Alias renderings are too hard to deal with

[rjrjr]

The recent introduction of the Wrapper interface (#920) made it easier to create things like NamedScreen, where one rendering is used to modify how a wrapped one is displayed; but made it harder to implement "alias" types: custom renderings that are most easily displayed by transforming them into existing types.

This is a problem because we encourgage apps to use BodyAndOverlaysScreen as a building block to craft their own, stricter rules.

e.g., it should be trivial to register a ScreenViewFactory that handles this:

class MyAppRootUI(
  val body: Screen,
  val maybeExactlyOnePopupScreen: MySpecificOverlayType?
  val maybeAnAlertOnTopOfThat: AlertOverlay?
)

by transforming it into this:

BodyAndOverlaysScreen(
  body = myRoot.body,
  overlays = listOfNotNull(
    myRoot.maybeExactlyOnePopupScreen,
    myRoot.maybeAnAlertOnTopOfThat
)

By coupling the ScreenViewFactory.forWrapper to the new Wrapper type, we have actually made that much harder.

So, at least two outcomes needed here:

• Convenient API for this use case
• Introduce a sample demonstrating this pattern, both to document it and to make it painfully obvious when we work against it.