swift - 为 ObservedObject 和 State 对象配置预览

Question

所以我仍在学习 Swift，出于某种原因，我在预览以及如何配置方面遇到了最大的麻烦。

所以我有以下代码:

struct MainView: View {
    
    // The app's model that the containing scene passes in
    @ObservedObject var model: MainViewModel
    @State var activeTab = 0
    
    var body: some View {
        VStack {
            TabView(selection: $activeTab) {
                Group {
                    WorldView(model: model, activeTab: $activeTab)
                        .tabItem {
                            Label(Tabs.explore.rawValue, systemImage: Tabs.explore.icon)
                                .environment(\.symbolVariants, .none)
                        }
                        .tag(0)
                    ListView(model: model, activeTab: $activeTab)
                        .tabItem {
                            Label(Tabs.list.rawValue, systemImage: Tabs.list.icon)
                                .environment(\.symbolVariants, .none)
                        }
                        .tag(1)
                    FavoritesView(activeTab: $activeTab)
                        .tabItem {
                            Label(Tabs.favorite.rawValue, systemImage: Tabs.favorite.icon)
                                .environment(\.symbolVariants, .none)
                        }
                        .tag(2)
                    ProfileView(model: model, activeTab: $activeTab)
                        .tabItem {
                            Label(Tabs.profile.rawValue, systemImage: Tabs.profile.icon)
                                .environment(\.symbolVariants, .none)
                        }
                        .tag(3)
                }
                .environmentObject(model)
            }
            .tint(.accentColor)
            .onChange(of: activeTab, perform: { value in
                log.info("\n 🟢: (MainView: 46) - User has selected tab: \(value).")
                print("")
            })
        }
        .onAppear() {
            model.fetchPlaces()
        }
    }
}

然后我有这样的预览:

struct MainView_Previews: PreviewProvider {
    static var previews: some View {
        MainView(model: model, activeTab: activeTab)
    }
}

我在预览中遇到了两个错误:

Cannot find 'activeTab' in scope
Cannot find 'model' in scope

如果我这样定义它:

struct MainView_Previews: PreviewProvider {
    @ObservedObject var model: MainViewModel
    @State var activeTab = 0
    static var previews: some View {
        MainView(model: model, activeTab: activeTab)
    }
}

我收到以下错误:

Instance member 'activeTab' cannot be used on type 'MainView_Previews'
Instance member 'model' cannot be used on type 'MainView_Previews'

有谁知道如何配置预览才能使其正常工作且不会崩溃？

Answer 1

发生这种情况是因为您在预览中传递了不存在的参数，或者因为您无法在预览中初始化对象。相反，这样做:

struct MyExamplePreviews: PreviewProvider{
   static var previews: some View {
        MainView(model: MainViewModel(), activeTab: 0)
    }
}

这将允许您预览 UI。这是做什么的:

• 创建一个在顶层传入的新模型 - 同样，您不能在任何地方创建它，只能在预览中创建
• 使 activeTab 设置为 0 - 如果需要，您可以拥有多个具有不同选项卡的预览设备。查看docs for Previews了解更多。