SwiftUI 状态管理

状态的概念

在 SwiftUI 中，状态是驱动用户界面更新的关键机制。当状态发生变化时，SwiftUI 会自动更新相关的视图。

@State

@State 用于管理视图内部的简单状态：

struct CounterView: View {
    @State private var count = 0

    var body: some View {
        VStack {
            Text("计数: \(count)")

            Button("增加") {
                count += 1
            }
        }
    }
}

@Binding

@Binding 允许子视图修改父视图的状态：

struct ToggleButton: View {
    @Binding var isOn: Bool

    var body: some View {
        Button(isOn ? "开启" : "关闭") {
            isOn.toggle()
        }
    }
}

struct ParentView: View {
    @State private var isEnabled = false

    var body: some View {
        ToggleButton(isOn: $isEnabled)
    }
}

可观察对象

ObservableObject

对于更复杂的状态管理，使用 ObservableObject：

class UserSettings: ObservableObject {
    @Published var username = ""
    @Published var isLoggedIn = false

    func login() {
        username = "张三"
        isLoggedIn = true
    }

    func logout() {
        username = ""
        isLoggedIn = false
    }
}

@StateObject 和 @ObservedObject

struct SettingsView: View {
    @StateObject private var settings = UserSettings()

    var body: some View {
        VStack {
            if settings.isLoggedIn {
                Text("欢迎, \(settings.username)!")
                Button("登出") {
                    settings.logout()
                }
            } else {
                Button("登录") {
                    settings.login()
                }
            }
        }
    }
}

环境对象

@EnvironmentObject

使用环境对象在视图层次结构中共享数据：

class AppState: ObservableObject {
    @Published var theme = "light"
    @Published var fontSize = 14
}

struct RootView: View {
    @StateObject private var appState = AppState()

    var body: some View {
        ContentView()
            .environmentObject(appState)
    }
}

struct ContentView: View {
    @EnvironmentObject var appState: AppState

    var body: some View {
        VStack {
            Text("当前主题: \(appState.theme)")
            Button("切换主题") {
                appState.theme = appState.theme == "light" ? "dark" : "light"
            }
        }
    }
}

数据流最佳实践

状态提升

当多个视图需要共享状态时，将状态提升到它们的共同父视图：

struct ParentView: View {
    @State private var selectedTab = 0

    var body: some View {
        VStack {
            TabView(selection: $selectedTab) {
                FirstTabView()
                    .tabItem { Text("第一页") }
                    .tag(0)
                SecondTabView()
                    .tabItem { Text("第二页") }
                    .tag(1)
            }

            // 共享状态的控制器
            TabController(selectedTab: $selectedTab)
        }
    }
}

依赖注入

使用依赖注入来提高代码的可测试性和可维护性：

protocol DataService {
    func fetchData() async throws -> [String]
}

class UserDataService: DataService {
    func fetchData() async throws -> [String] {
        // 实际的网络请求
        return ["数据1", "数据2"]
    }
}

class ViewModel: ObservableObject {
    private let dataService: DataService
    @Published var items: [String] = []

    init(dataService: DataService) {
        self.dataService = dataService
    }

    func loadData() async {
        do {
            items = try await dataService.fetchData()
        } catch {
            print("加载失败: \(error)")
        }
    }
}

练习

创建一个简单的待办事项应用，练习状态管理：

struct TodoItem: Identifiable {
    let id = UUID()
    var title: String
    var isCompleted: Bool
}

class TodoStore: ObservableObject {
    @Published var items: [TodoItem] = []

    func addItem(_ title: String) {
        items.append(TodoItem(title: title, isCompleted: false))
    }

    func toggleItem(_ item: TodoItem) {
        if let index = items.firstIndex(where: { $0.id == item.id }) {
            items[index].isCompleted.toggle()
        }
    }
}

struct TodoListView: View {
    @StateObject private var store = TodoStore()
    @State private var newItemTitle = ""

    var body: some View {
        VStack {
            List(store.items) { item in
                HStack {
                    Text(item.title)
                    Spacer()
                    Image(systemName: item.isCompleted ? "checkmark.circle.fill" : "circle")
                }
                .onTapGesture {
                    store.toggleItem(item)
                }
            }

            HStack {
                TextField("新任务", text: $newItemTitle)
                Button("添加") {
                    if !newItemTitle.isEmpty {
                        store.addItem(newItemTitle)
                        newItemTitle = ""
                    }
                }
            }
            .padding()
        }
    }
}

通过这个练习，你可以学习如何：

使用 @StateObject 管理应用状态
使用 @State 处理局部状态
实现数据的增加和更新
在视图中响应状态变化