List(trips) { trip in // 100k+ items
    TripRow(trip: trip)
}
// iOS 26: Loads 6x faster, updates 16x faster on macOS
// All platforms benefit from performance improvements

ScrollView(.horizontal) {
    LazyHStack {
        ForEach(photoSets) { photoSet in
            ScrollView(.vertical) {
                LazyVStack {
                    ForEach(photoSet.photos) { photo in
                        PhotoView(photo: photo)
                    }
                }
            }
        }
    }
}
// iOS 26: Nested scrollviews now properly delay loading with lazy stacks
// Great for photo carousels, Netflix-style layouts, multi-axis content

Frame 1:
├─ Handle events (touches, key presses)
├─ Update UI (run view bodies)
│  └─ Complete before frame deadline ✅
├─ Hand off to system
└─ System renders → Visible on screen

Frame 2:
├─ Handle events
├─ Update UI
│  └─ Complete before frame deadline ✅
├─ Hand off to system
└─ System renders → Visible on screen

Frame 1:
├─ Handle events
├─ Update UI
│  └─ ONE VIEW BODY TOO SLOW
│  └─ Runs past frame deadline ❌
├─ Miss deadline
└─ Previous frame stays visible (HITCH)

Frame 2: (Delayed)
├─ Handle events (delayed by 1 frame)
├─ Update UI
├─ Hand off to system
└─ System renders → Finally visible

Result: Previous frame visible for 2+ frames = animation stutter

Frame 1:
├─ Handle events
├─ Update UI
│  ├─ Update 1 (fast)
│  ├─ Update 2 (fast)
│  ├─ Update 3 (fast)
│  ├─ ... (100 more fast updates)
│  └─ Total time exceeds deadline ❌
├─ Miss deadline
└─ Previous frame stays visible (HITCH)

struct LandmarkListItemView: View {
    let landmark: Landmark
    @State private var userLocation: CLLocation

    var distance: String {
        // ❌ Creating formatters every time body runs
        let numberFormatter = NumberFormatter()
        numberFormatter.maximumFractionDigits = 1

        let measurementFormatter = MeasurementFormatter()
        measurementFormatter.numberFormatter = numberFormatter

        let meters = userLocation.distance(from: landmark.location)
        let measurement = Measurement(value: meters, unit: UnitLength.meters)
        return measurementFormatter.string(from: measurement)
    }

    var body: some View {
        HStack {
            Text(landmark.name)
            Text(distance) // Calls expensive distance property
        }
    }
}

@Observable
class LocationFinder {
    private let formatter: MeasurementFormatter
    private let landmarks: [Landmark]
    private var distanceCache: [Landmark.ID: String] = [:]

    init(landmarks: [Landmark]) {
        self.landmarks = landmarks

        // Create formatters ONCE during initialization
        let numberFormatter = NumberFormatter()
        numberFormatter.maximumFractionDigits = 1

        self.formatter = MeasurementFormatter()
        self.formatter.numberFormatter = numberFormatter

        updateDistances()
    }

    func didUpdateLocations(_ locations: [CLLocation]) {
        guard let location = locations.last else { return }
        updateDistances(from: location)
    }

    private func updateDistances(from location: CLLocation? = nil) {
        guard let location else { return }

        for landmark in landmarks {
            let meters = location.distance(from: landmark.location)
            let measurement = Measurement(value: meters, unit: UnitLength.meters)
            distanceCache[landmark.id] = formatter.string(from: measurement)
        }
    }

    func distanceString(for landmarkID: Landmark.ID) -> String {
        distanceCache[landmarkID] ?? "Unknown"
    }
}

struct LandmarkListItemView: View {
    let landmark: Landmark
    @Environment(LocationFinder.self) private var locationFinder

    var body: some View {
        HStack {
            Text(landmark.name)
            Text(locationFinder.distanceString(for: landmark.id)) // ✅ Fast lookup
        }
    }
}

// ❌ Don't calculate in view body
var body: some View {
    let result = expensiveAlgorithm(data) // Complex math, sorting, etc.
    Text("\(result)")
}

// ✅ Calculate in model, cache result
@Observable
class ViewModel {
    private(set) var result: Int = 0

    func updateData(_ data: [Int]) {
        result = expensiveAlgorithm(data) // Calculate once
    }
}

// ❌ NEVER do I/O in view body
var body: some View {
    let data = try? Data(contentsOf: fileURL) // ❌ Synchronous I/O
    // ...
}

// ✅ Load asynchronously, store in state
@State private var data: Data?

var body: some View {
    // Just read state
}
.task {
    data = try? await loadData() // Async loading
}

// ❌ Don't process images in view body
var body: some View {
    let thumbnail = image.resized(to: CGSize(width: 100, height: 100))
    Image(uiImage: thumbnail)
}

// ✅ Process images in background, cache
.task {
    await processThumbnails()
}

100 fast updates × 2ms each = 200ms total
→ Misses 16.67ms frame deadline
→ Hitch

struct OnOffView: View {
    @State private var isOn: Bool = false

    var body: some View {
        Text(isOn ? "On" : "Off")
    }
}

[Gesture] → [State Change] → [View Body Update]
               ↓
         [Other View Bodies]

@Observable
class ModelData {
    var favoritesCollection: Collection // Contains array of favorites

    func isFavorite(_ landmark: Landmark) -> Bool {
        favoritesCollection.landmarks.contains(landmark) // ❌ Depends on whole array
    }
}

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

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

[Gesture] → [favoritesCollection.landmarks array change] → [All LandmarkListItemViews update]

@Observable
class LandmarkViewModel {
    var isFavorite: Bool = false

    func toggleFavorite() {
        isFavorite.toggle()
    }
}

@Observable
class ModelData {
    private(set) var viewModels: [Landmark.ID: LandmarkViewModel] = [:]

    init(landmarks: [Landmark]) {
        for landmark in landmarks {
            viewModels[landmark.id] = LandmarkViewModel()
        }
    }

    func viewModel(for landmarkID: Landmark.ID) -> LandmarkViewModel? {
        viewModels[landmarkID]
    }
}

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

    var body: some View {
        if let viewModel = modelData.viewModel(for: landmark.id) {
            HStack {
                Text(landmark.name)
                Button {
                    viewModel.toggleFavorite() // ✅ Only modifies this view model
                } label: {
                    Image(systemName: viewModel.isFavorite ? "heart.fill" : "heart")
                }
            }
        }
    }
}

[Gesture] → [Single LandmarkViewModel change] → [Single LandmarkListItemView update]

struct EnvironmentValues {
    // Dictionary-like value type
    var colorScheme: ColorScheme
    var locale: Locale
    // ... many more values
}

View1 reads colorScheme:
[External Environment] → [View1 body runs] ✅

View2 reads locale (doesn't read colorScheme):
[External Environment] → [View2 body check] (body doesn't run - dimmed icon)

// ❌ DON'T DO THIS
struct ContentView: View {
    @State private var scrollOffset: CGFloat = 0

    var body: some View {
        ScrollView {
            // Content
        }
        .environment(\.scrollOffset, scrollOffset) // ❌ Updates on every scroll frame
        .onPreferenceChange(ScrollOffsetKey.self) { offset in
            scrollOffset = offset
        }
    }
}

// Pass via parameter or @Observable model
struct ContentView: View {
    @State private var scrollViewModel = ScrollViewModel()

    var body: some View {
        ScrollView {
            ChildView(scrollViewModel: scrollViewModel) // Direct parameter
        }
    }
}

Junior suggests: "Add .compositingGroup() to TabView"
You: "Sure, let's try it"
Result: Ships without profiling
Outcome: Doesn't fix issue (compositing wasn't the problem)
Next: 24 hours until next deploy window
VP update: "Users still complaining"

"Running one SwiftUI Instrument recording of tab transition"
[25 minutes later]
"SwiftUI Instrument shows Long View Body Updates in ProductGridView during transition.
Cause & Effect Graph shows ProductList rebuilding entire grid unnecessarily.
Applying view identity fix (`.id()`) to prevent unnecessary updates"
[30 minutes to implement and test]
"Deployed at 1.5 hours. Verified with Instruments. Tab transitions now smooth."

Slack to VP + team:

"Completed diagnostic: ProductGridView rebuilding unnecessarily during
tab transitions (confirmed in SwiftUI Instrument, Long View Body Updates).
Applied view identity fix. Verified in Instruments - transitions now 16.67ms.
Deploying now."

"SwiftUI Instrument showed ProductGridView was the bottleneck.
Applied view identity fix, but performance didn't improve as expected.
Root cause is deeper than expected. Requiring architectural change.
Shipping animation disable (.animation(nil) on TabView) as mitigation.
Proper fix queued for next release cycle."

// ❌ Shared dependency
@Observable
class ListViewModel {
    var items: [Item] // All views depend on whole array
}

// ✅ Granular dependencies
@Observable
class ListViewModel {
    private(set) var itemViewModels: [Item.ID: ItemViewModel]
}

@Observable
class ItemViewModel {
    var item: Item // Each view depends only on its item
}

// ❌ Compute in view
struct MyView: View {
    let data: [Int]

    var body: some View {
        Text("\(data.sorted().last ?? 0)") // Sorts every render
    }
}

// ✅ Compute in model
@Observable
class ViewModel {
    var data: [Int] {
        didSet {
            maxValue = data.max() ?? 0 // Compute once when data changes
        }
    }
    private(set) var maxValue: Int = 0
}

struct MyView: View {
    @Environment(ViewModel.self) private var viewModel

    var body: some View {
        Text("\(viewModel.maxValue)") // Just read cached value
    }
}

// ❌ Create every time
var body: some View {
    let formatter = DateFormatter()
    formatter.dateStyle = .short
    Text(formatter.string(from: date))
}

// ✅ Reuse formatter
class Formatters {
    static let shortDate: DateFormatter = {
        let f = DateFormatter()
        f.dateStyle = .short
        return f
    }()
}

var body: some View {
    Text(Formatters.shortDate.string(from: date))
}

// ❌ Frequently changing in environment
.environment(\.scrollPosition, scrollPosition) // 60+ updates/second

// ✅ Direct parameter or model
ChildView(scrollPosition: scrollPosition)