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name: ios-swiftui description: Build declarative UI and manage data flow with SwiftUI in iOS. Use when building declarative SwiftUI views or managing data flow with property wrappers. metadata: triggers: files: - '**/*View.swift' keywords: - View - SwiftUI @State - SwiftUI State - Binding - EnvironmentObject

SwiftUI Expert

Priority: P0 (CRITICAL)

Role: iOS UI Expert. Prioritize smooth 60fps, clean data flow.

Implementation Guidelines

• Views: Small, composable structs. Extract subviews often to keep body clean.
• State Selection (iOS 17+, default):
• @State for local simple data (Booleans, Strings, local view toggles).
• @State for VMs marked @Observable (owning parent creates it once: @State private var vm = MyViewModel()).
• Plain let/property for passed-in @Observable VMs — no wrapper needed, child views auto-track only the fields they read.
• Legacy (pre-iOS 17 or Combine ObservableObject VMs):
• @StateObject for VMs (initialized only once in parent view).
• @ObservedObject for passed-in VMs (initialized by parent).
• Prefer migrating ObservableObject VMs to @Observable when the deployment target allows it — see ios-state-management.
• Modifiers: Order matters sequentially. Apply layout modifiers before visual ones (e.g., .padding().background()).
• Preview: Always provide PreviewProvider or #Preview for every view.

Verification Checklist (Mandatory)

• Body Property: body property computationally cheap? (No complex logic or calculations).
• State Flow: @State/@StateObject VM initialized only once (in parent)?
• Identity: Lists/ForEach stable id?
• Main Actor: UI updates strictly on Main Actor?

Anti-Patterns

• No Logic in Body: Move calculations to ViewModel or computed vars. Keep body for UI composition only.
• No ObservedObject Init: NOT init @ObservedObject inside View settings — this causes leaks and performance issues.
• No New ObservableObject VMs on iOS 17+: Use @Observable for new code; reserve ObservableObject/@Published for code that must support pre-17 or already depends on Combine operators.
• No Hardcoded Sizes: Use flexible frames and spacers for responsive UI.

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• @State for local simple data