMore = decodedData.count > numOfMatchesToLoad
            }
        } catch let error {
            ...
        }
    }
}

然后更新MatchesView的引用：

List(appState.matchesData) { match in
    // ... 原有代码
}

方案二：让AppState监听Matches的内部变更 #

保留Matches类，通过Combine监听Matches的objectWillChange事件，当Matches内部属性变更时，手动触发AppState的objectWillChange通知：

import Combine

class AppState: ObservableObject {
    @Published var user: User?
    var matches = Matches(
        status: .NOT_LOADED,
        data: [],
        performedFullLoad: false,
        canFetchMore: false
    )
    
    private var cancellables = Set<AnyCancellable>()
    
    init() {
        // 监听Matches的变更，转发给AppState的订阅者
        matches.objectWillChange
            .sink { [weak self] _ in
                self?.objectWillChange.send()
            }
            .store(in: &cancellables)
    }
}

这样当Matches内部的data等属性更新时，AppState会同步发出变更通知，视图就会重新渲染并展示最新数据。

内容的提问来源于stack exchange，提问作者Sam