iOS13下SwiftUI中如何仅初始化一次ContainerViewModel？

核心思路 #

由于iOS13不支持StateObject，我们需要将ContainerViewModel的实例提前创建并保存在父视图的状态中，避免每次视图重建时重新初始化。这样无论tab切换还是showHeader状态变化，ViewModel实例都会保持存活，网络请求只会执行一次。

具体修改步骤 #

1. 在MainView中预创建并保存ViewModel实例
   用@State存储三个ContainerViewModel实例，iOS13中@State可以维持引用类型的生命周期，不会因body重建而销毁实例：

struct MainView: View {
    @EnvironmentObject var viewModel: MainViewModel
    @State private var selectedCategory: CategoryType = .first
    // 预创建三个ViewModel实例，通过@State保持生命周期
    @State private var firstVM = ContainerViewModel(selectedCategory: .first)
    @State private var secondVM = ContainerViewModel(selectedCategory: .second)
    @State private var thirdVM = ContainerViewModel(selectedCategory: .third)
    
    var body: some View {
        HeaderView() { category in
            selectedCategory = category
        }
        TabView(selection: $selectedCategory) {
            // 传递预创建的ViewModel，而非每次新建
            ContainerView()
                .environmentObject(firstVM)
                .tag(CategoryType.first)
            ContainerView()
                .environmentObject(secondVM)
                .tag(CategoryType.second)
            ContainerView()
                .environmentObject(thirdVM)
                .tag(CategoryType.third)
        }
    }
}

2. 修正ContainerView的递归问题
   你原代码中ContainerView内部嵌套了自身，会导致无限递归，调整为实际内容展示逻辑：

struct ContainerView: View {
    @EnvironmentObject var viewModel: ContainerViewModel
    @State private var showHeader = true
    
    var body: some View {
        VStack {
            if showHeader {
                HeaderView() { category in
                    viewModel.selectedCategory = category
                }
            }
            // 替换为你实际的内容视图，比如分类列表
            Text("当前分类: \(viewModel.selectedCategory.rawValue)")
        }
    }
}

3. 确保网络请求仅执行一次
   将网络请求放在ContainerViewModel的初始化方法中，由于实例只会创建一次，请求也只会执行一次：

class ContainerViewModel: ObservableObject {
    @Published var selectedCategory: CategoryType
    
    init(selectedCategory: CategoryType) {
        self.selectedCategory = selectedCategory
        // 网络请求仅在ViewModel初始化时执行一次
        fetchCategoryData()
    }
    
    private func fetchCategoryData() {
        // 执行具体的网络请求操作
        print("开始请求分类\(selectedCategory)的数据")
    }
}

可选优化：延迟加载ViewModel #

如果不想初始时就创建三个ViewModel，可以通过onAppear延迟初始化，仅在对应tab首次被选中时创建实例：

struct MainView: View {
    @EnvironmentObject var viewModel: MainViewModel
    @State private var selectedCategory: CategoryType = .first
    @State private var firstVM: ContainerViewModel?
    @State private var secondVM: ContainerViewModel?
    @State private var thirdVM: ContainerViewModel?
    
    var body: some View {
        HeaderView() { category in
            selectedCategory = category
        }
        TabView(selection: $selectedCategory) {
            ContainerView()
                .environmentObject(firstVM ?? .init(selectedCategory: .first))
                .onAppear {
                    if firstVM == nil {
                        firstVM = ContainerViewModel(selectedCategory: .first)
                    }
                }
                .tag(CategoryType.first)
            
            ContainerView()
                .environmentObject(secondVM ?? .init(selectedCategory: .second))
                .onAppear {
                    if secondVM == nil {
                        secondVM = ContainerViewModel(selectedCategory: .second)
                    }
                }
                .tag(CategoryType.second)
            
            ContainerView()
                .environmentObject(thirdVM ?? .init(selectedCategory: .third))
                .onAppear {
                    if thirdVM == nil {
                        thirdVM = ContainerViewModel(selectedCategory: .third)
                    }
                }
                .tag(CategoryType.third)
        }
    }
}

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