The user interface provides the bridge between the user interacting with the device and your application code. It is typically implemented in a very platform specific manner using classes provided by the platform.

At this point in time it is already a common practice for Android developers to protect their application code from the platform specific view code: logic is separated into different classes to provide a clear boundary between the application logic and the view.

In Acorn, the entire platform acts as a plugin to your application. Instead of depending on the Android platform to provide Activities or Fragments so you can start your logic, Acorn lets you view the Activity as a 'window' into your application. Regardless of whether the Activity showing your application is currently in the foreground or in the background, or whether the user rotates the device or changes the system language, your application and navigational state will just keep running until the OS decides to kill the app.

To be able to do this, Acorn lets the Activity subscribe to changes in the navigational state, and makes it its sole responsibility to provide the user interface. Upon starting the Activity the Activity will receive the current navigational state from Acorn, display the proper user interface and attach it to the currently active Scene. When the Activity disappears, albeit due to the user navigating to another application or rotating the device, it detaches the user interface from the Scene and stops listening to the navigational state.

The ext-acorn-android artifact provides several components that help bridge the gap between bare navigational state and displaying the user interface on Android.