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David Vávra

based in Prague, Czechia
Google Developer Expert for Android, Play, and Pay
Android Dev Lead at Air Bank Germany
CEO at Settle Up

Twitter: @vavradav

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Why to use Jetpack Navigation?

All navigation at one place (XML)
No more Fragment transactions
Fragments have guaranteed arguments (SafeArgs)
Back and Up navigation handled correctly
Support for deep links
Android Studio support
Recommended, stable (2.1.0)

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Air Bank Germany app architecture

100% Kotlin
single activity
multi-module
Android ViewModels (MVVM)
Koin DI
RxJava

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Where to put navigation XML?

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How to navigate from ViewModels?

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NavigationCommand.kt

sealed class NavigationCommand {

data class To(val directions: NavDirections): NavigationCommand()

object Back: NavigationCommand()

data class BackTo(val destinationId: Int): NavigationCommand()

object ToRoot: NavigationCommand()

}

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BaseViewModel.kt

// Command is SingleLiveEvent: http://bit.ly/arch-blueprints

val navigationCommands = Command<NavigationCommand>()

fun navigate(directions: NavDirections) {

navigationCommands.postValue(NavigationCommand.To(directions))

}

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BaseFragment.kt

override fun onActivityCreated(savedInstanceState: Bundle?) {

super.onActivityCreated(savedInstanceState)

vm.navigationCommands.observe { command ->

when (command) {

is NavigationCommand.To ->

findNavController().navigate(command.directions)

…

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CardListViewModel.kt

navigate(CardListFragmentDirections.cardDetail(cardId))

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How to use arguments in ViewModels?

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BaseFragment.kt

override fun onCreate(savedInstanceState: Bundle?) {

super.onCreate(savedInstanceState)

if (savedInstanceState == null) {

vm.setArguments(arguments)

vm.loadData()

}

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CardDetailViewModel.kt

override fun loadData() {

val cardId = CardDetailFragmentArgs.fromBundle(args).cardId

load(cardsRepository.getCard(cardId)) {

…

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How to show a login screen with deep links?

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Documentation on conditional navigation

ProfileFragment →LoginFragment → ProfileFragment
Disadvantages

LoginFragment doesn’t behave like a root destination
ProfileFragment starts initializing

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MainActivity.kt

override fun onStart() {

super.onStart()

if (sessionRepository.isLoggedOut()) {

startActivity<LoginActivity>()

finish()

}

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MainActivity.kt

val intent = Intent(this, LoginActivity::class.java)

intent.putExtra("deepLinkExtras", this.intent.extras)

startActivity(intent)

finish()

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LoginActivity.kt

val intent = Intent(this, MainActivity::class.java)

intent.putExtras(this.intent.getBundleExtra("deepLinkExtras"))

startActivity(intent)

finish()

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How to navigate back with a result?

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NavigationResult.kt

interface NavigationResult {

fun onNavigationResult(result: Bundle)

}

Documentation: shared ViewModel

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BaseActivity.kt

fun navigateBackWithResult(result: Bundle) {

val childFragmentManager = supportFragmentManager.findFragmentById(R.id.nav_host_fragment)?.childFragmentManager

var backStackListener: FragmentManager.OnBackStackChangedListener by Delegates.notNull()

backStackListener = FragmentManager.OnBackStackChangedListener {

(childFragmentManager?.fragments?.get(0) as NavigationResult).onNavigationResult(result)

childFragmentManager.removeOnBackStackChangedListener(backStackListener)

}

childFragmentManager?.addOnBackStackChangedListener(backStackListener)

navController().popBackStack()

}

http://bit.ly/nav-comp

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How to plug in a complex screen with a lot of animations?

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Our dashboard

Multi-account dashboard: ProductListFragment
Product detail: ProductDetailFragment
Single-account dashboard: ProductDetailFragment with all navigation as ProductListFragment
Very custom animations between those fragments
Custom account switcher

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DashboardFragment

only this Fragment is in the nav graph
ProductListFragment and ProductDetailFragment are child fragments
We handle the fragment transactions �and animations ourselves

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TL;DR

multi-module = put nav XML into common-android
navigate from ViewModels using SingleLiveEvents, put all boilerplate to base classes
pass SafeArgs arguments to ViewModel
different Activity for login screen, pass intent extras for deep link support
Use our solution for navigation with result
You can handle Fragment transactions yourself in some complex cases

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Thank you

Medium post with all the code:

http://bit.ly/nav-comp

Twitter: @vavradav