Performance guardrails

Intent

Use these rules when a SwiftUI screen is large, scroll-heavy, frequently updated, or at risk of unnecessary recomputation.

Core rules

Give ForEach and list content stable identity. Do not use unstable indices as identity when the collection can reorder or mutate.
Keep expensive filtering, sorting, and formatting out of body; precompute or move it into a model/helper when it is not trivial.
Narrow observation scope so only the views that read changing state need to update.
Prefer lazy containers for larger scrolling content and extract subviews when only part of a screen changes frequently.
Avoid swapping entire top-level view trees for small state changes; keep a stable root view and vary localized sections or modifiers.

Example: stable identity

ForEach(items) { item in
  Row(item: item)
}

Prefer that over index-based identity when the collection can change order:

ForEach(Array(items.enumerated()), id: \.offset) { _, item in
  Row(item: item)
}

Example: move expensive work out of body

struct FeedView: View {
  let items: [FeedItem]

  private var sortedItems: [FeedItem] {
    items.sorted(using: KeyPathComparator(\.createdAt, order: .reverse))
  }

  var body: some View {
    List(sortedItems) { item in
      FeedRow(item: item)
    }
  }
}

If the work is more expensive than a small derived property, move it into a model, store, or helper that updates less often.

When to investigate further

Janky scrolling in long feeds or grids
Typing lag from search or form validation
Overly broad view updates when one small piece of state changes
Large screens with many conditionals or repeated formatting work

Pitfalls

Recomputing heavy transforms every render
Observing a large object from many descendants when only one field matters
Building custom scroll containers when List, LazyVStack, or LazyHGrid would already solve the problem

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