Day 5 · 2부 · @Environment와 마이그레이션

import SwiftUI

@Observable final class Library {
    var books: [String] = ["A", "B"]
    var availableCount: Int { books.count }
}

@main
struct BookReaderApp: App {
    @State private var library = Library()       // 진실은 여기 하나

    var body: some Scene {
        WindowGroup {
            RootView().environment(library)       // 타입 자체가 키
        }
    }
}

struct RootView: View {
    var body: some View { DetailBadge() }         // library를 모른다
}

struct DetailBadge: View {
    @Environment(Library.self) private var library

    var body: some View {
        Text("\(library.availableCount)권")
    }
}

import SwiftUI

private struct AccentThemeKey: EnvironmentKey {
    static let defaultValue: Color = .blue          // 넣지 않으면 이 값
}

extension EnvironmentValues {
    var accentTheme: Color {
        get { self[AccentThemeKey.self] }
        set { self[AccentThemeKey.self] = newValue }
    }
}

struct PriceTag: View {
    @Environment(\.accentTheme) private var accent   // 키패스로 읽는다

    var body: some View {
        Text("₩12,000").foregroundStyle(accent)
    }
}

struct CheckoutView: View {
    var body: some View {
        PriceTag().environment(\.accentTheme, .orange) // 하위 트리만 덮어쓴다
    }
}

import SwiftUI

final class Library: ObservableObject {
    @Published var books: [String] = []
    @Published var searchText = ""
}

struct LibraryScreen: View {
    @StateObject private var library = Library()

    var body: some View {
        VStack {
            TextField("검색", text: $library.searchText)
            BookCountLabel()
        }
        .environmentObject(library)
    }
}

struct BookCountLabel: View {
    @EnvironmentObject var library: Library

    var body: some View {
        Text("\(library.books.count)권")
        // searchText를 읽지 않는데도 타이핑마다 갱신된다
    }
}

import SwiftUI

@Observable final class Library {
    var books: [String] = []
    var searchText = ""
    @ObservationIgnored var cacheDir: URL?      // UI와 무관 → 추적 제외
}

struct LibraryScreen: View {
    @State private var library = Library()       // StateObject → State

    var body: some View {
        @Bindable var library = library          // $searchText용
        VStack {
            TextField("검색", text: $library.searchText)
            BookCountLabel()
        }
        .environment(library)                    // environmentObject → environment
    }
}

struct BookCountLabel: View {
    @Environment(Library.self) private var library

    var body: some View {
        Text("\(library.books.count)권")          // books만 추적
    }
}

import SwiftUI

@Observable final class Library {              // : ObservableObject를 먼저 지웠다
    var books: [String] = []
}

struct LibraryScreen: View {
    @StateObject private var library = Library()
    // error: generic struct 'StateObject' requires that 'Library' conform to 'ObservableObject'
    var body: some View { Text("\(library.books.count)권") }
}

import SwiftUI

@Observable final class Library: ObservableObject {   // 1단계: 채택은 아직 남긴다
    var books: [String] = []                           // @Published는 지웠다
}

struct OldScreen: View {
    @StateObject private var library = Library()       // 아직 안 바꾼 뷰 — 컴파일된다
    var body: some View {
        NewBadge().environmentObject(library)
    }
}

struct NewBadge: View {
    @EnvironmentObject var library: Library            // 이것도 컴파일된다
    var body: some View { Text("\(library.books.count)권") }
}

struct NewScreen: View {
    @State private var library = Library()             // 2단계: 바꾼 뷰
    var body: some View { Text("\(library.books.count)권") }
}

import SwiftUI

final class Counter: ObservableObject {
    @Published var count = 0
    @Published var step = 1
}

struct CounterScreen: View {
    @StateObject private var counter = Counter()

    var body: some View {
        VStack(spacing: 16) {
            CountText(counter: counter)
            Stepper("간격 \(counter.step)", value: $counter.step, in: 1...10)
            Button("더하기") { counter.count += counter.step }
        }
        .padding()
    }
}

struct CountText: View {
    @ObservedObject var counter: Counter
    var body: some View {
        let _ = print("CountText 갱신")
        Text("\(counter.count)").font(.largeTitle)
    }
}

#Preview { CounterScreen() }