Difference Between a Cold Start and a Hot Restart?

Overview

Cold Start and Hot Restart describe different ways a Flutter app begins running — with major differences in startup speed and what state is preserved.

Cold Start

A Cold Start is when the app launches from scratch — the process is completely fresh.

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Device → OS launches app process → Flutter engine initializes
       → Dart VM starts → main() runs → App displays first frame

What happens in a Cold Start:

• ✅ Full app process starts fresh
• ✅ All dependencies initialized
• ✅ All state fully freshly created
• ❌ Slowest — everything is done from zero
• ❌ App data from previous session is not in memory (must be loaded from storage)

When it occurs:

• First app launch after install
• After device reboot
• After the app is force-killed
• After too long in the background (OS killed the process)

dartCopy
// In cold start, this runs from the beginning
void main() {
  WidgetsFlutterBinding.ensureInitialized();
  // All initialization happens fresh
  runApp(MyApp());
}

Hot Restart (Flutter Dev Tool)

A Hot Restart is a Flutter development feature that re-runs the app within the existing Dart VM, without restarting the OS process.

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Flutter CLI → Recompiles Dart code → Reinitializes Dart state
           → Re-executes main() → App rebuilds from scratch (visually)

What happens in a Hot Restart:

• ✅ Dart VM stays alive — faster than cold start
• ✅ Code changes are picked up
• ❌ All Dart state is lost —
  textCopy

  StatefulWidget

  state, variables reset
• ❌
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  initState()

  is called again
• ✅ Faster than cold start (no OS process init)

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# Trigger via CLI
flutter run  # then press 'R' for Hot Restart

# Or keyboard shortcut in IDE: Shift+R

Hot Reload vs Hot Restart (Bonus)

Full Comparison

Improving Cold Start Performance

dartCopy
// 1. Use const widgets to reduce build time
const MyWidget();

// 2. Defer non-critical initialization
void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(MyApp()); // Show UI first

  // Initialize non-critical things after first frame
  WidgetsBinding.instance.addPostFrameCallback((_) async {
    await Analytics.init();
    await RemoteConfig.fetch();
  });
}

// 3. Use AOT compilation for production
// flutter build apk --release

Summary: Cold Start = full fresh launch from OS (slowest, production scenario). Hot Restart = re-runs Dart code in existing VM (dev tool, loses all state). Hot Reload = updates UI without re-running anything (fastest, preserves state).