Day 3 · 2부 · @Binding과 파생값

struct EditorConfig {
    var isEditorPresented = false
    var note = ""
    var progress = 0.0

    mutating func present(initialProgress: Double) {
        progress = initialProgress
        note = ""
        isEditorPresented = true
    }
}

struct ProgressEditor: View {
    @State var config: EditorConfig       // 부모 것과 별개의 새 진실

    var body: some View {
        TextEditor(text: $config.note)    // 자기 복사본만 바뀐다
    }
}

struct ProgressEditor: View {
    @Binding var config: EditorConfig     // 소유하지 않는다

    var body: some View {
        Form {
            TextEditor(text: $config.note)            // Binding에서 Binding을 꺼낸다
            Slider(value: $config.progress, in: 0...1)
        }
    }
}

struct BookView: View {
    @State private var editorConfig = EditorConfig()   // 진실은 여기

    var body: some View {
        Button("진행률 업데이트") {
            editorConfig.present(initialProgress: 0.3)
        }
        .sheet(isPresented: $editorConfig.isEditorPresented) {
            ProgressEditor(config: $editorConfig)
        }
    }
}

struct PlayButton: View {
    @Binding var isPlaying: Bool
    var body: some View {
        Button(isPlaying ? "Pause" : "Play") { isPlaying.toggle() }
    }
}

#Preview("재생 중") { PlayButton(isPlaying: .constant(true)) }
#Preview("정지") { PlayButton(isPlaying: .constant(false)) }

private struct PlayButtonPreviewHost: View {
    @State private var isPlaying = false
    var body: some View {
        VStack {
            PlayButton(isPlaying: $isPlaying)
            Text(isPlaying ? "재생 중" : "정지")
        }
    }
}

#Preview("동작 확인") { PlayButtonPreviewHost() }

struct TagFilterBad: View {
    @State private var selected: Set<String> = []
    @State private var swiftOn = false        // selected와 중복된 진실
    @State private var uikitOn = false        // 〃

    var body: some View {
        Form {
            Toggle("Swift", isOn: $swiftOn)
            Toggle("UIKit", isOn: $uikitOn)
        }
        .onChange(of: swiftOn) {              // glue logic이 돌아왔다
            if swiftOn { selected.insert("Swift") } else { selected.remove("Swift") }
        }
    }
}

struct TagFilter: View {
    let allTags = ["Swift", "UIKit", "SwiftUI"]
    @State private var selected: Set<String> = []

    var body: some View {
        Form {
            ForEach(allTags, id: \.self) { tag in
                Toggle(tag, isOn: binding(for: tag))
            }
            Text("선택: \(selected.sorted().joined(separator: ", "))")
        }
    }

    private func binding(for tag: String) -> Binding<Bool> {
        Binding(
            get: { selected.contains(tag) },               // 읽기: Set에서 계산
            set: { isOn in                                 // 쓰기: Set을 고친다
                if isOn { selected.insert(tag) } else { selected.remove(tag) }
            }
        )
    }
}

struct Todo: Identifiable {
    let id = UUID()
    var title: String
    var isDone = false
}

struct TodoListBad: View {
    @State private var items: [Todo] = [Todo(title: "우유"), Todo(title: "빵")]
    @State private var query = ""
    @State private var filtered: [Todo] = []      // 파생값을 저장 → 진실 2개

    var body: some View {
        List(filtered) { Text($0.title) }
            .searchable(text: $query)
            .onChange(of: query, initial: true) { refilter() }
            .onChange(of: items.count) { refilter() }   // 제목 편집은 놓친다
    }

    private func refilter() {
        filtered = query.isEmpty ? items : items.filter { $0.title.contains(query) }
    }
}

struct TodoList: View {
    @State private var items: [Todo] = [Todo(title: "우유"), Todo(title: "빵")]
    @State private var query = ""

    private var visible: [Todo] {                 // 저장하지 않는다
        query.isEmpty ? items : items.filter { $0.title.contains(query) }
    }
    private var doneCount: Int { items.filter(\.isDone).count }

    var body: some View {
        List(visible) { Text($0.title) }
            .searchable(text: $query)
            .navigationTitle("완료 \(doneCount)/\(items.count)")
    }
}

struct TemperatureConverter: View {
    @State private var celsius = 20.0
    @State private var fahrenheit = 68.0      // celsius에서 계산할 수 있는 값

    var body: some View {
        Form {
            Slider(value: $celsius, in: -30...50) { Text("섭씨") }
            Text("섭씨 \(celsius, specifier: "%.1f")°")
            Slider(value: $fahrenheit, in: -22...122) { Text("화씨") }
            Text("화씨 \(fahrenheit, specifier: "%.1f")°")
        }
    }
}

#Preview { TemperatureConverter() }