swiftui-data-flow

name	swiftui-data-flow
description	Validates SwiftUI data flow, shared state, and property wrappers — @Observable, @State, @Bindable, @Environment, bindings, and SwiftData/CloudKit constraints. Use when reviewing state management or designing data architecture in SwiftUI apps.

SwiftUI Data Flow

Keep body code and logic code separate. Extract logic into methods or @Observable classes — never let business logic and view layout share a function.

Shared state

• All shared data should use @Observable classes with @State (for ownership) and @Bindable / @Environment (for passing).
• @Observable classes must be marked @MainActor unless the project enables Main Actor default actor isolation. Flag any unmarked @Observable.
• Strongly prefer not to use ObservableObject, @Published, @StateObject, @ObservedObject, or @EnvironmentObject unless unavoidable (e.g., legacy or integration code where rearchitecting is impractical).

Local state

• @State should be private and owned only by the view that creates it.
• @State can also hold an expensive-to-recompute class instance (e.g., a CIContext) as a cache. There's no change tracking, but the value persists across body re-evaluations.

Bindings

• Avoid Binding(get:set:) in view body code. Use a binding from @State / @Binding, then trigger side effects with .onChange():

  // Bad
  TextField("Username", text: Binding(
      get: { model.username },
      set: { model.username = $0; model.save() }
  ))
  
  // Good
  TextField("Username", text: $model.username)
      .onChange(of: model.username) { model.save() }
  

• For numeric TextField input, bind to the numeric type and supply a format and keyboard:

  TextField("Enter your score", value: $score, format: .number)
      .keyboardType(.numberPad)        // or .decimalPad for floating point
  

  The keyboard modifier alone is not sufficient.

Working with data

• Prefer making structs Identifiable over passing id: \.someProperty in SwiftUI APIs.
• Never use @AppStorage inside an @Observable class — even with @ObservationIgnored, it will not trigger view updates.

SwiftData

• If you only need a count, ModelContext.fetchCount(_:) with a fetch descriptor is cheaper than fetching items. Note: it will not live-update unless another driver (like @Query) re-renders the view.
• For deeper SwiftData guidance, suggest the SwiftData Pro agent skill.

SwiftData with CloudKit

• Never use @Attribute(.unique).
• Model properties must have default values or be optional.
• All relationships must be optional.

Learn More

This guidance is adapted from the SwiftUI Pro agent skill by Paul Hudson (MIT-licensed).