To simplify how your app communicates with extensions,
Flutter plugins wrap these APIs.
To find plugins that wrap extension APIs,
check out Leveraging Apple’s System APIs and Frameworks or
search pub.dev.

Sharing resources

To share resources between your Flutter app and your app extension, put
the Runner app target and the extension target in the same
App Group.

Note:
  You must be signed in to your Apple Developer account.

To add a target to an App Group:

Open the target settings in Xcode.

Navigate to the Signing & Capabilities tab.

Select + Capability then App Groups.

Choose which App Group you want to add the target from one of two options:
    
      Select an App Group from the list.
      Click + to add a new App Group.