为何@Published属性变更未触发SwiftUI子视图更新？

核心问题 #

你遇到的视图不更新问题，根源有两个：

• CurrentChallengeView中用@State修饰taskStatus，这会让子视图持有一份独立的状态副本，父视图传递的taskStatus更新时，子视图不会同步刷新。
• 若TaskStatus是引用类型（class），修改其passed属性时，@Published的currentTaskStatus引用未改变，SwiftUI不会触发视图更新。

另外调试时无法查看@Published属性，可通过调试器查看带$前缀的属性（如$currentTaskStatus）解决。

解决方案 #

方案1：让子视图直接依赖ViewModel #

修改CurrentChallengeView，直接引用ChallengeViewModel，这样能直接响应@Published属性的更新：

struct CurrentChallengeView: View {
    @Binding var result: String
    @ObservedObject var challengeViewModel: ChallengeViewModel
    var validateResult: () -> Void
    var nextTask: () -> Void
    
    var body: some View {
        NavigationView {
            GeometryReader { proxy in
                VStack {
                    if let taskStatus = challengeViewModel.currentTaskStatus {
                        TaskView(task: taskStatus.task, width: proxy.size.width * 0.8, result: $result)
                        
                        if taskStatus.passed == nil {
                            Button {
                                validateResult()
                            } label: {
                                Text("Check")
                                    .foregroundStyle(.white)
                                    .padding()
                                    .frame(width: proxy.size.width * 0.8)
                                    .background(.green)
                                    .cornerRadius(30)
                            }
                        } else {
                            Button {
                                nextTask()
                            } label: {
                                Text("Next")
                                    .foregroundStyle(.white)
                                    .padding()
                                    .frame(width: proxy.size.width * 0.8)
                                    .background(taskStatus.passed! ? .green : .red)
                                    .cornerRadius(30)
                            }
                        }
                    }
                    Spacer()
                }
            }
            .background {
                Image("background")
                    .resizable()
                    .scaledToFill()
                    .ignoresSafeArea()
            }
        }
    }
}

更新ChallengeView的调用代码：

struct ChallengeView: View {
    @StateObject var challengeViewModel: ChallengeViewModel
    @State var result: String = ""
    
    var body: some View {
        if challengeViewModel.currentTaskStatus != nil {
            CurrentChallengeView(
                result: $result,
                challengeViewModel: challengeViewModel,
                validateResult:  {
                    if let intRes = Int(result) {
                        challengeViewModel.validateResult(result: intRes)
                    }
                },
                nextTask: { _ = challengeViewModel.nextTask() }
            )
        } else {
            Text("No task set")
        }
    }
}

方案2：将子视图的taskStatus改为@Binding #

如果不想让子视图依赖ViewModel，可将taskStatus改为@Binding，绑定到ViewModel的currentTaskStatus：

struct CurrentChallengeView: View {
    @Binding var result: String
    @Binding var taskStatus: TaskStatus
    var validateResult: () -> Void
    var nextTask: () -> Void
    
    var body: some View {
        NavigationView {
            GeometryReader { proxy in
                VStack {
                    TaskView(task: taskStatus.task, width: proxy.size.width * 0.8, result: $result)
                    
                    if taskStatus.passed == nil {
                        Button {
                            validateResult()
                        } label: {
                            Text("Check")
                                .foregroundStyle(.white)
                                .padding()
                                .frame(width: proxy.size.width * 0.8)
                                .background(.green)
                                .cornerRadius(30)
                        }
                    } else {
                        Button {
                            nextTask()
                        } label: {
                            Text("Next")
                                .foregroundStyle(.white)
                                .padding()
                                .frame(width: proxy.size.width * 0.8)
                                .background(taskStatus.passed! ? .green : .red)
                                .cornerRadius(30)
                        }
                    }
                    Spacer()
                }
            }
            .background {
                Image("background")
                    .resizable()
                    .scaledToFill()
                    .ignoresSafeArea()
            }
        }
    }
}

更新ChallengeView的调用：

struct ChallengeView: View {
    @StateObject var challengeViewModel: ChallengeViewModel
    @State var result: String = ""
    
    var body: some View {
        if let taskStatusBinding = $challengeViewModel.currentTaskStatus {
            CurrentChallengeView(
                result: $result,
                taskStatus: taskStatusBinding,
                validateResult:  {
                    if let intRes = Int(result) {
                        challengeViewModel.validateResult(result: intRes)
                    }
                },
                nextTask: { _ = challengeViewModel.nextTask() }
            )
        } else {
            Text("No task set")
        }
    }
}

额外注意：确保TaskStatus是值类型 #

如果TaskStatus是class（引用类型），即使修改passed属性，@Published的引用未改变，SwiftUI不会触发更新。需将其改为struct：

struct TaskStatus {
    var task: Task
    var result: Int
    var passed: Bool?
}

若必须使用class，则需在修改属性后手动发送更新通知：

func validateResult(result: Int) {
    if let status = currentTaskStatus, let index = self.currentTaskIndex {
        status.passed = result == status.result
        self.taskStatus[index] = status
        objectWillChange.send() // 手动触发视图更新
    }
}

调试@Published属性的方法 #

在Xcode调试器中，查看challengeViewModel.$currentTaskStatus（注意前缀$），即可看到@Published属性的当前值。也可在ViewModel的方法中添加打印语句，跟踪状态变化。

内容的提问来源于stack exchange，提问作者Gerrit