d2be634870
Add fourteen skills from Dimillian/Skills, integrate the merged Snowflake and WordPress updates into the maintainer sync, and refresh registry metadata, attributions, walkthrough notes, and the 8.9.0 release notes while keeping validation warnings within budget.
63 lines
1.9 KiB
Markdown
63 lines
1.9 KiB
Markdown
# Performance guardrails
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## Intent
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Use these rules when a SwiftUI screen is large, scroll-heavy, frequently updated, or at risk of unnecessary recomputation.
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## Core rules
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- Give `ForEach` and list content stable identity. Do not use unstable indices as identity when the collection can reorder or mutate.
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- Keep expensive filtering, sorting, and formatting out of `body`; precompute or move it into a model/helper when it is not trivial.
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- Narrow observation scope so only the views that read changing state need to update.
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- Prefer lazy containers for larger scrolling content and extract subviews when only part of a screen changes frequently.
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- Avoid swapping entire top-level view trees for small state changes; keep a stable root view and vary localized sections or modifiers.
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## Example: stable identity
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```swift
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ForEach(items) { item in
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Row(item: item)
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}
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```
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Prefer that over index-based identity when the collection can change order:
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```swift
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ForEach(Array(items.enumerated()), id: \.offset) { _, item in
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Row(item: item)
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}
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```
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## Example: move expensive work out of body
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```swift
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struct FeedView: View {
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let items: [FeedItem]
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private var sortedItems: [FeedItem] {
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items.sorted(using: KeyPathComparator(\.createdAt, order: .reverse))
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}
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var body: some View {
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List(sortedItems) { item in
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FeedRow(item: item)
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}
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}
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}
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```
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If the work is more expensive than a small derived property, move it into a model, store, or helper that updates less often.
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## When to investigate further
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- Janky scrolling in long feeds or grids
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- Typing lag from search or form validation
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- Overly broad view updates when one small piece of state changes
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- Large screens with many conditionals or repeated formatting work
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## Pitfalls
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- Recomputing heavy transforms every render
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- Observing a large object from many descendants when only one field matters
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- Building custom scroll containers when `List`, `LazyVStack`, or `LazyHGrid` would already solve the problem
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