Screen based animation overrides

[stagg]

About

Building out a surface where navigation transitions can be overridden at navigation time based on the type of navigation event and the source/target screens.

Due to how shared element transitions work with AnimatedContent, we need some ability to completely replace the the transition spec before the target shared elements are loaded into the composition. Can look at PetDetailAnimatedNavigationOverride for an example of this.

AnimatedNavigationTransform usage

Transition Spec

• This method receives a AnimatedNavigationTransform.NavigationEvent indicating the type of
  navigation event that is occurring (GoTo, Pop, RootReset).
• It also has access to the current AnimatedContentTransitionScope, from which you can get the
  initialState and targetState of the transition, as well as use the helper functions.
• Return a ContentTransform if you want to override the default transition for the given
  NavigationEvent and state, return null otherwise.

Transition Selection

• When a transition is happening, the AnimatedNavDecoration will compare each of the available
  AnimatedNavigationTransforms and select the first one that is able to handle the current
  transition. If no AnimatedNavigationTransform is able to handle the transition, then the
  default transition provided by the AnimatedNavDecorator.defaultTransform will be used.

Example

object PetDetailAnimatedNavigationOverride : AnimatedNavigationTransform {

  override fun AnimatedContentTransitionScope<AnimatedNavState>.transitionSpec(
    navigationEvent: NavigationEvent
  ): ContentTransform? {
    return when {
      navigationEvent == NavigationEvent.GoTo &&
        initialState.screen is HomeScreen &&
        targetState.screen.isSharedElementDetailScreen() -> {
        // We're expecting shared elements to exist between the source and target so remove the
        // default animations.
        EnterTransition.None togetherWith ExitTransition.None
      }
      else -> null
    }
  }
}

The example checks if the user is navigating to the PetDetailScreen from the HomeScreen, with the information needed to tie together a shared element transition. If that is the case no transitions are used, otherwise it returns null to continue checking other AnimatedNavigationTransforms or fallback to the AnimatedNavDecorator.defaultTransform.

Demo

Notice how on the forward navigation the grid view is sliding out, on the pop AndroidPredictiveBackNavDecorator already removes the animation.

Before	After
before.mp4	after.mp4

Questions

• Does AnimatedNavigationTransform make sense as a general interceptor?
  • I'm a bit split on iterating over all of them at the moment in the animatedNavigationOverride method
• Should the override be directly tied to a particular Screen?
  • Like how Ui and Presenters are via CircuitInject, or other navigation setups.
• Should a user be able to provide an override at navigation time?
  • Think this is some extra property or context on the Navigator methods
• If the above all makes sense what should the DI story for this look like?
  • Thinking it'd be nice to also "CircuitInject" the AnimatedNavigationTransform implementations into the Circuit instance