Hot Reload & Hot Restart

One of the main reasons developers love Flutter is its speed of development. Traditionally, when you make changes in your code, you must rebuild the application, which can take several minutes depending on the project size. Flutter changes this completely with its Hot Reload and Hot Restart features, making development faster, smoother, and more interactive.

In this detailed guide, we’ll explore:

What Hot Reload and Hot Restart mean
How they work internally
Key differences between the two
Practical scenarios where each should be used
Benefits for productivity and developer experience
Common issues and troubleshooting
Best practices for using them effectively

What is Hot Reload?

Hot Reload is a feature in Flutter that allows developers to inject updated source code directly into a running app without needing to restart the whole application.

When you press Hot Reload:

Flutter preserves the app state.
It rebuilds only the widget tree with the new changes.
You instantly see UI updates within milliseconds.

👉 Example: If you change the color of a button or text in your code, you’ll immediately see the new color in your running app without restarting.

What is Hot Restart?

Hot Restart is similar to Hot Reload but with a critical difference: it restarts the app from scratch without requiring a full rebuild.

When you press Hot Restart:

The entire application state is reset.
All widgets are rebuilt from the beginning.
It’s faster than a full restart but slower than hot reload.

👉 Example: If you add a new variable at the root of your app, you’ll need a Hot Restart to initialize and see the change.

How Hot Reload Works Internally

Flutter uses the Dart Virtual Machine (Dart VM).
The updated source code is injected into the VM.
Flutter’s widget tree is rebuilt using the new code.
The state objects are preserved, so user input, navigation, or data remain the same.

How Hot Restart Works Internally

The Dart VM discards the current app state.
A new instance of the main function is executed.
The entire widget tree is rebuilt from scratch.
Any temporary data is lost.

Key Differences Between Hot Reload & Hot Restart

Feature	Hot Reload	Hot Restart
Preserves App State	✅ Yes	❌ No
Speed	⚡ Instant	🚀 Fast but slower
Rebuilds Widget Tree	✅ Yes	✅ Yes
Re-executes `main()`	❌ No	✅ Yes
Use Case	UI changes, minor edits	New variables, logic changes

When to Use Hot Reload

UI modifications (colors, layouts, themes).
Fixing minor bugs in widgets.
Iterating on animations and styling.
Making quick design adjustments.

When to Use Hot Restart

Adding new classes, variables, or methods that need re-initialization.
Changing app-level logic (like main() or provider initialization).
When Hot Reload doesn’t reflect the change.
Debugging app lifecycle events.

Benefits for Productivity

Rapid Iteration – Test changes instantly without losing time.
State Preservation – Continue where you left off (great for forms and navigation).
Reduced Build Time – Avoid rebuilding the app completely.
Interactive UI Development – Experiment with designs in real time.

Common Issues and Fixes

Hot Reload not working
- Ensure the app is running in Debug Mode, not Release.
- Some code changes (like generics or enums) may require a restart.
State not updating
- Use Hot Restart if stateful changes are not reflected.
Crashes after reload
- Sometimes widget trees can get inconsistent. A Hot Restart usually fixes this.

Best Practices

Prefer Hot Reload for frequent UI changes.
Use Hot Restart when working on initialization logic.
Always save your files (Ctrl + S or Cmd + S) before reloading.
Get familiar with shortcuts:
- In Android Studio: Hot Reload (lightning bolt icon), Hot Restart (green arrow icon).
- In terminal: type r for reload, R for restart.
Combine with Flutter DevTools for smooth debugging.