SwiftUI中ForEach更新时仅执行特定次数的问题排查

点击「Go Back」和「Save & Continue」按钮更新currentIndex时，LeftView中的ForEach出现异常行为：

• 初始渲染时执行5次let _ = print(index)
• 后续修改currentIndex时，每次仅执行两次不同索引的循环

BulletView内的打印日志：
初始状态：

Customer Information true
System Information false
Payment false
Add Template false
Receipt false

currentIndex增至1时：

Customer Information true
System Information false

currentIndex增至2时：

System Information false
Payment false

相关代码：

fileprivate let titlesList = ["Customer Information",
                              "System Information",
                              "Payment",
                              "Add Template",
                              "Receipt"]

struct InstallationView: View {
    
    @ObservedObject private var viewModel = InstallationViewModel()
    
    var body: some View {
        NavigationView {
            GeometryReader { metrics in
                HStack(alignment: .center, spacing: 0) {
                    LeftView(currentIndex: $viewModel.currentIndex)
                        .frame(width: metrics.size.width * 0.35)
                        .frame(maxHeight: .infinity)
                    RightView(currentIndex: $viewModel.currentIndex)
                        .frame(width: metrics.size.width * 0.65)
                        .frame(maxHeight: .infinity)
                }
            }
        }.navigationBarHidden(true).navigationViewStyle(StackNavigationViewStyle())
    }
}

fileprivate struct LeftView: View {
    @Binding var currentIndex: Int
     
    var body: some View {
        VStack(alignment: .center, spacing: 0) {
            ForEach(0..<titlesList.count, id: \.self) { index in
                let _ = print(index)
                BulletView(count: index + 1,
                           title: titlesList[index],
                           isActive: (currentIndex == index),
                           showTopSeparator: (index != 0),
                           showBottomSeparator: (index != titlesList.count - 1))
            }
        }.padding(EdgeInsets.init(top: 0, leading: 100, bottom: 0, trailing: 0))
    }
}

fileprivate struct RightView: View {
    
    @Binding var currentIndex: Int

    private let edges = EdgeInsets.init(top: 0, leading: 0, bottom: 20, trailing: 20)
    private let leftEdges = EdgeInsets.init(top: 0, leading: 10, bottom: 0, trailing: 0)
    @Environment(\.presentationMode) var presentationMode
    
    var body: some View {
        ScrollView {
            VStack(alignment: .leading, spacing: 0) {
                Text(titlesList[currentIndex])
                    .font(Font(UIFont.boldSystemFont(ofSize: 24)))
                    .foregroundColor(Color.black)
                    .padding(EdgeInsets(top: 20, leading: 0, bottom: 0, trailing: 0))
                
                containedView()
                
                HStack {
                    Button(action: { currentIndex -= 1 }) {
                        Text("Go Back")
                            .frame(width: 200, height: 70)
                            .font(Font(UIFont.boldSystemFont(ofSize: 24)))
                            .foregroundColor(.white)
                            .background(Color(Constants.Colors.main))
                            .cornerRadius(5)
                    }
                    Spacer()
                    Button(action: { currentIndex += 1 }) {
                        Text("Save & Continue")
                            .frame(width: 200, height: 70)
                            .font(Font(UIFont.boldSystemFont(ofSize: 24)))
                            .foregroundColor(.white)
                            .background(Color(Constants.Colors.main))
                            .cornerRadius(5)
                    }
                }
                .padding(edges)
            }.padding(EdgeInsets.init(top: 80, leading: 0, bottom: 0, trailing: 0))
        }
    }
    
    func containedView() -> AnyView {
        switch currentIndex {
        case 0:
            return AnyView(CustomerView())
        case 1:
            return AnyView(SystemView())
        case 2:
            return AnyView(PaymentView())
        default:
            return AnyView(CustomerView())
        }
    }
}

struct InstallationView_Previews: PreviewProvider {
    static var previews: some View {
        InstallationView().previewInterfaceOrientation(.landscapeLeft)
    }
}

这是SwiftUI差异化视图更新机制导致的正常行为：

1. SwiftUI的核心优化逻辑是仅重新渲染依赖状态发生变化的视图，以此避免不必要的性能消耗。
2. 对于BulletView，只有isActive参数直接依赖currentIndex。当currentIndex变化时，仅两个BulletView的状态会改变：
   • 上一个选中项的isActive从true变为false
   • 新选中项的isActive从false变为true
3. 其余BulletView的所有参数（count/title/showTopSeparator/showBottomSeparator）均未发生变化，因此SwiftUI不会重新渲染这些视图，自然也不会执行视图内部的print语句，以及ForEach中对应索引的print(index)。

如果需要让所有BulletView都响应currentIndex变化（比如存在其他依赖currentIndex的UI逻辑），可以采用以下两种方式：

• 显式添加依赖参数：给BulletView新增一个接收currentIndex的参数（即使暂时不使用），强制SwiftUI感知到该视图依赖currentIndex：

BulletView(count: index + 1,
           title: titlesList[index],
           isActive: (currentIndex == index),
           showTopSeparator: (index != 0),
           showBottomSeparator: (index != titlesList.count - 1),
           currentIndex: currentIndex) // 新增依赖参数

• 强制视图重建（不推荐，影响性能）：给LeftView添加.id(currentIndex)修饰符，让currentIndex变化时整个LeftView重新构建：

LeftView(currentIndex: $viewModel.currentIndex)
    .frame(width: metrics.size.width * 0.35)
    .frame(maxHeight: .infinity)
    .id(viewModel.currentIndex) // 强制视图重建

如果只是调试阶段的打印需求，该现象属于SwiftUI的正常优化，无需额外处理。

内容的提问来源于stack exchange，提问作者Muhammad Umar