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import SwiftUI

struct ContentView: View {
    var body: some View {
        VStack(spacing: 12) {
            Text("Hello, SwiftUI")
                .font(.title)
            Text("声明式界面会让状态成为主角")
                .foregroundStyle(.secondary)
        }
        .padding()
    }
}

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

// 界面效果：窗口里居中一块两行文字，第一行大标题，第二行灰色副标题

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import SwiftUI

struct StatusBadge: View {
    let on: Bool

    var body: some View {
        HStack {
            Image(systemName: on ? "checkmark.circle" : "xmark.circle")
            Text(on ? "已开启" : "已关闭")
            if on {
                Text("(刚刚更新)").font(.caption)
            }
        }
    }
}

// 界面效果：一行图标 + 文字；on 为 true 时末尾多一小段灰色小字

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$ xcrun --sdk macosx swiftc -typecheck -swift-version 6 View.swift

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import SwiftUI

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

    var body: some View {
        VStack {
            Text("计数：\(count)")
            Button("加一") { count += 1 }
        }
    }
}

// 界面效果：初始显示"计数：0"；每点一次"加一"，数字加一

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import SwiftUI

struct BrokenCounter: View {
    private var count = 0        // ❌ 普通存储属性

    var body: some View {
        VStack {
            Text("计数：\(count)")
            Button("加一") {
                // count += 1  ← 这里连编译都过不去
            }
        }
    }
}

// 报错：cannot assign to property: 'self' is immutable
// （在视图的 body 里，self 是只读的，改不了自己的存储属性）

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初始显示：计数：0
点击按钮后：计数：1

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import SwiftUI

struct ToggleRow: View {
    @Binding var isOn: Bool

    var body: some View {
        Toggle("开启通知", isOn: $isOn)
    }
}

struct SettingsView: View {
    @State private var notifications = false

    var body: some View {
        Form {
            ToggleRow(isOn: $notifications)
            Text(notifications ? "通知已开启" : "通知已关闭")
                .foregroundStyle(.secondary)
        }
    }
}

// 界面效果：一行开关；拨动后下面那行灰字在"已开启/已关闭"之间切换

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import SwiftUI
import Observation

@Observable
final class CartModel {
    var items: [String] = []
    var total = 0

    func add(_ item: String) {
        items.append(item)
        total += 1
    }
}

struct CartView: View {
    @State private var cart = CartModel()

    var body: some View {
        VStack {
            Text("商品数：\(cart.total)")
            Button("添加") { cart.add("书") }
        }
    }
}

// 界面效果：初始"商品数：0"；每点一次"添加"数字加一

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import SwiftUI

final class LegacyCart: ObservableObject {
    @Published var total = 0
}

struct LegacyView: View {
    @StateObject private var cart = LegacyCart()

    var body: some View {
        Text("总数：\(cart.total)")
    }
}

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import SwiftUI

struct Item: Identifiable, Hashable {
    let id = UUID()
    let name: String
}

struct ItemListView: View {
    let items = [Item(name: "第一项"), Item(name: "第二项")]

    var body: some View {
        NavigationStack {
            List(items) { item in
                NavigationLink(item.name, value: item)
            }
            .navigationTitle("项目")
            .navigationDestination(for: Item.self) { item in
                Text("详情：\(item.name)")
            }
        }
    }
}

// 界面效果：一个标题为"项目"的列表，两行；点任意一行滑入详情页

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import SwiftUI

struct LoadView: View {
    @State private var text = "加载中"

    var body: some View {
        Text(text)
            .task {
                try? await Task.sleep(for: .milliseconds(50))
                text = "加载完成"
            }
    }
}

// 界面效果：进入页面先是"加载中"，约 0.05 秒后变成"加载完成"