How to debug Flutter apps

It is possible to do most things, including source-level debugging, profiling and hot reload when you debug Flutter apps on this embedder. Note that you need to use libflutter_engine.so built with debug mode.

Getting the Observatory port

You will find the following message in the console when you run Flutter apps. You need to attach a debugger to this port when you want to debug it. Note that the URI changes every time after launching.

flutter: Observatory listening on http://127.0.0.1:40409/k8IUol2dnPI=/

Debugging

Command line

$ cd <path_to_flutter_project>/
$ flutter attach --device-id=flutter-tester --debug-uri http://127.0.0.1:40409/k8IUol2dnPI=/

How to use command line options in the Dart VM

You can use the environment variables such as FLUTTER_ENGINE_SWITCHES, FLUTTER_ENGINE_SWITCH_* to use Dart VM command line options. If you want to fix always same observatory URI, you can use the following command. Note that this option is only available in debug mode. See also: How to debug the embedder

$ FLUTTER_ENGINE_SWITCHES=2 \
    FLUTTER_ENGINE_SWITCH_1="observatory-port=12345"  \
    FLUTTER_ENGINE_SWITCH_2="disable-service-auth-codes" \
    ./flutter-x11-clinet --bundle=sample/build/linux/x64/debug/bundle
flutter: Observatory listening on http://127.0.0.1:12345/

VS Code

Create a launch.json file like the following.

{
  "version": "0.2.0"
  "configurations": [
    {
      "type": "dart",
      "name": "Flutter Embedded Linux Attach",
      "request": "attach",
      "deviceId": "flutter-tester",
      "observatoryUri": "http://127.0.0.1:40409/k8IUol2dnPI=/"
    }
  ]
}