Gatekeeper workflow pattern

Gatekeeper.kt

/**
 * In our production code, this typealias is more interesting,
 * something like AlertContainerScreen<PanelContainerScreen<R>>
 */
typealias MetaRootRendering<R>: AlertContainerScreen<R>

data class ActivityAndRoot<R> (
  val activity: Activity,
  val root: R
)

/**
 * Receives our root UI and the current Activity as props.
 * When rendered, can choose to 
 *   - wrap the given root rendering with modals
 *   - replace it entirely
 *   - return `null` to indicate that it doesn't want to interfere
 */
interface GatekeeperWorkflow: 
  Workflow<ActivityAndRoot<Any>, Nothing, MetaRootRendering<Any>?>

class MetaRootWorkflow<R>(
  private val realRootWorkflow: Workflow<R>,
  val systemPermissionWorkflow: GatekeeperWorkflow,
  val oneTrustWorkflow: GatekeeperWorkflow
): StatelessWorkflow<Activity, Nothing, MetaRootRendering<R>> {
  /** In priority order. Dagger injected in real life. */
  private val gatekeepers = listOf(
    systemPermissionWorkflow, oneTrustWorkflow
  )

  override fun render(
    renderProps: Activity,
    context: RenderContext
  ): RootRendering<GatekeeperScreen> {
    // In this sketch we're always running the main UI, whether or
    // not we show its rendering.
    // 
    // You might instead want to prevent it from running until
    // both Gatekeepers render null, or run some blocking 
    // Gatekeepers first and another set after.

    val root = context.renderChild(realRootWorkflow)

    // Note that we are careful always to render all gatekeepers, even if upstream
    // ones are blocking downstream via non-null renderings.
    val gateOrNull = gatekeepers
      .mapNotNull { context.renderChild(it, ActivityAndRoot(renderProps, root) }
      .firstOrNull()

    return gateOrNull ?: AlertContainerScreen(root)
  }
}