/performance

View Identity

How SwiftUI decides whether two views are the same view (update in place) or different views (destroy and recreate). Getting identity wrong causes lost state, broken animations, and unnecessary work.

Two Kinds of Identity

Structural Identity (Implicit)

SwiftUI assigns identity based on a view's position in the view hierarchy. Two views at the same position in the same parent are considered the same view across renders.

// These two Text views have different structural identities
// because they are at different positions in the VStack
VStack {
    Text("First")   // position 0
    Text("Second")  // position 1
}

Explicit Identity

You override structural identity using .id() or ForEach identifiers. This tells SwiftUI: "this specific value identifies this view."

ForEach(items, id: \.id) { item in
    ItemRow(item: item)  // identity = item.id
}

DetailView(item: selectedItem)
    .id(selectedItem.id)  // identity = selectedItem.id

How SwiftUI Uses Identity

Key consequence: when identity changes, all @State is lost and the view is recreated from scratch.

.id() Modifier

When to Use .id()

Force SwiftUI to treat a view as a completely new instance:

// Force recreation when switching between items
// Without .id(), SwiftUI would try to update the same view,
// which can show stale state or skip the appear animation
DetailView(item: selectedItem)
    .id(selectedItem.id)

Stable vs Unstable Identifiers

// ✅ Stable: database ID, UUID, or model-provided unique key
ForEach(items, id: \.databaseID) { item in
    ItemRow(item: item)
}

// ✅ Stable: using Identifiable conformance
ForEach(items) { item in  // uses item.id automatically
    ItemRow(item: item)
}

// ❌ Unstable: array index on a mutable array
// When items are inserted/deleted, indices shift and
// SwiftUI matches the wrong data to the wrong view
ForEach(Array(items.enumerated()), id: \.offset) { index, item in
    ItemRow(item: item)
}

// ❌ Unstable: random value or Date()
// Generates a new identity every render, destroying all state
DetailView(item: item)
    .id(UUID())  // NEVER do this -- recreates view every body call

Performance Impact of Unstable .id()

An .id() that changes every render destroys and recreates the entire subtree -- state, onAppear, animations, and in-flight tasks all reset as if the view were brand new. This is the single most expensive identity mistake.

Conditional View Branching

The if/else Identity Problem

if/else branches create different structural identities:

// ❌ Problem: toggling isLoading destroys and recreates ContentView
// because the two branches are different structural positions
if isLoading {
    ProgressView()
} else {
    ContentView()
}

When isLoading changes from true to false:

• ProgressView is destroyed (it existed at the "if-true" branch)
• ContentView is created from scratch (it appears at the "if-else" branch)
• All @State inside ContentView resets

When Destruction Is Acceptable

If the two branches show completely different content (like loading vs loaded), this destruction is fine and expected. The problem arises when you use if/else to toggle modifiers on the same view:

// ❌ Bad: destroys and recreates the same Text view
if isHighlighted {
    Text(item.title)
        .foregroundStyle(.yellow)
        .bold()
} else {
    Text(item.title)
        .foregroundStyle(.primary)
}

// ✅ Good: same structural identity, just different modifier values
Text(item.title)
    .foregroundStyle(isHighlighted ? .yellow : .primary)
    .bold(isHighlighted)

Ternary Expressions vs if/else in View Body

// ✅ Ternary: single structural identity, SwiftUI diffs the values
RoundedRectangle(cornerRadius: 12)
    .fill(isSelected ? Color.blue : Color.gray)
    .frame(height: isExpanded ? 200 : 80)

// ❌ if/else: two different structural identities
if isSelected {
    RoundedRectangle(cornerRadius: 12).fill(.blue)
} else {
    RoundedRectangle(cornerRadius: 12).fill(.gray)
}

AnyView and Identity

AnyView erases type information, which breaks structural identity tracking:

// ❌ AnyView: SwiftUI cannot track structural identity across renders
func makeView() -> AnyView {
    if condition {
        return AnyView(ViewA())
    } else {
        return AnyView(ViewB())
    }
}

// ✅ @ViewBuilder: preserves structural identity
@ViewBuilder
func makeView() -> some View {
    if condition {
        ViewA()
    } else {
        ViewB()
    }
}

See common-pitfalls.md for more on AnyView performance costs.

ForEach Identity

How ForEach Uses Identifiers

ForEach maps each data element to a view using the id key path. SwiftUI uses these identifiers to:

• Match existing views to updated data (minimizing recreations)
• Animate insertions, removals, and reorderings
• Preserve @State in each row

// ✅ Identifiable conformance (cleanest)
struct Item: Identifiable {
    let id: UUID
    var title: String
}
ForEach(items) { item in
    ItemRow(item: item)
}

// ✅ Explicit id key path
ForEach(items, id: \.uniqueKey) { item in
    ItemRow(item: item)
}

Duplicate Identifiers

If two items share the same id, SwiftUI behavior is undefined -- it may show duplicates, skip items, or crash:

// ❌ Dangerous: if two items have the same title, behavior is undefined
ForEach(items, id: \.title) { item in
    ItemRow(item: item)
}

// ✅ Always use a truly unique identifier
ForEach(items, id: \.id) { item in
    ItemRow(item: item)
}

Debugging Identity Issues

Symptoms of Identity Problems

Using _printChanges to Detect Recreation

var body: some View {
    let _ = Self._printChanges()
    // If you see "@identity changed" in the output,
    // SwiftUI considers this a brand-new view instance
    Text("Content")
}

Output @identity changed means the view was destroyed and recreated, not updated in place. This usually means an unstable .id() or a structural identity change from if/else branching.

Verifying with onAppear / onDisappear

DetailView(item: item)
    .id(item.id)
    .onAppear { print("DetailView appeared: \(item.id)") }
    .onDisappear { print("DetailView disappeared") }

If you see rapid appear/disappear cycles for the same content, the identity is unstable.

Fix Patterns Summary