ios-performance-engineeringlisted

# iOS Performance Engineering ## When to invoke - Diagnosing UI slowness, scroll hitches, or app hangs. - Investigating high memory usage, leaks, or large binary size. - Wiring MetricKit to receive field performance data from real devices. - Setting up `XCTMetric` / `measure {}` baselines in CI. - Deciding whether to move work off `@MainActor` and how to do it safely. - Evaluating launch time before a release. ## Instruments — the primary measurement tool Never guess at a performance problem; profile first. Instruments ships with Xcode. | Symptom | Instrument / template | Metric to read | |---|---|---| | High CPU / slow interactive path | Time Profiler | inverted call tree, self time > 5 ms on main thread | | Unbounded memory growth | Allocations (Generation) | allocations that grow across repeated actions | | Scroll / animation stutter | Animation Hitches | `hitch rate` (ms of hitch per second) | | Main-thread freeze / spin | Hangs | block duration ≥ 250 ms | | Slow cold launch | App Launch | time to first committed frame | | Excessive SwiftUI re-renders | SwiftUI instrument | body invocation count, triggering property | Key templates, condensed (full walkthrough of each template's UI: `references/instruments-templates.md`): - **Time Profiler**: a function taking >5 ms on the main thread in an interactive path is a candidate for offloading. - **Allocations**: the "Leaks" instrument detects reference cycles automatically but misses logical leaks (objects kept alive lon