What is the difference between hot reload and hot restart?

Behind the scenes: reload swaps class definitions while keeping object instances alive. Restart blows away the heap and starts fresh.

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What each gets you

class _CounterState extends State<Counter> {
  int _count = 5;

  @override
  Widget build(BuildContext context) =>
      Text('Count: $_count', style: TextStyle(color: Colors.red));
}

// You change Colors.red → Colors.blue and press:
// • Hot reload  → text turns blue, _count still 5
// • Hot restart → text turns blue, _count back to 0

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When you need a hot restart instead

If you see a "Some changes won't take effect" warning, that's Flutter telling you a restart is needed.

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When to reach for which

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Common mistakes to avoid

// ❌ "Hot reload didn't pick up my fix" — when you changed initState
@override
void initState() {
  super.initState();
  _loadConfig();          // edited this body → reload won't re-run it
}
// ✅ Hot restart, or temporarily call it from build() while iterating

// ❌ Confusing UI not updating with broken code
// Check the terminal — reload prints "1 Reloaded" or warnings
// If you don't see your widget update, something didn't apply.

// ❌ Hot reloading after upgrading flutter / dart SDK
// Always do a full restart (and sometimes `flutter clean`) after SDK changes

// ❌ Expecting reload to clear caches or rerun network calls
// Anything inside a one-time future / stream stays as-is until restart

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Interview follow-ups

1. Why does hot reload preserve state but hot restart doesn't? Reload patches class definitions in the running VM — existing objects keep their fields and identities, but next call to a method uses the new code. Restart blows away the isolate's heap and re-runs main(), so every object is newly constructed.

2. Why doesn't hot reload re-run initState or main? Because those are one-time setup. Reload assumes objects are still valid — it just refreshes their behaviour. Re-running setup would defeat the "preserve state" goal.

3. Why do enum changes require a restart? Enum values are canonical instances baked into existing objects. Changing the enum could leave objects holding stale references that no longer match the new definition. Restart guarantees consistency.

4. What's the difference between hot restart and a full app restart? Hot restart restarts the Dart VM only — keeping the embedder, native plugins, and engine warm. A full restart (stop and re-run) rebuilds native code too — needed when you touch iOS/Android sources or update Flutter itself.

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