Combine vs Async/Await 比較

// Combine - ライブ検索によるフォームバリデーション
import Combine
import Foundation

class SearchViewModel: ObservableObject {
    @Published var searchText = ""
    @Published var results: [String] = []
    @Published var isLoading = false
    @Published var errorMessage: String?

    private var cancellables = Set<AnyCancellable>()
    private let searchService: SearchService

    init(searchService: SearchService) {
        self.searchService = searchService
        setupSearch()
    }

    private func setupSearch() {
        $searchText
            .debounce(for: .milliseconds(300), scheduler: RunLoop.main)
            .removeDuplicates()
            .filter { $0.count >= 2 }
            .handleEvents(receiveOutput: { [weak self] _ in
                self?.isLoading = true
                self?.errorMessage = nil
            })
            .flatMap { [weak self] query -> AnyPublisher<[String], Never> in
                guard let self else { return Just([]).eraseToAnyPublisher() }
                return self.searchService.search(query: query)
                    .catch { [weak self] error -> Just<[String]> in
                        self?.errorMessage = error.localizedDescription
                        return Just([])
                    }
                    .eraseToAnyPublisher()
            }
            .receive(on: DispatchQueue.main)
            .sink { [weak self] results in
                self?.isLoading = false
                self?.results = results
            }
            .store(in: &cancellables)
    }
}

// Async/Await - 並列APIコールとエラー処理
import Foundation

// Actorによるスレッドセーフなキャッシュ
actor NetworkCache {
    private var cache: [URL: Data] = [:]

    func get(_ url: URL) -> Data? { cache[url] }
    func set(_ url: URL, data: Data) { cache[url] = data }
}

struct UserDashboard {
    let user: User
    let posts: [Post]
    let notifications: [Notification]
}

@MainActor
class DashboardViewModel: ObservableObject {
    @Published var dashboard: UserDashboard?
    @Published var isLoading = false
    @Published var error: Error?

    private let cache = NetworkCache()

    func loadDashboard(userId: String) async {
        isLoading = true
        error = nil

        do {
            // async letによる並列読み込み
            async let user = fetchUser(id: userId)
            async let posts = fetchPosts(userId: userId)
            async let notifications = fetchNotifications(userId: userId)

            // すべて同時に開始し、すべての完了を待つ
            dashboard = UserDashboard(
                user: try await user,
                posts: try await posts,
                notifications: try await notifications
            )
        } catch {
            self.error = error
        }

        isLoading = false
    }

    private func fetchUser(id: String) async throws -> User {
        let url = URL(string: "https://api.example.com/users/\(id)")!
        let (data, _) = try await URLSession.shared.data(from: url)
        return try JSONDecoder().decode(User.self, from: data)
    }

    private func fetchPosts(userId: String) async throws -> [Post] {
        let url = URL(string: "https://api.example.com/users/\(userId)/posts")!
        let (data, _) = try await URLSession.shared.data(from: url)
        return try JSONDecoder().decode([Post].self, from: data)
    }

    private func fetchNotifications(userId: String) async throws -> [Notification] {
        let url = URL(string: "https://api.example.com/users/\(userId)/notifications")!
        let (data, _) = try await URLSession.shared.data(from: url)
        return try JSONDecoder().decode([Notification].self, from: data)
    }
}