Modifying state during view update 🍎 iOS

SwiftUI: Modifying state during view update, this will cause undefined behavior

A @State/@Published value was changed while SwiftUI was computing a view’s body.

Seen on: iOS

Meaning

Changing state inside body, inside a computed property used by body, or synchronously in a binding getter causes this runtime warning and unpredictable UI. Move mutations to actions, .onAppear, .task or .onChange.

Common causes

  • Assigning state inside var body
  • Mutating in a computed property or Binding get
  • Calling a method that changes state during layout (GeometryReader)

⚡ Quick fix

  1. Move the change into .onAppear/.task/.onChange
  2. Derive values instead of storing them
  3. Defer with Task { @MainActor in … } as a last resort

Detailed fix by platform

Swift

  1. swift
    var body: some View {
    Text(title)
        .onChange(of: size) { _, newValue in layoutWidth = newValue.width }
}

How to diagnose

  1. Location — Which state changes during body?
  2. Trigger — GeometryReader/computed property?

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