SwiftUI Environment & Preferences

How it works — down

• EnvironmentValues — value-typed bag keyed by EnvironmentKey types. Read @Environment(\.colorScheme); set .environment(\.key, v); augment the inherited value with .transformEnvironment.
• Custom key: type with static let defaultValue + computed property on EnvironmentValues over self[Key.self]. @Entry (Xcode 16) writes both. A keyed read never crashes — it falls back to defaultValue.
• Objects: ObservableObject → .environmentObject(o) + @EnvironmentObject (any @Published invalidates every reader). @Observable (iOS 17) → .environment(o) + @Environment(Session.self): per-property tracking. Missing object = runtime crash; type it Session? to make it optional.
• Re-render: only views whose body reads the changed value — read churny values low.
• DI: @Entry var api: APIClient = .live; previews/tests inject .mock. Ambient concerns only; the item a view shows stays in init.

How it works — up

• PreferenceKey: defaultValue + reduce(value: inout V, nextValue: () -> V). Emit .preference(key:value:); read .onPreferenceChange (Equatable), or build UI with overlayPreferenceValue / backgroundPreferenceValue.
• reduce folds siblings in tree order; non-emitting views may contribute defaultValue, so it must be the identity of reduce (0 for max, [] for append). Only descendants below the reader are aggregated.
• Uses: equal-width labels, child size (GeometryReader in .background — no layout effect), scroll offset, custom tab bars, tooltips, connector lines.

Example — custom key + child-size preference

extension EnvironmentValues { @Entry var theme: Theme = .light }
struct MaxWidth: PreferenceKey {
  static let defaultValue: CGFloat = 0     // identity of max
  static func reduce(value: inout CGFloat,
                     nextValue: () -> CGFloat) {
    value = max(value, nextValue()) } }
Text(label).background(GeometryReader { g in Color.clear
  .preference(key: MaxWidth.self, value: g.size.width) })
.onPreferenceChange(MaxWidth.self) { w in   // on the ancestor
  if w != labelWidth { labelWidth = w } }  // only on change

Interview traps

• Not injected: @EnvironmentObject / non-optional @Environment(T.self) compiles, then crashes at render. Culprits: #Preview, a new UIHostingController (fresh environment), injecting on a sibling branch or inside a NavigationLink destination.
• Down only — a modifier below the reader is invisible to it; inject at a common ancestor.
• Reference type in an EnvironmentKey: mutating its insides invalidates nobody. Keys = immutable config (theme, formatter, flags, clients); mutable shared state = @Observable.
• Preference loop: size → @State → child resizes → new size… “Bound preference …tried to update multiple times per frame”. Store only on change, measure in .background; newer SDKs: onGeometryChange.
• reduce that overwrites (value = nextValue()) or ignores nextValue() keeps one child.
• Swift 6: static var defaultValue is global mutable state — use static let.
• \.dismiss in a root with no presenter is a silent no-op.

Choosing the channel

Remember

“Config rains down, measurements bubble up — and the default is the identity.”

Likely questions

1. Why can @Environment(\.x) never crash? — keys resolve to defaultValue.
2. @EnvironmentObject vs @Environment(T.self)? — whole-object vs per-property invalidation (17).
3. Why anchors, not global frames? — resolved in the reader’s space after every transform.
4. DI through the environment? — .live default, .mock in previews/tests.