---
title: Measure TTI (Time to Interactive)
impact: HIGH
tags: tti, startup, performance, markers
---

# Skill: Measure TTI (Time to Interactive)

Set up performance markers to measure app startup time and track TTI improvements.

## Quick Command

```bash
npm install react-native-performance
```

```tsx
// Mark when screen is interactive
import performance from 'react-native-performance';

useEffect(() => {
  performance.mark('screenInteractive');
}, []);
```

## When to Use

- App startup feels slow
- Need baseline metrics for optimization
- Setting up performance monitoring
- Comparing TTI across releases

## Prerequisites

- `react-native-performance` library (recommended)

```bash
npm install react-native-performance
```

> **Note**: This skill involves visual timeline diagrams and profiler output. Use `agent-device` for cold-start evidence; install it through the environment's approved/trusted path or ask the user if verification needs it and it is missing. Timeline interpretation may still require exported metrics or human review.

## Understanding TTI

**Time to Interactive**: Time from app icon tap to displaying usable content.

### Startup Types

| Type | Description | Measure? |
|------|-------------|----------|
| Cold | App not in memory, full init | ✅ Yes |
| Warm | Process exists, activity recreated | ❌ Skip |
| Hot | App in background, resumed | ❌ Skip |
| Prewarmed (iOS) | iOS pre-initialized app | ❌ Filter out |

**Only measure cold starts** for consistent metrics.

## React Native Startup Pipeline

![TTI Warm Start Diagram](images/tti-warm-start-diagram.png)

The diagram shows a warm start (app was in memory):

**UI Thread:**
1. `init native process` → `init native app`
2. Gap while user is away (e.g., "5h break from using the app")
3. `JS bundle load` → `RootView render`

**JS Thread (runs in parallel):**
- `init entrypoint` → `registerComponent`

**Pipeline markers:**
```
1. Native Process Init     (nativeLaunchStart → nativeLaunchEnd)
2. Native App Init         (appCreationStart → appCreationEnd)  
3. JS Bundle Load          (runJSBundleStart → runJSBundleEnd)
4. RN Root View Render     (contentAppeared)
5. React App Interactive   (screenInteractive) ← This is TTI
```

## Step-by-Step Implementation

### 1. Detect Cold Start

**iOS (Swift):**

```swift
let isColdStart = ProcessInfo.processInfo.environment["ActivePrewarm"] != "1"
```

**Android (Kotlin):**

```kotlin
class MainApplication : Application() {
    var isColdStart = false
    
    override fun onCreate() {
        super.onCreate()
        
        var firstPostEnqueued = true
        Handler().post { firstPostEnqueued = false }
        
        registerActivityLifecycleCallbacks(object : ActivityLifecycleCallbacks {
            override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) {
                unregisterActivityLifecycleCallbacks(this)
                if (firstPostEnqueued && savedInstanceState == null) {
                    isColdStart = true
                }
            }
            // ... other callbacks
        })
    }
}
```

### 2. Check Foreground State

Only measure when app starts in foreground.

**iOS:**

```swift
var isForegroundProcess = false

override func application(_ application: UIApplication, 
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    if application.applicationState == .active {
        isForegroundProcess = true
    }
    return true
}
```

**Android:**

```kotlin
private fun isForegroundProcess(): Boolean {
    val processInfo = ActivityManager.RunningAppProcessInfo()
    ActivityManager.getMyMemoryState(processInfo)
    return processInfo.importance == IMPORTANCE_FOREGROUND
}
```

### 3. Set Up Performance Markers

Using `react-native-performance`:

**Native (iOS):**

```swift
import ReactNativePerformance

RNPerformance.sharedInstance().mark("appCreationStart")
// ... app init ...
RNPerformance.sharedInstance().mark("appCreationEnd")
```

**Native (Android):**

```kotlin
import com.oblador.performance.RNPerformance

RNPerformance.getInstance().mark("appCreationStart")
// ... app init ...
RNPerformance.getInstance().mark("appCreationEnd")
```

### 4. Mark Screen Interactive (JavaScript)

```tsx
import performance from 'react-native-performance';

export default function HomeScreen() {
    useEffect(() => {
        // Mark when meaningful content is displayed
        performance.mark('screenInteractive');
    }, []);
    
    return <TabNavigator />;
}
```

### 5. Collect and Report Metrics

```tsx
import performance from 'react-native-performance';

const collectTTIMetrics = () => {
    const entries = performance.getEntriesByType('mark');
    
    // Calculate durations
    const metrics = {
        nativeInit: getMarkDuration('nativeLaunchStart', 'nativeLaunchEnd'),
        appCreation: getMarkDuration('appCreationStart', 'appCreationEnd'),
        jsBundleLoad: getMarkDuration('runJSBundleStart', 'runJSBundleEnd'),
        tti: getMarkDuration('nativeLaunchStart', 'screenInteractive'),
    };
    
    // Send to analytics
    analytics.track('app_performance', metrics);
};
```

## Built-in Markers

`react-native-performance` provides automatic markers:

| Marker | Description |
|--------|-------------|
| `nativeLaunchStart` | Process start (pre-main) |
| `nativeLaunchEnd` | Native init complete |
| `runJSBundleStart` | JS bundle loading starts |
| `runJSBundleEnd` | JS bundle loaded |
| `contentAppeared` | RN root view rendered |

## Listening to Native Events

**iOS (JS Bundle Load):**

```swift
NotificationCenter.default.addObserver(
    self,
    selector: #selector(onJSLoad),
    name: NSNotification.Name("RCTJavaScriptDidLoadNotification"),
    object: nil
)
```

**Android (JS Bundle Load):**

```kotlin
ReactMarker.addListener { name ->
    when (name) {
        RUN_JS_BUNDLE_START -> { /* mark start */ }
        RUN_JS_BUNDLE_END -> { /* mark end */ }
        CONTENT_APPEARED -> { /* mark content */ }
    }
}
```

## Target Metrics

| Metric | Good | Acceptable | Needs Work |
|--------|------|------------|------------|
| TTI | < 2s | 2-4s | > 4s |
| JS Bundle Load | < 500ms | 500ms-1s | > 1s |
| Native Init | < 500ms | 500ms-1s | > 1s |

**Note**: Targets vary by app complexity and device tier.

## Common Pitfalls

- **Including prewarmed starts**: iOS prewarming skews metrics
- **Measuring warm/hot starts**: Only cold starts are meaningful
- **Wrong screenInteractive placement**: Mark when truly interactive, not just mounted
- **Not filtering background launches**: Push notifications can start app in background

## Related Skills

- [bundle-analyze-js.md](./bundle-analyze-js.md) - Reduce JS bundle load time
- [native-profiling.md](./native-profiling.md) - Profile native init
- [bundle-hermes-mmap.md](./bundle-hermes-mmap.md) - Improve Android TTI
