Smth 客户端开发：从 MVVM 架构到多平台适配的 SwiftUI 实践

@MainActor
final class TopicListViewModel: ObservableObject {
    @Published private(set) var topics: [Topic] = []
    @Published private(set) var isLoadingPage = false
    @Published private(set) var isRefreshing = false
    @Published private(set) var errorMessage: String?

    private let repository: TopicRepositoryProtocol
    private var paginationState = PaginationState<Topic>()

    init(repository: TopicRepositoryProtocol = AppContainer.shared.resolve(TopicRepositoryProtocol.self)) {
        self.repository = repository
    }

    func loadInitialIfNeeded() async {
        if topics.isEmpty {
            await loadInitialPage()
        }
    }

    func loadNextPageIfNeeded(currentItem item: Topic?) {
        guard let item else { return }
        let thresholdIndex = topics.index(topics.endIndex, offsetBy: -5, limitedBy: topics.startIndex) ?? topics.startIndex
        if topics.firstIndex(where: { $0.id == item.id }) == thresholdIndex {
            Task { await loadNextPage() }
        }
    }

    private func loadPage() async {
        guard let nextPage = paginationState.startLoadingNextPage() else { return }
        do {
            let newItems = try await repository.fetchTopics(in: boardID, page: nextPage, pageSize: pageSize)
            paginationState.completeLoading(with: newItems, pageSize: pageSize)
            topics = paginationState.items
        } catch {
            errorMessage = error.localizedDescription
        }
    }
}

struct TopicRepository: TopicRepositoryProtocol {
    private let apiService: APIService

    func fetchTopics(in boardID: String, page: Int, pageSize: Int) async throws -> [Topic] {
        let endpoint = APIEndpoint.topicList(boardID: boardID, page: page, pageSize: pageSize).toEndpoint()
        let response: TopicResponse = try await apiService.request(endpoint)
        return response.data.topics
    }
}

final class AppContainer: DependencyContainer {
    static let shared = AppContainer()

    private lazy var apiService: APIService = DefaultAPIService()
    private lazy var topicRepository: TopicRepositoryProtocol = TopicRepository(apiService: apiService)

    func resolve<T>(_ type: T.Type) -> T {
        if type == TopicRepositoryProtocol.self {
            return topicRepository as! T
        }
        // ... 其他依赖
    }
}

struct PaginationState<Item: Identifiable & Hashable> {
    private(set) var items: [Item] = []
    private(set) var currentPage: Int = 0
    private(set) var isLoadingPage = false
    private(set) var canLoadMorePages = true

    mutating func startLoadingNextPage() -> Int? {
        guard !isLoadingPage, canLoadMorePages else { return nil }
        isLoadingPage = true
        currentPage += 1
        return currentPage
    }

    mutating func completeLoading(with newItems: [Item], pageSize: Int) {
        items.append(contentsOf: newItems)
        isLoadingPage = false
        canLoadMorePages = !newItems.isEmpty
    }
}

var body: some View {
    ScrollView {
        LazyVStack(spacing: AppTheme.compactSpacing) {
            ForEach(viewModel.topics) { topic in
                NavigationLink(value: topic) {
                    TopicRowView(topic: topic)
                }
                .onAppear {
                    viewModel.loadNextPageIfNeeded(currentItem: topic)
                }
            }
        }
    }
}

struct HomeView: View {
    @EnvironmentObject private var browsingHistory: BrowsingHistoryStore
    @Environment(\.colorScheme) private var colorScheme
    @StateObject private var viewModel = NaviTopicListViewModel()
    @State private var selectedIndex: Int = 0
}

extension View {
    func smthScaffoldBackground() -> some View {
        modifier(ScaffoldBackgroundModifier())
    }

    func smthSurfaceBackground(subdued: Bool = false) -> some View {
        modifier(SurfaceBackgroundModifier(subdued: subdued))
    }
}

.onAppear {
    Task {
        await navigationViewModel.loadNavigationsIfNeeded()
    }
}

var body: some View {
    #if os(iOS)
    ImageViewerUIKit(images: images, initialIndex: initialIndex, isPresented: $isPresented)
    #else
    ImageViewerSwiftUI(images: images, initialIndex: initialIndex, isPresented: $isPresented)
    #endif
}

struct TopicRowView: View {
    let topic: Topic
    let isVisited: Bool

    var body: some View {
        VStack(alignment: .leading, spacing: 5) {
            Text(topic.subject)
                .font(.headline)
            // ... 其他 UI 元素
        }
        .background(AppTheme.surfaceBackground(for: colorScheme))
    }
}

var body: some View {
    Group {
        #if os(macOS)
        macSidebarLayout
        #else
        if horizontalSizeClass == .compact {
            tabLayout
        } else {
            sidebarLayout
        }
        #endif
    }
}

private var tabLayout: some View {
    TabView(selection: $selection) {
        NavigationStack {
            HomeView()
        }
        .tabItem { Label("首页", systemImage: "house") }
        // ... 其他 Tab
    }
}

#if os(macOS)
private var macSidebarLayout: some View {
    NavigationSplitView(columnVisibility: $columnVisibility) {
        macSidebar.frame(minWidth: 240, idealWidth: 280)
    } content: {
        macContentStack.frame(minWidth: 300, idealWidth: 380)
    } detail: {
        macDetailPlaceholder.frame(minWidth: 500, idealWidth: 600)
    }
}
#endif

#if os(iOS)
private struct ImageViewerUIKit: UIViewControllerRepresentable {
    func makeUIViewController(context: Context) -> ImagePageViewController {
        ImagePageViewController(images: images, initialIndex: initialIndex)
    }
}
#endif

#if !os(iOS)
private struct ImageViewerSwiftUI: View {
    @State private var currentIndex: Int
    @State private var scale: CGFloat = 1.0

    var body: some View {
        ScrollView([.horizontal, .vertical]) {
            CachedAsyncImage(url: URL(string: images[currentIndex]))
                .scaleEffect(scale)
        }
        .gesture(MagnificationGesture())
    }
}
#endif

#if os(iOS)
.sheet(isPresented: $showProfileView) {
    ProfileView()
        .presentationDetents([.large])
}
#endif

#elseif os(macOS)
.sheet(isPresented: $showProfileView) {
    ProfileView()
        .frame(minWidth: 600, minHeight: 500)
}
#endif

final class TopicListViewModelTests: XCTestCase {
    func testInitialLoadFetchesTopics() async throws {
        let repository = StubTopicRepository(
            hotTopics: { page, size in Self.mockTopics(page: page, pageSize: size) }
        )
        let viewModel = TopicListViewModel(repository: repository)

        await viewModel.loadInitialIfNeeded()

        XCTAssertEqual(viewModel.topics.count, 20)
        XCTAssertFalse(viewModel.isLoadingPage)
    }
}