How do I define default animations for Navigation Actions?

Question

I\'m using Android Studio 3.2 Canary 14 and The Navigation Architecture Component. With this you can define transition animations pretty much as you would when using Intents

Answer 1

It is possible with custom androidx.navigation.fragment.Navigator.

I will demonstrate how to override fragment navigation. Here is our custom navigator. Pay attention to setAnimations() method

@Navigator.Name("fragment")
class MyAwesomeFragmentNavigator(
    private val context: Context,
    private val manager: FragmentManager, // Should pass childFragmentManager.
    private val containerId: Int
): FragmentNavigator(context, manager, containerId) {
private val backStack by lazy {
    this::class.java.superclass!!.getDeclaredField("mBackStack").let {
        it.isAccessible = true
        it.get(this) as ArrayDeque
    }
}

override fun navigate(destination: Destination, args: Bundle?, navOptions: NavOptions?, navigatorExtras: Navigator.Extras?): NavDestination? {
    if (manager.isStateSaved) {
        logi("Ignoring navigate() call: FragmentManager has already"
                + " saved its state")
        return null
    }
    var className = destination.className
    if (className[0] == '.') {
        className = context.packageName + className
    }
    val frag = instantiateFragment(context, manager,
            className, args)
    frag.arguments = args
    val ft = manager.beginTransaction()

    navOptions?.let { setAnimations(it, ft) }

    ft.replace(containerId, frag)
    ft.setPrimaryNavigationFragment(frag)

    @IdRes val destId = destination.id
    val initialNavigation = backStack.isEmpty()
    // TODO Build first class singleTop behavior for fragments
    val isSingleTopReplacement = (navOptions != null && !initialNavigation
            && navOptions.shouldLaunchSingleTop()
            && backStack.peekLast()?.toInt() == destId)

    val isAdded: Boolean
    isAdded = if (initialNavigation) {
        true
    } else if (isSingleTopReplacement) { // Single Top means we only want one 
instance on the back stack
        if (backStack.size > 1) { // If the Fragment to be replaced is on the FragmentManager's
// back stack, a simple replace() isn't enough so we
// remove it from the back stack and put our replacement
// on the back stack in its place
            manager.popBackStack(
                    generateBackStackName(backStack.size, backStack.peekLast()!!.toInt()),
                    FragmentManager.POP_BACK_STACK_INCLUSIVE)
            ft.addToBackStack(generateBackStackName(backStack.size, destId))
        }
        false
    } else {
        ft.addToBackStack(generateBackStackName(backStack.size + 1, destId))
        true
    }
    if (navigatorExtras is Extras) {
        for ((key, value) in navigatorExtras.sharedElements) {
            ft.addSharedElement(key!!, value!!)
        }
    }
    ft.setReorderingAllowed(true)
    ft.commit()
    // The commit succeeded, update our view of the world
    return if (isAdded) {
        backStack.add(Integer(destId))
        destination
    } else {
        null
    }
}

private fun setAnimations(navOptions: NavOptions, transaction: FragmentTransaction) {
    transaction.setCustomAnimations(
            navOptions.enterAnim.takeIf { it != -1 } ?: android.R.anim.fade_in,
            navOptions.exitAnim.takeIf { it != -1 } ?: android.R.anim.fade_out,
            navOptions.popEnterAnim.takeIf { it != -1 } ?: android.R.anim.fade_in,
            navOptions.popExitAnim.takeIf { it != -1 } ?: android.R.anim.fade_out
    )
}

private fun generateBackStackName(backStackIndex: Int, destId: Int): String? {
    return "$backStackIndex-$destId"
}
}

In the next step we have to add the navigator to NavController. Here is an example how to set it:

 override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    val navHostFragment = supportFragmentManager.findFragmentById(R.id.fragmentContainer)!!
    with (findNavController(R.id.fragmentContainer)) {
        navigatorProvider += MyAwesomeFragmentNavigator(this@BaseContainerActivity, navHostFragment.childFragmentManager, R.id.fragmentContainer)
        setGraph(navGraphId)
    }
}

And nothing special in xml :)

Now each fragment from graph will have alpha transitions