SwiftUI 状态管理完全指南

1状态管理到底在管什么

声明式 UI 的核心公式：UI = f(state)。你改的是数据，SwiftUI 负责重算 body。新手最常卡在：该用哪个包装器、对象该谁创建、为啥一转屏数据没了。

2@State：视图私有状态

最基础的工具。适合开关、输入框草稿、计数器——只给当前 View 用的简单值。

struct CounterView: View {
  @State private var count = 0          // 建议 private
  @State private var name = ""

  var body: some View {
    VStack(spacing: 12) {
      Text("点了 \(count) 次")
      Button("加一") { count += 1 }

      TextField("名字", text: $name)   // $name → Binding<String>
      Text(name.isEmpty ? "还没填" : "你好，\(name)")
    }
    .padding()
  }
}

3@Binding：子视图改父状态

子视图不拥有数据，只拿着「通往父状态的引用」。父传 $xxx，子声明 @Binding。

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

  var body: some View {
    VStack {
      Text(isOn ? "开" : "关")
      ToggleRow(isOn: $isOn)   // 注意 $
    }
  }
}

struct ToggleRow: View {
  @Binding var isOn: Bool      // 引用，不是拷贝

  var body: some View {
    Toggle("开关", isOn: $isOn)
  }
}

4ObservableObject + @Published（iOS 13+ 传统写法）

业务一复杂（列表、登录、网络结果），别堆几十个 @State。做成可观察的类，视图订阅它。

import Combine

final class UserViewModel: ObservableObject {
  @Published var name = "张三"
  @Published var age = 25

  func updateName(_ newName: String) {
    name = newName   // @Published 会通知订阅者刷新
  }
}

5@StateObject vs @ObservedObject（必懂）

这是新手翻车第一名：对象被反复重建，列表清空、登录态丢失。

struct ContentView: View {
  @StateObject private var viewModel = UserViewModel()  // 拥有

  var body: some View {
    ChildView(viewModel: viewModel)  // 往下传同一实例
  }
}

struct ChildView: View {
  @ObservedObject var viewModel: UserViewModel          // 只观察

  var body: some View {
    Text(viewModel.name)
    Button("改名") { viewModel.updateName("李四") }
  }
}

6@EnvironmentObject：跨多层共享（传统）

登录态、主题配置等「整棵树都可能用到」的对象，不必一层层参数往下传。

final class AppState: ObservableObject {
  @Published var isLoggedIn = false
}

@main
struct MyApp: App {
  @StateObject private var appState = AppState()

  var body: some Scene {
    WindowGroup {
      ContentView()
        .environmentObject(appState)   // 注入
    }
  }
}

struct ProfileView: View {
  @EnvironmentObject var appState: AppState  // 任意深层读取

  var body: some View {
    Text(appState.isLoggedIn ? "已登录" : "未登录")
  }
}

7@Environment：系统环境值

读系统/框架提供的值（深浅色、尺寸类、dismiss 等），不是你自己的业务 Store。

struct ThemeAwareView: View {
  @Environment(\.colorScheme) private var colorScheme
  @Environment(\.dismiss) private var dismiss

  var body: some View {
    VStack {
      Text(colorScheme == .dark ? "深色模式" : "浅色模式")
      Button("关闭") { dismiss() }
    }
  }
}

8@AppStorage / @SceneStorage（轻量持久化）

简单开关、用户名这类小数据可直接绑 UserDefaults；场景级临时状态用 SceneStorage。大列表请看第 09 章文件存储。

struct SettingsView: View {
  @AppStorage("username") private var username = ""
  @AppStorage("isDarkMode") private var isDarkMode = false
  @SceneStorage("draft") private var draft = ""  // 随场景，偏临时

  var body: some View {
    Form {
      TextField("用户名", text: $username)
      Toggle("深色模式", isOn: $isDarkMode)
      TextField("草稿", text: $draft)
    }
  }
}

9@Observable（iOS 17+ 推荐）

现代写法：一个宏搞定可观察类，不用 ObservableObject / @Published。新项目优先学这个。

import SwiftUI

@Observable
final class UserModel {
  var name = "张三"
  var age = 25
  var email = "zhangsan@example.com"
}

struct ContentView: View {
  @State private var user = UserModel()   // 用 @State 持有（拥有）

  var body: some View {
    VStack {
      Text("姓名：\(user.name)")
      TextField("修改姓名", text: $user.name)
    }
  }
}

10@Bindable：给 @Observable 做双向绑定

父把 @Observable 实例传给子；子要用 $settings.xxx 时，给参数加 @Bindable。

@Observable
final class Settings {
  var isDarkMode = false
}

struct ParentView: View {
  @State private var settings = Settings()

  var body: some View {
    ChildView(settings: settings)
  }
}

struct ChildView: View {
  @Bindable var settings: Settings

  var body: some View {
    Toggle("深色模式", isOn: $settings.isDarkMode)
  }
}

11@Environment + @Observable（现代全局态）

@Observable
final class AppState {
  var isLoggedIn = false
  var userName = ""
}

@main
struct MyApp: App {
  @State private var appState = AppState()

  var body: some Scene {
    WindowGroup {
      ContentView()
        .environment(appState)   // 注意：不是 environmentObject
    }
  }
}

struct ProfileView: View {
  @Environment(AppState.self) private var appState

  var body: some View {
    Text(appState.isLoggedIn ? "欢迎 \(appState.userName)" : "未登录")
  }
}

12怎么选：一张表

13从 ObservableObject 迁到 @Observable

final class OldVM: ObservableObject {
  @Published var text = ""
}
struct OldView: View {
  @StateObject private var vm = OldVM()
  var body: some View {
    TextField("", text: $vm.text)
  }
}

@Observable
final class NewVM {
  var text = ""
}
struct NewView: View {
  @State private var vm = NewVM()
  var body: some View {
    TextField("", text: $vm.text)
  }
}

14迷你实战：待办 Store（现代写法）

把「状态 + 行为」收进一个模型；界面只负责渲染与调用。完整持久化版见第 12 章。

@Observable
final class TodoViewModel {
  struct Todo: Identifiable, Equatable {
    let id = UUID()
    var title: String
    var isCompleted = false
  }

  var todos: [Todo] = []
  var newTitle = ""

  func addTodo() {
    let t = newTitle.trimmingCharacters(in: .whitespacesAndNewlines)
    guard !t.isEmpty else { return }
    todos.insert(Todo(title: t), at: 0)
    newTitle = ""
  }

  func toggle(_ todo: Todo) {
    guard let i = todos.firstIndex(of: todo) else { return }
    todos[i].isCompleted.toggle()
  }
}

struct TodoListView: View {
  @State private var vm = TodoViewModel()

  var body: some View {
    NavigationStack {
      List {
        ForEach(vm.todos) { todo in
          HStack {
            Image(systemName: todo.isCompleted ? "checkmark.circle.fill" : "circle")
              .onTapGesture { vm.toggle(todo) }
            Text(todo.title)
              .strikethrough(todo.isCompleted)
          }
        }
      }
      .navigationTitle("待办")
      .toolbar {
        ToolbarItem(placement: .bottomBar) {
          HStack {
            TextField("新待办", text: $vm.newTitle)
            Button("添加") { vm.addTodo() }
          }
        }
      }
    }
  }
}

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