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Debugging.md

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Debugging Desktop Flutter Applications

While the current workflow for debugging on desktop is not ideal, it is possible to do most things, including source-level debugging and hot reload.

Because this relies on workarounds for desktop not being a supported device, Flutter changes may break the workflows described below. We hope to extend the flutter tool to improve support, but in the meantime please file bugs in this project if you encounter issues with these instructions.

Getting the Observatory Port

For all of these approaches, you will need the Observatory port of the desktop application. Usually, flutter run would handle this, but there's currently no way to flutter run a desktop application, so you will need to provide it manually.

There are two options:

  1. Find the port in the console. After launching your application, check the console output (your terminal in Linux, Console in Xcode, etc.) for a line like:

    Observatory listening on http://127.0.0.1:49494/

    The port in this case is 49494. This will change on every run of the application, so you'll need to repeat this every time you relaunch.

  2. Hard-code the port. In your embedder code, add --observatory-port=49494 (substituting a port of your choice) to the list of arguments passed to the engine. If you are using example/, or code based on it, look for the line that adds --disable-dart-asserts for an example of adding arguments. (Be sure not to add the observatory-port argument inside the #if, however.)

Debugging

Command Line

The flutter attach command can connect to a desktop Flutter application and provide the same interactive command line that flutter run would provide. In the directory of the Flutter portion of your application (e.g., /example/flutter_app) run:

$ flutter attach --device-id=flutter-tester --debug-port=49494

Use the port you obtained above in place of 49494. The device-id is necessary to bypass checks for an attached iOS or Android device.

VS Code

Open the Flutter portion of your application (e.g., /example/flutter_app). Add a launch configuration like the following, substituting your Observatory port:

  {
    "name": "Flutter Desktop Attach",
    "request": "attach",
    "deviceId": "flutter-tester",
    "observatoryUri": "http://127.0.0.1:49494/",
    "type": "dart"
  }

You will likely want to hard-code your observatory port, otherwise you will need to change launch.json every time you relaunch the app.

In addition to the Flutter debug commands, source-level debugging with pause, continue, variable inspection, etc. should all work.

IntelliJ

TBD. If you test IntelliJ and are able to attach successfully, please contribute instructions!