PROTOCOL

SingleScreenContaining

Defines a type that semantically ‘contains’ a Screen instance.

The motivating use case for this protocol is to expose a means of unifying various types that provide access to Screens in a way that can be used without knowing their static type.

For example, without this protocol, we cannot identify that a UIViewController is an instance of ScreenViewController<S> without specifying the associated screen’s type.

func makeUninspectableScreenVC() -> UIViewController {
  struct LocalScreen: Screen { ... } // `LocalScreen` symbol is only visible in this function
  return ScreenViewController(screen: LocalScreen(), environment: .empty)
}

let isSVC = makeUninspectableScreenVC() is ScreenViewController<???> // no generic can be specified here that makes this true

Conceptually this API is intended to enable runtime traversal of a hierarchy of SingleScreenContaining instances such that an ‘inner’ Screen value can be found at runtime. For example, if we had an instance of ScreenViewController<AnyScreen> but only statically knew it was a UIViewController, we can use the conformances to this protocol to conditionally cast & traverse the contained screens to find the inner wrappedScreen.

Properties

primaryScreen

The primary Screen the conforming type contains. Note that this may be ambiguous in some cases, for instance, if the conforming type logically contains multiple screens. Implementors should return the Screen which most appropriately reflects the ‘primary’ one for a given domain.