Disabled by default, to enable use fl_dart_project_get_ui_thread_policy:

  g_autoptr(FlDartProject) project = fl_dart_project_new();
  fl_dart_project_set_ui_thread_policy(project, FL_UI_THREAD_POLICY_RUN_ON_PLATFORM_THREAD);

With this PR, the UI isolate now runs on platform thread, which allows for synchronous calls to Gtk, as well as synchronous calls from Gtk to Flutter. For example something like this is now possible:

typedef GtkWindow = Pointer<Void>;

void setWindowPosition() {
  final library = DynamicLibrary.executable();
  // Hypothetical function to get pointer to GtkWindow
  final getMainWindow = library
      .lookupFunction<GtkWindow Function(), GtkWindow Function()>(
        'get_main_window',
      );
  final gtkWindowResize = library.lookupFunction<
    Void Function(GtkWindow, Int32, Int32),
    void Function(GtkWindow, int, int)
  >('gtk_window_resize');
  final window = getMainWindow();
  gtkWindowResize(window, 200, 200);
}

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.