Hot reload and hot restart in Flutter - Deep Dive

Overview - Hot reload and hot restart

What is it?

Hot reload and hot restart are features in Flutter that help developers see changes in their app instantly without losing all the app's state. Hot reload updates the code and UI quickly while keeping the app running, so you don't have to start over. Hot restart reloads the entire app from scratch, resetting everything to the initial state. These tools speed up development by making it easier to try changes and fix bugs fast.

Why it matters

Without hot reload and hot restart, developers would have to stop the app and start it again every time they make a change, which wastes a lot of time. These features let you see your changes immediately, making the development process smoother and more fun. This means apps get built faster and with fewer mistakes, improving the experience for both developers and users.

Where it fits

Before learning hot reload and hot restart, you should understand how Flutter apps run and the basics of Flutter widgets. After mastering these features, you can explore advanced Flutter debugging, state management, and performance optimization to build better apps.

Mental Model

Core Idea

Hot reload updates your app's code instantly without restarting, while hot restart reloads the whole app, resetting its state.

Think of it like...

Imagine you are writing a story on paper. Hot reload is like quickly erasing and rewriting a sentence without starting the whole page over. Hot restart is like tearing off the page and starting a fresh new page from the beginning.

┌───────────────┐       ┌───────────────┐
│  Hot Reload   │──────▶│ Update Code & │
│               │       │ UI Instantly  │
│ Keeps State   │       └───────────────┘
└───────────────┘
         │
         ▼
┌───────────────┐       ┌───────────────┐
│ Hot Restart   │──────▶│ Restart Whole │
│               │       │ App & Reset   │
│ Resets State  │       └───────────────┘
└───────────────┘

Build-Up - 7 Steps

1

Foundation

What is Hot Reload in Flutter

Concept: Hot reload lets you change your app's code and see the results immediately without restarting the app or losing its current state.

When you make a change in your Flutter code and save it, hot reload injects the updated code into the running app. This updates the UI and logic instantly, so you can see your changes right away. It works best for changes in the UI, like colors, layouts, or text.

Result

The app updates on screen immediately with your changes, and the app keeps running where you left off.

Understanding hot reload helps you develop faster by avoiding full app restarts and preserving your app's current state.

2

Foundation

What is Hot Restart in Flutter

Concept: Hot restart reloads the entire app from the beginning, clearing all current app data and state.

When you perform a hot restart, Flutter stops the app's Dart code and starts it fresh. This means all variables, states, and UI reset to their initial values. Hot restart is useful when changes affect app initialization or when the app is in a broken state after hot reload.

Result

The app restarts fully, showing the initial screen and state as if you just launched it.

Knowing when to use hot restart helps you fix issues that hot reload cannot, especially for deep changes.

3

Intermediate

Differences Between Hot Reload and Hot Restart

🤔Before reading on: Do you think hot reload and hot restart both reset the app state? Commit to yes or no.

Concept: Hot reload updates code and UI without resetting state; hot restart reloads the whole app and resets state.

Hot reload is fast and keeps your app running with current data. Hot restart is slower and resets everything. Hot reload works well for UI tweaks and logic changes that don't affect app startup. Hot restart is needed for changes in main(), global variables, or app initialization.

Result

You learn when to use each tool to save time and avoid bugs during development.

Understanding the difference prevents confusion and wasted time restarting unnecessarily.

4

Intermediate

How Hot Reload Preserves App State

🤔Before reading on: Do you think hot reload can update stateful widgets without losing their data? Commit to yes or no.

Concept: Hot reload injects updated code into the running app without rebuilding the whole app, so stateful widgets keep their data.

Flutter keeps the widget tree and state objects intact during hot reload. It only updates the code for widgets that changed. This means user input, scroll positions, and other stateful data stay as they are, making it easy to test UI changes without losing progress.

Result

Your app UI updates but user data and interactions remain unchanged.

Knowing how state is preserved helps you trust hot reload and use it confidently.

5

Intermediate

When Hot Reload Cannot Apply Changes

🤔Before reading on: Can hot reload update changes in app startup code like main()? Commit to yes or no.

Concept: Hot reload cannot apply changes that affect app startup or global state; hot restart is needed for those.

If you change code in main(), global variables, or static initializers, hot reload won't apply those changes because the app is already running. You must do a hot restart to reload the entire app and apply those changes.

Result

You learn to recognize when hot reload won't work and avoid confusion.

Understanding hot reload limits saves time and prevents frustration when changes don't appear.

6

Advanced

How Flutter Implements Hot Reload Internally

🤔Before reading on: Do you think hot reload replaces the entire app code or just parts? Commit to one answer.

Concept: Flutter's hot reload injects updated source code into the running Dart Virtual Machine, updating classes and functions without restarting the app.

Flutter uses the Dart VM's ability to load new code dynamically. When you hot reload, Flutter sends the updated source code to the VM, which updates the running app's classes and functions. The widget tree rebuilds with new code, but the app's state objects remain intact. This process is fast because it avoids restarting the whole app.

Result

You understand the technical magic behind hot reload's speed and state preservation.

Knowing the internal mechanism explains why some changes require hot restart and others don't.

7

Expert

Limitations and Edge Cases of Hot Reload

🤔Before reading on: Can hot reload fix all bugs instantly? Commit to yes or no.

Concept: Hot reload has limits: it can't update native code, change app initialization, or fix all bugs instantly.

Hot reload only updates Dart code. Changes in native Android/iOS code, assets, or plugins require a full app restart. Also, some state changes or bugs may persist after hot reload, needing hot restart or full rebuild. Understanding these limits helps you choose the right tool and avoid wasted effort.

Result

You gain realistic expectations and strategies for using hot reload and restart effectively.

Knowing hot reload's boundaries helps you troubleshoot faster and avoid confusion in complex apps.

Under the Hood

Flutter runs Dart code inside a Dart Virtual Machine (VM) during development. Hot reload works by sending updated Dart source code to the VM, which dynamically updates classes and functions without stopping the app. The Flutter framework then rebuilds the widget tree using the new code, preserving existing state objects. Hot restart stops the Dart VM and restarts the app from main(), resetting all state and variables.

Why designed this way?

Flutter's hot reload was designed to speed up development by reducing the time between code changes and seeing results. Using the Dart VM's dynamic code loading avoids full app restarts, which are slow. This design balances fast feedback with the complexity of preserving app state. Alternatives like full rebuilds were too slow, and partial reloads without state preservation would hurt developer productivity.

┌───────────────┐       ┌───────────────┐       ┌───────────────┐
│ Developer     │──────▶│ Dart VM       │──────▶│ Widget Tree   │
│ Changes Code  │       │ Injects Code  │       │ Rebuilds UI   │
└───────────────┘       └───────────────┘       └───────────────┘
         │                      │                      │
         │                      │                      ▼
         │                      │               ┌───────────────┐
         │                      │               │ State Objects │
         │                      │               │ Preserved     │
         │                      │               └───────────────┘
         ▼                      ▼                      
┌───────────────┐       ┌───────────────┐             
│ Hot Reload    │       │ Hot Restart   │             
│ Injects Code  │       │ Restarts App  │             
│ Keeps State   │       │ Resets State  │             
└───────────────┘       └───────────────┘             

Myth Busters - 4 Common Misconceptions

Quick: Does hot reload reset your app's state? Commit to yes or no.

Common Belief: Hot reload restarts the app and resets all data.

Reality: Hot reload updates code and UI without resetting the app's current state or data.

Why it matters: Believing this causes developers to avoid hot reload and waste time restarting the app unnecessarily.

Quick: Can hot reload apply changes to the main() function? Commit to yes or no.

Common Belief: Hot reload can update any part of the app, including main().

Reality: Hot reload cannot update main() or global variables; hot restart is required for those changes.

Why it matters: Misunderstanding this leads to confusion when changes to startup code don't appear after hot reload.

Quick: Does hot restart keep your app's current screen and data? Commit to yes or no.

Common Belief: Hot restart keeps the app running exactly where you left off.

Reality: Hot restart reloads the entire app and resets all state, returning to the initial screen.

Why it matters: Expecting state preservation causes frustration and lost work when hot restart resets the app.

Quick: Can hot reload update native Android or iOS code? Commit to yes or no.

Common Belief: Hot reload updates all code, including native platform code.

Reality: Hot reload only updates Dart code; native code changes require a full app restart.

Why it matters: Not knowing this causes wasted time trying hot reload for native code changes that won't appear.

Expert Zone

1

Hot reload preserves state by keeping existing State objects and only rebuilding widgets, but changes to State class fields may not update unless the widget rebuilds.

2

Hot restart clears the Dart VM's memory, so static variables and singletons reset, which can fix hard-to-find bugs caused by stale state.

3

Hot reload speed depends on the size of the app and complexity of changes; very large apps may see slower reloads, requiring optimization.

When NOT to use

Avoid relying on hot reload when changing native platform code, app initialization logic, or global state. Use full app restart or rebuild instead. Also, for production testing, always do a full restart to ensure app behaves correctly from a fresh start.

Production Patterns

Developers use hot reload for quick UI tweaks and logic fixes during development. Hot restart is used when changing app startup code or after complex state changes. Continuous integration systems and release builds always use full rebuilds to ensure clean app state.

Connections

Live Programming

Hot reload is a form of live programming where code changes reflect immediately in the running program.

Understanding hot reload connects to the broader idea of live programming, which improves developer feedback loops across many languages.

State Management

Hot reload preserves app state, so understanding state management helps predict how changes affect the app during reload.

Knowing state management concepts clarifies why some changes appear instantly and others require restart.

Software Development Feedback Loops

Hot reload shortens the feedback loop between writing code and seeing results, a key principle in agile development.

Recognizing hot reload as a feedback loop tool helps appreciate its role in faster, iterative app building.

Common Pitfalls

#1 Expecting hot reload to apply changes in main() or global variables.

Wrong approach:

Change main() function code and press hot reload expecting the app to restart automatically.

Correct approach:

After changing main() or global variables, perform a hot restart to reload the entire app.

Root cause:

Misunderstanding that hot reload only updates running code and cannot restart app initialization.

#2 Using hot reload to fix bugs caused by stale or corrupted app state.

Wrong approach:

Try hot reload repeatedly to fix UI glitches or logic errors without restarting the app.

Correct approach:

Perform a hot restart or full app restart to clear app state and fix persistent bugs.

Root cause:

Not realizing that hot reload preserves state, so some bugs require resetting the app.

#3 Trying to hot reload changes in native Android or iOS code.

Wrong approach:

Modify native platform code and press hot reload expecting changes to appear immediately.

Correct approach:

Stop the app and rebuild or restart it fully to apply native code changes.

Root cause:

Confusing Dart code hot reload with native platform code updates.

Key Takeaways

Hot reload updates your Flutter app's code and UI instantly without restarting or losing app state.

Hot restart reloads the entire app from scratch, resetting all state and variables to initial values.

Use hot reload for quick UI and logic changes; use hot restart for changes in app startup or global state.

Hot reload works by injecting updated Dart code into the running Dart VM, preserving state objects.

Knowing the limits of hot reload helps avoid confusion and speeds up your Flutter development workflow.