Day 9 · 2부 · 성능 진단 루프

import SwiftUI

struct CounterView: View {
    @State private var count = 0

    var body: some View {
        let _ = Self._printChanges()   // 예: "CounterView: _count changed."
        Button("탭 \(count)") { count += 1 }
    }
}

import SwiftUI

struct Dog {
    var name: String
    var favoriteTreat: String
    var image: Image
}

struct ScalableDogImage: View {
    let dog: Dog                          // 필요한 건 image 하나뿐
    @State private var scaleToFill = false

    var body: some View {
        dog.image
            .resizable()
            .aspectRatio(contentMode: scaleToFill ? .fill : .fit)
            .onTapGesture { scaleToFill.toggle() }
    }
}

import SwiftUI

struct ScalableDogImage: View {
    let image: Image
    @State private var scaleToFill = false

    var body: some View {
        image
            .resizable()
            .aspectRatio(contentMode: scaleToFill ? .fill : .fit)
            .onTapGesture { scaleToFill.toggle() }
    }
}

// 부모: ScalableDogImage(image: dog.image)

import SwiftUI

struct Landmark: Identifiable, Hashable {
    let id: Int
    var name: String
}

@Observable final class ModelData {
    var favorites: [Landmark] = []

    func isFavorite(_ landmark: Landmark) -> Bool {
        favorites.contains(landmark)          // 배열 전체 읽기 = 배열 전체에 의존
    }
    func toggleFavorite(_ landmark: Landmark) {
        if let i = favorites.firstIndex(of: landmark) { favorites.remove(at: i) }
        else { favorites.append(landmark) }
    }
}

struct LandmarkRow: View {
    @Environment(ModelData.self) private var model
    let landmark: Landmark

    var body: some View {
        HStack {
            Text(landmark.name)
            Spacer()
            Button {
                model.toggleFavorite(landmark)
            } label: {
                Image(systemName: model.isFavorite(landmark) ? "heart.fill" : "heart")
            }
        }
    }
}

import SwiftUI

struct Landmark: Identifiable, Hashable {
    let id: Int
    var name: String
}

@Observable final class LandmarkRowModel: Identifiable {
    let landmark: Landmark
    var isFavorite = false
    var id: Int { landmark.id }
    init(landmark: Landmark) { self.landmark = landmark }
}

@Observable final class ModelData {
    let rows: [LandmarkRowModel]
    var favorites: [Landmark] { rows.filter(\.isFavorite).map(\.landmark) }

    init(landmarks: [Landmark]) {
        rows = landmarks.map(LandmarkRowModel.init)
    }
}

struct LandmarkRow: View {
    let row: LandmarkRowModel               // 자기 모델만 읽는다

    var body: some View {
        HStack {
            Text(row.landmark.name)
            Spacer()
            Button {
                row.isFavorite.toggle()
            } label: {
                Image(systemName: row.isFavorite ? "heart.fill" : "heart")
            }
        }
    }
}

struct LandmarkList: View {
    let model: ModelData
    var body: some View {
        List(model.rows) { row in LandmarkRow(row: row) }
    }
}

import SwiftUI

struct DistanceRow: View {
    let name: String
    let meters: Double

    private var distance: String {
        let number = NumberFormatter()
        number.maximumFractionDigits = 1
        let formatter = MeasurementFormatter()
        formatter.numberFormatter = number
        return formatter.string(from: Measurement(value: meters, unit: UnitLength.meters))
    }

    var body: some View {
        Text("\(name) · \(distance)")      // 스크롤 중 수십 행 × 매번 생성
    }
}

import SwiftUI

@Observable final class DistanceCache {
    @ObservationIgnored private let formatter: MeasurementFormatter = {
        let number = NumberFormatter()
        number.maximumFractionDigits = 1
        let formatter = MeasurementFormatter()
        formatter.numberFormatter = number
        return formatter
    }()

    private(set) var texts: [Int: String] = [:]    // landmark id → "1.2 km"

    func update(meters: [Int: Double]) {            // 위치가 바뀔 때만 호출
        texts = meters.mapValues {
            formatter.string(from: Measurement(value: $0, unit: UnitLength.meters))
        }
    }
}

struct DistanceRow: View {
    let id: Int
    let name: String
    let cache: DistanceCache

    var body: some View {
        Text("\(name) · \(cache.texts[id] ?? "—")")   // 읽기만
    }
}

import SwiftUI

struct Card: Identifiable {
    let id: Int
    var isHidden: Bool { id % 3 == 0 }
}

struct CardShelf: View {
    let cards = (0..<30).map(Card.init)

    var body: some View {
        ScrollView(.horizontal) {
            LazyHStack {
                ForEach(cards) { card in
                    if !card.isHidden {
                        RoundedRectangle(cornerRadius: 12).fill(.blue)
                            .frame(width: 260, height: 160)
                            .overlay(Text("\(card.id)").foregroundStyle(.white))
                    }
                }
            }
        }
        .padding(.horizontal, 20)
    }
}

#Preview { CardShelf() }