Running, debugging & hot reload


Running, debugging, and hot reload

Once you’ve integrated the Flutter module to your project and used Flutter’s platform APIs to run the Flutter engine and/or UI, you can then build and run your Android or iOS app the same way you run normal Android or iOS apps.

However, Flutter is now powering the UI in places where you’re showing a FlutterActivity or FlutterViewController.


You may be used to having your suite of favorite Flutter debugging tools available to you automatically when running flutter run or an equivalent command from an IDE. But you can also use all your Flutter debugging functionalities such as hot reload, performance overlays, DevTools, and setting breakpoints in add-to-app scenarios.

These functionalities are provided by the flutter attach mechanism. flutter attach can be initiated through different pathways, such as through the SDK’s CLI tools, through VSCode or IntelliJ/Android Studio.

flutter attach via terminal
flutter attach via terminal
flutter attach via VSCode
flutter attach via VSCode
flutter attach via IntelliJ
flutter attach via IntelliJ

flutter attach can connect as soon as you run your FlutterEngine, and remains attached until your FlutterEngine is disposed. But you can invoke flutter attach before starting your engine. flutter attach waits for the next available Dart VM that is hosted by your engine.

In IntelliJ or VSCode, you should select the device on which the Flutter module runs so flutter attach filters for the right start signals.

Debugging specific instances of Flutter

It’s possible to add multiple instances of Flutter (root isolates) to an app. flutter attach connects to all of the available isolates by default. Any commands sent from the attached CLI are then forwarded to each of the attached isolates.

You can list all the attached isolates by typing l from an attached flutter CLI tool. If unspecified, then the isolate names are automatically generated from the dart entrypoint file and function name.

Example l output for an application that is displaying two Flutter isolates simultaneously:

Connected views:
  main.dart$main-517591213 (isolates/517591213)
  main.dart$main-332962855 (isolates/332962855)

In order to attach to specific isolates instead, do the following:

  1. Name the Flutter root isolate of interest in its Dart source.

     // main.dart
     import 'dart:ui' as ui;
     void main() {
       ui.window.setIsolateDebugName("debug isolate");
       // ...
  2. Run flutter attach with the --isolate-filter option.

     $ flutter attach --isolate-filter='debug'
     Waiting for a connection from Flutter...
     Syncing files to device...      1.1s
     🔥  To hot reload changes while running, press "r".
     To hot restart (and rebuild state), press "R".
     An Observatory debugger and profiler is available
     For a more detailed help message,
     press "h". To detach, press "d"; to quit, press "q".
     Connected view:
       debug isolate (isolates/642101161)
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