Creating app extension UIs with Flutter

Some app extensions display a user interface.
For example, iMessage extensions allow users to access your app’s
content directly from the Messages app.

Flutter does not support 
building Flutter UI for app extensions. 
To create the UI for 
an app extension using Flutter, you must compile 
a custom engine and embed the FlutterViewController
as described in the following section.

Note:
  Creating an app extension user interface with Flutter
  requires you to compile a custom build of the Flutter engine.
  The Flutter team cautions that only advanced users
  should try to create a custom build.
  To learn more, check out Compiling the engine.

Create a custom build of the Flutter engine that removes uses of
sharedApplication and corrects the path for the bundle.
Check out an example from the community on GitHub.

Open the Flutter app project settings in Xcode to share build
configurations.