Initial commit
This commit is contained in:
Zhongwei Li
562
skills/expertise/iphone-apps/references/performance.md
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562
skills/expertise/iphone-apps/references/performance.md
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# Performance
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Instruments, memory management, launch optimization, and battery efficiency.
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## Instruments Profiling
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### Time Profiler
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Find CPU-intensive code:
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```bash
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# Profile from CLI
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xcrun xctrace record \
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--template 'Time Profiler' \
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--device-name 'iPhone 16' \
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--launch MyApp.app \
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--output profile.trace
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```
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Common issues:
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- Main thread work during UI updates
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- Expensive computations in body
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- Synchronous I/O
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### Allocations
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Track memory usage:
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```bash
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xcrun xctrace record \
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--template 'Allocations' \
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--device-name 'iPhone 16' \
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--launch MyApp.app \
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--output allocations.trace
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```
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Look for:
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- Memory growth over time
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- Abandoned memory
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- High transient allocations
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### Leaks
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Find retain cycles:
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```bash
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xcrun xctrace record \
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--template 'Leaks' \
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--device-name 'iPhone 16' \
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--launch MyApp.app \
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--output leaks.trace
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```
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Common causes:
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- Strong reference cycles in closures
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- Delegate patterns without weak references
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- Timer retain cycles
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## Memory Management
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### Weak References in Closures
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```swift
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// Bad - creates retain cycle
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class ViewModel {
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var timer: Timer?
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func startTimer() {
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timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { _ in
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self.update() // Strong capture
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}
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}
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}
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// Good - weak capture
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class ViewModel {
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var timer: Timer?
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func startTimer() {
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timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] _ in
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self?.update()
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}
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}
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deinit {
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timer?.invalidate()
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}
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}
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```
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### Async Task Cancellation
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```swift
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class ViewModel {
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private var loadTask: Task<Void, Never>?
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func load() {
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loadTask?.cancel()
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loadTask = Task { [weak self] in
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guard let self else { return }
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let items = try? await fetchItems()
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// Check cancellation before updating
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guard !Task.isCancelled else { return }
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await MainActor.run {
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self.items = items ?? []
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}
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}
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}
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deinit {
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loadTask?.cancel()
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}
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}
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```
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### Large Data Handling
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```swift
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// Bad - loads all into memory
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let allPhotos = try await fetchAllPhotos()
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for photo in allPhotos {
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process(photo)
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}
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// Good - stream processing
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for await photo in fetchPhotosStream() {
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process(photo)
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// Allow UI updates
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if shouldYield {
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await Task.yield()
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}
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}
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```
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## SwiftUI Performance
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### Avoid Expensive Body Computations
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```swift
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// Bad - recomputes on every body call
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struct ItemList: View {
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let items: [Item]
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var body: some View {
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let sortedItems = items.sorted { $0.date > $1.date } // Every render!
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List(sortedItems) { item in
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ItemRow(item: item)
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}
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}
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}
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// Good - compute once
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struct ItemList: View {
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let items: [Item]
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var sortedItems: [Item] {
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items.sorted { $0.date > $1.date }
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}
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var body: some View {
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List(sortedItems) { item in
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ItemRow(item: item)
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}
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}
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}
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// Better - use @State or computed in view model
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struct ItemList: View {
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@State private var sortedItems: [Item] = []
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let items: [Item]
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var body: some View {
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List(sortedItems) { item in
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ItemRow(item: item)
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}
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.onChange(of: items) { _, newItems in
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sortedItems = newItems.sorted { $0.date > $1.date }
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}
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}
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}
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```
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### Optimize List Performance
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```swift
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// Use stable identifiers
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struct Item: Identifiable {
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let id: UUID // Stable identifier
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var name: String
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}
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// Explicit id for efficiency
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List(items, id: \.id) { item in
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ItemRow(item: item)
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}
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// Lazy loading for large lists
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LazyVStack {
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ForEach(items) { item in
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ItemRow(item: item)
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}
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}
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```
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### Equatable Conformance
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```swift
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// Prevent unnecessary re-renders
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struct ItemRow: View, Equatable {
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let item: Item
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static func == (lhs: ItemRow, rhs: ItemRow) -> Bool {
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lhs.item.id == rhs.item.id &&
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lhs.item.name == rhs.item.name
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}
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var body: some View {
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Text(item.name)
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}
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}
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// Use in ForEach
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ForEach(items) { item in
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ItemRow(item: item)
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.equatable()
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}
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```
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### Task Modifier Optimization
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```swift
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// Bad - recreates task on any state change
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struct ContentView: View {
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@State private var items: [Item] = []
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@State private var searchText = ""
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var body: some View {
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List(filteredItems) { item in
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ItemRow(item: item)
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}
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.task {
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items = await fetchItems() // Reruns when searchText changes!
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}
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}
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}
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// Good - use task(id:)
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struct ContentView: View {
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@State private var items: [Item] = []
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@State private var searchText = ""
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@State private var needsLoad = true
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var body: some View {
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List(filteredItems) { item in
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ItemRow(item: item)
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}
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.task(id: needsLoad) {
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if needsLoad {
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items = await fetchItems()
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needsLoad = false
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}
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}
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}
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}
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```
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## Launch Time Optimization
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### Measure Launch Time
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```bash
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# Cold launch measurement
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xcrun simctl spawn booted log stream --predicate 'subsystem == "com.apple.os.signpost" && category == "PointsOfInterest"'
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```
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In Instruments: App Launch template
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### Defer Non-Critical Work
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```swift
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@main
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struct MyApp: App {
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init() {
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// Critical only
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setupErrorReporting()
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}
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var body: some Scene {
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WindowGroup {
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ContentView()
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.task {
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// Defer non-critical
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await setupAnalytics()
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await preloadData()
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}
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}
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}
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}
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```
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### Avoid Synchronous Work
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```swift
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// Bad - blocks launch
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@main
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struct MyApp: App {
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let database = Database.load() // Synchronous I/O
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var body: some Scene {
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WindowGroup {
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ContentView()
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}
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}
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}
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// Good - async initialization
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@main
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struct MyApp: App {
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@State private var database: Database?
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var body: some Scene {
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WindowGroup {
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if let database {
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ContentView()
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.environment(database)
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} else {
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LaunchScreen()
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}
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}
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.task {
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database = await Database.load()
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}
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}
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}
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```
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### Reduce Dylib Loading
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- Minimize third-party dependencies
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- Use static linking where possible
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- Merge frameworks
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## Network Performance
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### Request Batching
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```swift
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// Bad - many small requests
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for id in itemIDs {
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let item = try await fetchItem(id)
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items.append(item)
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}
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// Good - batch request
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let items = try await fetchItems(ids: itemIDs)
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```
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### Image Loading
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```swift
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// Use AsyncImage with caching
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AsyncImage(url: imageURL) { phase in
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switch phase {
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case .empty:
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ProgressView()
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case .success(let image):
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image.resizable().scaledToFit()
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case .failure:
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Image(systemName: "photo")
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@unknown default:
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EmptyView()
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}
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}
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// For better control, use custom caching
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actor ImageCache {
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private var cache: [URL: UIImage] = [:]
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func image(for url: URL) async throws -> UIImage {
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if let cached = cache[url] {
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return cached
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}
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let (data, _) = try await URLSession.shared.data(from: url)
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let image = UIImage(data: data)!
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cache[url] = image
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return image
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}
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}
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```
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### Prefetching
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```swift
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struct ItemList: View {
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let items: [Item]
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let prefetcher = ImagePrefetcher()
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var body: some View {
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List(items) { item in
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ItemRow(item: item)
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.onAppear {
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// Prefetch next items
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let index = items.firstIndex(of: item) ?? 0
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let nextItems = items.dropFirst(index + 1).prefix(5)
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prefetcher.prefetch(urls: nextItems.compactMap(\.imageURL))
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}
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}
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}
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}
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```
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## Battery Optimization
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### Location Updates
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```swift
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import CoreLocation
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class LocationService: NSObject, CLLocationManagerDelegate {
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private let manager = CLLocationManager()
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func startUpdates() {
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// Use appropriate accuracy
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manager.desiredAccuracy = kCLLocationAccuracyHundredMeters // Not kCLLocationAccuracyBest
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// Allow deferred updates
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manager.allowsBackgroundLocationUpdates = false
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manager.pausesLocationUpdatesAutomatically = true
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// Use significant change for background
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manager.startMonitoringSignificantLocationChanges()
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}
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}
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```
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### Background Tasks
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```swift
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import BackgroundTasks
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func scheduleAppRefresh() {
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let request = BGAppRefreshTaskRequest(identifier: "com.app.refresh")
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request.earliestBeginDate = Date(timeIntervalSinceNow: 15 * 60) // 15 minutes
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do {
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try BGTaskScheduler.shared.submit(request)
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} catch {
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print("Could not schedule app refresh: \(error)")
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}
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}
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func handleAppRefresh(task: BGAppRefreshTask) {
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// Schedule next refresh
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scheduleAppRefresh()
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let refreshTask = Task {
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do {
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try await syncData()
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task.setTaskCompleted(success: true)
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} catch {
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task.setTaskCompleted(success: false)
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}
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}
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task.expirationHandler = {
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refreshTask.cancel()
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}
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}
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```
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### Network Efficiency
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```swift
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// Use background URL session for large transfers
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let config = URLSessionConfiguration.background(withIdentifier: "com.app.background")
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config.isDiscretionary = true // System chooses optimal time
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config.allowsCellularAccess = false // WiFi only for large downloads
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let session = URLSession(configuration: config, delegate: self, delegateQueue: nil)
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```
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## Debugging Performance
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### Signposts
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```swift
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import os
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let signposter = OSSignposter()
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func processItems() async {
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let signpostID = signposter.makeSignpostID()
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let state = signposter.beginInterval("Process Items", id: signpostID)
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for item in items {
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signposter.emitEvent("Processing", id: signpostID, "\(item.name)")
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await process(item)
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}
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signposter.endInterval("Process Items", state)
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}
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```
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### MetricKit
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```swift
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import MetricKit
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class MetricsManager: NSObject, MXMetricManagerSubscriber {
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override init() {
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super.init()
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MXMetricManager.shared.add(self)
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}
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func didReceive(_ payloads: [MXMetricPayload]) {
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for payload in payloads {
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// Process CPU, memory, launch time metrics
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if let cpuMetrics = payload.cpuMetrics {
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print("CPU time: \(cpuMetrics.cumulativeCPUTime)")
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}
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}
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}
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func didReceive(_ payloads: [MXDiagnosticPayload]) {
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for payload in payloads {
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// Process crash and hang diagnostics
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}
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}
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}
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```
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## Performance Checklist
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### Launch
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- [ ] < 400ms to first frame
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- [ ] No synchronous I/O in init
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- [ ] Deferred non-critical setup
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### Memory
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||||
- [ ] No leaks
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||||
- [ ] Stable memory usage
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||||
- [ ] No abandoned memory
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||||
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### UI
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||||
- [ ] 60 fps scrolling
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||||
- [ ] No main thread blocking
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||||
- [ ] Efficient list rendering
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### Network
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||||
- [ ] Request batching
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- [ ] Image caching
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- [ ] Proper timeout handling
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||||
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||||
### Battery
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||||
- [ ] Minimal background activity
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||||
- [ ] Efficient location usage
|
||||
- [ ] Discretionary transfers
|
||||
Reference in New Issue
Block a user