What is view identity in SwiftUI, and why does it matter?

#TL;DR

SwiftUI views are structs that are created again and again. Identity tells SwiftUI "this is still the same view". There are two kinds: structural identity (position in the view tree) and explicit identity (id). Same identity: state is kept and changes animate smoothly. New identity: old state is destroyed and the view starts fresh.

#Structural identity

SwiftUI knows a view by its type and its place in body.

var body: some View {
    if isLoggedIn {
        HomeView()   // identity: "first branch of the if"
    } else {
        LoginView()  // identity: "second branch of the if"
    }
}

The two branches are different views. Switching between them removes one and inserts the other, with all their @State.

#The common bug: branching on a modifier

// Bad: two different views, the TextField loses focus and state
if isError {
    TextField("Email", text: $email).border(.red)
} else {
    TextField("Email", text: $email)
}

// Good: one view, only the modifier value changes
TextField("Email", text: $email)
    .border(isError ? .red : .clear)

Prefer one view with changing values over if/else that creates different views. The same applies to a custom .if(condition) modifier. It looks nice but it changes identity.

#Explicit identity

#ForEach and List

ForEach(messages) { message in   // Message: Identifiable
    MessageRow(message: message)
}

SwiftUI uses message.id to match rows between updates. It can then animate inserts and moves.

// Risky: identity is the index
ForEach(messages.indices, id: \.self) { i in
    MessageRow(message: messages[i])
}

If you insert at the top, every index shifts. Rows get the wrong state and animations break. IDs must be stable and unique. A new UUID() on each fetch is also bad.

#The .id(_:) modifier

Changing .id destroys the view and creates a new one.

ProfileView(userID: userID)
    .id(userID) // reset all @State when the user changes

Useful to reset a form or restart .task. Expensive if used carelessly, for example .id(UUID()) rebuilds the view on every update.

#Identity, lifetime and state

The lifetime of @State and .task follows identity, not the struct.

struct Stopwatch: View {
    @State private var seconds = 0 // lives as long as this identity
    var body: some View { Text("\(seconds)") }
}

The Stopwatch struct is created many times. seconds survives because the identity stays the same.

#Why AnyView hurts

AnyView hides the type. SwiftUI cannot compare structures easily, so it may rebuild more. Use @ViewBuilder or some View instead.

@ViewBuilder
func badge(for status: Status) -> some View {
    switch status {
    case .new: Text("New")
    case .done: Image(systemName: "checkmark")
    }
}

#Transitions need identity changes

.transition(...) only runs when a view is inserted or removed. That means an identity change. A changed modifier value animates instead.

#Common follow-up questions

• Why does my TextField lose focus when I type? Something gives it a new identity each update, often .id(UUID()) or an if around it.
• Does ForEach require Identifiable? No. You can pass id: \.someKeyPath. It must still be stable and unique.
• What does onAppear do on an identity change? The old view gets onDisappear and the new one gets onAppear.
• How do you reset @State from the parent? Change the child's .id.