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**Time cost**: 5 minutes
**Why**: Wrong Xcode version or Debug mode produces misleading profiling data

---

SwiftUI view issue after basic troubleshooting?
│
├─ View not updating?
│  ├─ Basic check: Add Self._printChanges() temporarily
│  │  ├─ Shows "@self changed" → View value changed
│  │  │  └─ Pattern D1: Analyze what caused view recreation
│  │  ├─ Shows specific state property → That state triggered update
│  │  │  └─ Verify: Should that state trigger update?
│  │  └─ Nothing logged → Body not being called at all
│  │     └─ Pattern D3: View Identity Investigation
│  └─ Advanced: Use SwiftUI Instrument
│     └─ Pattern D2: SwiftUI Instrument Investigation
│
├─ View updating too often?
│  ├─ Pattern D1: Self._printChanges() Analysis
│  │  └─ Identify unnecessary state dependencies
│  └─ Pattern D2: SwiftUI Instrument → Cause & Effect Graph
│     └─ Trace data flow, find broad dependencies
│
├─ Intermittent issues (works sometimes)?
│  ├─ Pattern D3: View Identity Investigation
│  │  └─ Check: Does identity change unexpectedly?
│  ├─ Pattern D4: Environment Dependency Check
│  │  └─ Check: Environment values changing frequently?
│  └─ Reproduce in preview 30+ times
│     └─ If can't reproduce: Likely timing/race condition
│
└─ Preview crashes (after basic fixes)?
    ├─ Pattern D5: Preview Diagnostics (Xcode 26)
    │  └─ Check diagnostics button, crash logs
    └─ If still fails: Pattern D2 (profile preview build)

struct MyView: View {
    @State private var count = 0
    @Environment(AppModel.self) private var model

    var body: some View {
        let _ = Self._printChanges()  // Add temporarily

        VStack {
            Text("Count: \(count)")
            Text("Model value: \(model.value)")
        }
    }
}

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**Common discoveries**:

1. **"@self changed" when you don't expect**
   - Parent recreating view unnecessarily
   - Check parent's state management

2. **Property shows changed but you didn't change it**
   - Indirect dependency (reading from object that changed)
   - Pattern D2: Use Instruments to trace

3. **Multiple properties changing together**
   - Broad dependency (e.g., reading entire array when only need one item)
   - Fix: Extract specific dependency

**Verification**:
- Remove `Self._printChanges()` call before committing
- Never ship to production with this code

**Cross-reference**: For complex cases, use Pattern D2 (SwiftUI Instrument)

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[Blue node] = Your code (gesture, state change, view body)
[System node] = SwiftUI/system work
[Arrow labeled "update"] = Caused this update
[Arrow labeled "creation"] = Caused view to appear

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**Click on nodes**:
- **State change node** → See backtrace of where value was set
- **View body node** → See which properties it read (dependencies)

**Verification**:
- Record new trace after fix
- Compare before/after update counts
- Verify red/orange bars reduced or eliminated

**Cross-reference**: `axiom-swiftui-performance` skill for detailed Instruments workflows

---

// ❌ PROBLEM: Identity changes with condition
if showDetails {
    CounterView()  // Gets new identity each time showDetails toggles
}

// ✅ FIX: Use .opacity()
CounterView()
    .opacity(showDetails ? 1 : 0)  // Same identity always

// ❌ PROBLEM: .id() changes when data changes
DetailView()
    .id(item.id + "-\(isEditing)")  // ID changes with isEditing

// ✅ FIX: Stable ID
DetailView()
    .id(item.id)  // Stable ID

// ❌ WRONG: Index-based ID
ForEach(Array(items.enumerated()), id: \.offset) { index, item in
    Text(item.name)
}

// ❌ WRONG: Non-unique ID
ForEach(items, id: \.category) { item in  // Multiple items per category
    Text(item.name)
}

// ✅ RIGHT: Unique, stable ID
ForEach(items, id: \.id) { item in
    Text(item.name)
}

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**Look for**:
```swift
// ❌ BAD: Frequently changing values
.environment(\.scrollOffset, scrollOffset)  // Updates 60+ times/second
.environment(model)  // If model updates frequently

// ✅ GOOD: Stable values
.environment(\.colorScheme, .dark)
.environment(appModel)  // If appModel changes rarely

// ❌ Before: Environment
.environment(\.scrollOffset, scrollOffset)

// ✅ After: Direct parameter
ChildView(scrollOffset: scrollOffset)

// Instead of storing primitive in environment:
@Observable class ScrollViewModel {
    var offset: CGFloat = 0
}

// Views depend on specific properties:
@Environment(ScrollViewModel.self) private var viewModel

var body: some View {
    Text("\(viewModel.offset)")  // Only updates when offset changes
}

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**What to look for**:
- Fatal errors (array out of bounds, force unwrap nil)
- Missing module imports
- Framework initialization failures

// Start with empty preview
#Preview {
    Text("Test")
}

// If this works, gradually add:
#Preview {
    MyView()  // Your actual view, but with mock data
        .environment(MockModel())  // Provide all dependencies
}

// Find which dependency causes crash

// Missing @Environment
#Preview {
    ContentView()
        .environment(AppModel())  // Provide dependency
}

// Missing @EnvironmentObject (pre-iOS 17)
#Preview {
    ContentView()
        .environmentObject(AppModel())
}

// Missing ModelContainer (SwiftData)
#Preview {
    let config = ModelConfiguration(isStoredInMemoryOnly: true)
    let container = try! ModelContainer(for: Item.self, configurations: config)

    return ContentView()
        .modelContainer(container)
}

// State with invalid defaults
@State var selectedIndex = 10  // ❌ Out of bounds
let items = ["a", "b", "c"]

// Fix: Safe default
@State var selectedIndex = 0  // ✅ Valid index

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**If can't reproduce**: Ask for video recording or device logs from affected users

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**Find**:
- Which view is expensive?
- What data change triggered it?
- How many views updated?

// Example finding: Formatter creation in body
// Fix: Move to cached formatter

// Example finding: All toggle views reading entire settings array
// Fix: Per-toggle view models with granular dependencies

// Example finding: Environment value updating every frame
// Fix: Remove from environment, use direct parameter

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Slack to VP + team:

"Diagnostic complete: Settings screen freeze caused by formatter creation
in ToggleRow body (confirmed via SwiftUI Instrument, Long View Body Updates).

Each toggle tap recreated NumberFormatter + DateFormatter for all visible
toggles (20+ formatters per tap).

Fix: Cached formatters in SettingsViewModel, pre-formatted strings.
Verified: Settings screen now responds in <16ms (was 200ms+).

Deploying build 2.1.1 now. Will monitor for next 24 hours."

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// Add temporarily to view body
var body: some View {
    let _ = Self._printChanges()  // Shows update reason

    // Your view code
}

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