swiftui-expert-skill

Audit the code against the correctness checklist: verify @State is private, ForEach uses stable identity, and animations use the value parameter. Then consult the performance-patterns reference to reduce unnecessary state updates and extract complex view bodies into subviews.

Run the analyze_trace.py script with --json-only to get time profiler, hangs, hitches, and SwiftUI lanes plus cross-lane correlations. Check main_running_coverage_pct and swiftui-causes top sources to find which views are invalidated and why.

Use @StateObject when the view owns and creates the object, and @ObservedObject when the object is injected from a parent. On iOS 17+, prefer @State with @Observable classes and @Bindable for injected observables that need bindings.

The SwiftUI Instruments template returns an empty SwiftUI lane on the iOS Simulator. Use the Time Profiler template instead, which still captures Time Profiler, Hangs, and Animation Hitches data; the SwiftUI template works on physical devices and the host Mac.

The Document protocol family replaces FileDocument on iOS 27, macOS 27, and visionOS 27. For earlier deployment targets, keep FileDocument since it is only soft-deprecated; migrate by converting to an @Observable class with DocumentReader and DocumentWriter.

Transitions require an animation context outside the conditional. Placing .animation inside the if-branch removes it with the view, so removal transitions never animate; wrap the state change in withAnimation or place .animation(_:value:) on the parent.