.task { } — とは何か、async修飾子とViewでのデータ読み込み

struct ArticlesView: View {
    @State var articles: [Article] = []
    @State var error: Error?
    
    var body: some View {
        List(articles) { article in
            Text(article.title)
        }
        .task {
            do {
                articles = await APIClient().fetchArticles()
            } catch {
                self.error = error
            }
        }
    }
}

// ❌ レガシーアプローチ: .onAppearでのキャンセルなしのTask
var loadTask: Task<Void, Never>?
func body() { var body: some View { Text("") }
    .onAppear {
        loadTask = Task { await loadData() }
    }
    .onDisappear { loadTask?.cancel() }

// ✅ モダンなアプローチ: .taskがキャンセルを管理
func body() { var body: some View { Text("") }
    .task { await loadData() }

struct CategoryView: View {
    let categoryId: Int
    @State var items: [Item] = []
    
    var body: some View {
        List(items) { item in
            Text(item.name)
        }
        .task(id: categoryId) {
            await loadItems(for: categoryId)
        }
    }
    
    func loadItems(for id: Int) async {
        do {
            items = await APIClient().fetchItems(categoryId: id)
        } catch {
            // エラーを処理
        }
    }
}

struct LoadingView: View {
    @State var progress: Double = 0
    
    var body: some View {
        ProgressView(value: progress)
            .task {
                for i in 0..<100 {
                    // キャンセルを確認
                    try Task.checkCancellation()
                    
                    await Task.sleep(nanoseconds: 50_000_000)
                    progress = Double(i + 1) / 100.0
                }
            }
    }
}

struct ProfileView: View {
    @State var profile: Profile?
    @State var isLoading = true
    
    var body: some View {
        Group {
            if isLoading {
                ProgressView()
            } else if let profile {
                Text(profile.name)
            } else {
                Text("読み込みに失敗")
            }
        }
        .task {
            defer { isLoading = false }
            do {
                profile = await APIClient().fetchProfile()
            } catch {
                // プロフィールがnilのまま
            }
        }
    }
}

struct DashboardView: View {
    @State var stats: DashboardStats?
    
    var body: some View {
        Text("ダッシュボード")
            .task {
                stats = await Task {
                    await withThrowingTaskGroup { group in
                        group.addTask { await API().fetchUsers() }
                        group.addTask { await API().fetchOrders() }
                        group.addTask { await API().fetchRevenue() }
                        
                        return DashboardStats(
                            users: try await group.next(),
                            orders: try await group.next(),
                            revenue: try await group.next()
                        )
                    }
                }.value
            }
    }
}

// ❌ エラー: エラーハンドリングなし
.task {
    let data = await fetchData() // エラー発生時にクラッシュ!
    items = data
}

// ✅ 正しい: do/catch
.task {
    do {
        items = await fetchData()
    } catch {
        errorMessage = error.localizedDescription
    }
}