Navigation

For navigable contents, we have a custom compose-based backstack implementation that the androidx folks shared with us. Navigation becomes two parts:

1. A BackStack, where we use a SaveableBackStack implementation that saves a stack of Screens and the ProvidedValues for each record on that stack (allowing us to save and restore on configuration changes automatically).
2. A Navigator, which is a simple interface that we can point at a BackStack and offers simple goTo(<screen>)/pop() semantics. These are offered to presenters to perform navigation as needed to other screens.

A new navigable content surface is handled via the NavigableCircuitContent functions.

setContent {
  val backStack = rememberSaveableBackStack(root = HomeScreen)
  val navigator = rememberCircuitNavigator(backStack)
  NavigableCircuitContent(navigator, backStack)
}

Warning

SaveableBackStack must have a size of 1 or more after initialization. It’s an error to have a backstack with zero items.

Presenters are then given access to these navigator instances via Presenter.Factory (described in Factories), which they can save if needed to perform navigation.

fun showAddFavorites() {
  navigator.goTo(
    AddFavorites(
      externalId = uuidGenerator.generate()
    )
  )
}

If you want to have custom behavior for when back is pressed on the root screen (i.e. backstack.size == 1), you should implement your own BackHandler and use it before creating the backstack.

setContent {
  val backStack = rememberSaveableBackStack(root = HomeScreen)
  BackHandler(onBack = { /* do something on root */ })
  // The Navigator's internal BackHandler will take precedence until it is at the root screen.
  val navigator = rememberCircuitNavigator(backstack)
  NavigableCircuitContent(navigator, backstack)
}

Results

In some cases, it makes sense for a screen to return a result to the previous screen. This is done by using the an answering Navigator pattern in Circuit.

The primary entry point for requesting a result is the rememberAnsweringNavigator API, which takes a Navigator or BackStack and PopResult type and returns a navigator that can go to a screen and await a result.

Result types must implement PopResult and are used to carry data back to the previous screen.

The returned navigator should be used to navigate to the screen that will return the result. The target screen can then pop the result back to the previous screen and Circuit will automatically deliver this result to the previous screen’s receiver.

var photoUri by remember { mutableStateOf<String?>(null) }
val takePhotoNavigator = rememberAnsweringNavigator<TakePhotoScreen.Result>(navigator) { result ->
  photoUri = result.uri
}

// Elsewhere
takePhotoNavigator.goTo(TakePhotoScreen)

// In TakePhotoScreen.kt
data object TakePhotoScreen : Screen {
  @Parcelize
  data class Result(val uri: String) : PopResult
}

class TakePhotoPresenter {
  @Composable fun present(): State {
    // ...
    navigator.pop(result = TakePhotoScreen.Result(photoUri))
  }
}

Circuit automatically manages saving/restoring result states and ensuring that results are only delivered to the original receiver that requested it. If the target screen does not pop back a result, the previous screen’s receiver will just never receive one.

When to use an Overlay vs navigating to a Screen with result?

See this doc in Overlays!

Nested Navigation

Navigation carries special semantic value in CircuitContent as well, where it’s common for UIs to want to curry navigation events emitted by nested UIs. For this case, there’s a CircuitContent overload that accepts an optional onNavEvent callback that you must then forward to a Navigator instance.

@Composable fun ParentUi(state: ParentState, modifier: Modifier = Modifier) {
  CircuitContent(NestedScreen, modifier = modifier, onNavEvent = { navEvent -> state.eventSink(NestedNav(navEvent)) })
}

@Composable fun ParentPresenter(navigator: Navigator): ParentState {
  return ParentState(...) { event ->
    when (event) {
      is NestedNav -> navigator.onNavEvent(event.navEvent)
    }
  }
}

@Composable 
fun NestedPresenter(navigator: Navigator): NestedState {
  // These are forwarded up!
  navigator.goTo(AnotherScreen)

  // ...
}