How does Hot Reload or Hot Restart actually works: An insight under the hood.

Ashish Sharma
3 min readNov 16, 2023

Flutter, Google’s open-source UI toolkit, has gained immense popularity among developers for its ability to create beautiful and fast apps for various platforms, including iOS, Android, and the web. One of its standout features is “Hot Reload” and “Hot Restart,” which offer developers a dynamic and efficient development experience. But how do these features actually work? Let’s take a closer look under the hood.

Understanding Hot Reload and Hot Restart

Hot Reload:

Hot Reload is like a refresh button for your app. It enables developers to inject code changes into a running app without restarting the entire application. Here’s a glimpse of what happens:

  1. JIT Compilation: Flutter employs a Just-In-Time (JIT) compiler, a crucial element in the Hot Reload saga. When code alterations occur, Flutter’s JIT compiler springs into action.
  2. Selective Recompilation: The JIT compiler selectively recompiles the modified sections of code while the app is still running. This swift compilation process ensures that only the updated parts are refreshed.
  3. Seamless Update: Flutter’s ingenious “widget-diffing” algorithm steps in next. It analyzes the differences between the app’s current state and the updated code, intelligently applies these changes, and seamlessly updates the user interface.
  4. Real-Time Gratification: The result? A near-instantaneous reflection of the code changes on the device or emulator, providing developers with real-time feedback.

Hot Restart: A Clean Slate

In contrast to Hot Reload, Hot Restart offers a more profound renovation of the app, akin to a fresh canvas:

  1. Complete Rerun: Hot Restart initiates a complete restart of the app. It re-executes the main() function and reconstructs the widget tree, revitalizing the app from the ground up.
  2. Optimized Speed: Remarkably, despite the thorough restart, Flutter’s optimized mechanisms ensure this process happens with incredible speed, minimizing downtime.

Why it’s Handy

Making Life Easier for Developers

  • Try and See: You can try out new stuff instantly. Imagine painting a picture and changing colors right away without waiting for the paint to dry.
  • Fixing Things Fast: If something’s not right, you can fix it quickly and see if it works better without going back to the beginning.

Conclusion: Making Coding Fun and Quick

Hot Reload and Hot Restart are like magic buttons for app developers. They help you change things quickly and see the results right away, which makes coding feel more like playing and less like waiting.

That’s it for this article! I hope you enjoyed it and leave a few claps if you did. Follow me for more Flutter articles and comment for any feedback you might have about this article.

I‘m Ashish Sharma aka theFlutterist. You can find me on LinkedIn or maybe follow me on GitHub as well.

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Ashish Sharma

TheFlutterist: Teaching Flutter in the most playful way possible. Join me in the land of widgets and hot reloads!