Day 12 · 2부 · @MainActor·Swift 6와 이벤트 대응

actor ThumbnailRenderer {
    func render(_ data: Data) -> Data {
        // 무거운 CPU 작업 — 이 actor의 실행 컨텍스트에서 돈다 (main actor 아님)
        Data(data.reversed())
    }
}

@MainActor
@Observable
final class GalleryModel {
    private(set) var thumbnail: Data?
    private let renderer = ThumbnailRenderer()

    func makeThumbnail(from data: Data) async {
        let result = await renderer.render(data)   // main actor는 기다리는 동안 양보
        thumbnail = result                          // 재개는 main actor — 바로 대입
    }
}

struct PulseText: View {
    @State private var pulse = false

    var body: some View {
        Text("Extracting…")
            .visualEffect { content, _ in
                content.blur(radius: pulse ? 2 : 0)   // self.pulse 접근
            }
            .onTapGesture { pulse.toggle() }
    }
}

struct PulseTextFixed: View {
    @State private var pulse = false

    var body: some View {
        Text("Extracting…")
            .visualEffect { [pulse] content, _ in
                content.blur(radius: pulse ? 2 : 0)   // 복사본만 읽는다
            }
            .onTapGesture { pulse.toggle() }
    }
}

@MainActor
@Observable
final class FeedModelWrong {
    var items: [String] = []
    var page = 0

    func loadNextPage() async {
        let next = page + 1
        let newItems = await FeedAPI.fetch(page: next)   // 여기서 양보 → 두 번째 호출이 끼어든다
        items += newItems
        page = next
    }
}

@MainActor
@Observable
final class FeedModel {
    var items: [String] = []
    private var page = 0
    private var isLoadingPage = false

    func loadNextPage() async {
        guard !isLoadingPage else { return }   // 동기 구간: 끼어들 틈이 없다
        isLoadingPage = true
        defer { isLoadingPage = false }

        let next = page + 1
        let newItems = await FeedAPI.fetch(page: next)
        items += newItems
        page = next
    }

    func reload() async {
        guard !isLoadingPage else { return }   // 첫 로딩 중 당기면 무시 → 인디케이터가 바로 사라진다
        items = []
        page = 0
        await loadNextPage()
    }
}

enum FeedAPI {
    static func fetch(page: Int) async -> [String] {
        (1...10).map { "Page \(page) · #\($0)" }
    }
}

struct FeedScreen: View {
    @State private var model = FeedModel()

    var body: some View {
        List(model.items, id: \.self) { Text($0) }
            .task { await model.loadNextPage() }
            .refreshable { await model.reload() }   // 끝날 때까지 인디케이터 유지
    }
}

struct TimerScreen: View {
    @Environment(\.scenePhase) private var scenePhase
    @State private var isRunning = true

    var body: some View {
        Text(isRunning ? "타이머 동작 중" : "일시정지")
            .onChange(of: scenePhase) { _, phase in
                isRunning = (phase == .active)
            }
    }
}

@main
struct StudyApp: App {
    @Environment(\.scenePhase) private var scenePhase

    var body: some Scene {
        WindowGroup {
            Text("Home")
        }
        .onChange(of: scenePhase) { _, phase in
            if phase == .background {
                // 곧 종료될 수 있다 — 저장·리소스 정리
            }
        }
    }
}

enum Category: String, CaseIterable, Identifiable {
    case life = "인생", work = "일", study = "공부"
    var id: Self { self }
}

enum QuoteAPI {
    static func fetch(_ category: Category) async throws -> [String] {
        try await Task.sleep(for: .seconds(1))
        if Bool.random() && category == .work { throw URLError(.timedOut) }
        return (1...5).map { "\(category.rawValue) 명언 #\($0)" }
    }
}

struct QuoteScreen: View {
    @State private var category: Category = .life
    @State private var quotes: [String] = []

    var body: some View {
        NavigationStack {
            List(quotes, id: \.self) { Text($0) }
                .toolbar {
                    Picker("카테고리", selection: $category) {
                        ForEach(Category.allCases) { Text($0.rawValue) }
                    }
                    .pickerStyle(.segmented)
                }
        }
    }
}

#Preview { QuoteScreen() }