Let the button do safe navigation Mao Yufeng 

Self-Introduction Mao Yufeng • @myf_pa • https://github.com/lcdsmao • https://medium.com/@lcdsmao • CyberAgent/CATS 

Ensure navigation is safe 

Navigation Graph: 

button_navigate_bar.setOnClickListener { findNavController().navigate(R.id.action_fooFragment_to_barFragment) } FooFragment.kt: 

java.lang.IllegalArgumentException: navigation destination com.github.lcdsmao.uievent:id action_fooFragment_to_barFragment is unknown to this NavController at androidx.navigation.NavController.navigate(NavController.java:789) at androidx.navigation.NavController.navigate(NavController.java:730) at androidx.navigation.NavController.navigate(NavController.java:716) at androidx.navigation.NavController.navigate(NavController.java:704) at com.github.lcdsmao.uievent.FooFragment$onCreateView$$inlined$apply$lambda$1.onClick(FooFragment.kt:22) at android.view.View.performClick(View.java:6256) at android.view.View$PerformClick.run(View.java:24710) at android.os.Handler.handleCallback(Handler.java:789) at android.os.Handler.dispatchMessage(Handler.java:98) at android.os.Looper.loop(Looper.java:251) at android.app.ActivityThread.main(ActivityThread.java:6572) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) This exception occurs when calling Action that does not belong to the current Fragment 

nav_main.xml: 

nav_main.xml: 

button_navigate_bar.setOnClickListener { findNavController().navigate(R.id.action_fooFragment_to_barFragment) findNavController().navigate(R.id.action_fooFragment_to_barFragment) } FooFragment.kt: 

NavController holds its own back stack of navigation info and check the state before each navigation private final Deque mBackStack = new ArrayDeque<>(); NavController.java 

button_navigate_bar.setOnClickListener { [email protected]("Click NavigateBarMaybeCrash") [email protected]("Before Call Navigate") findNavController().navigate(R.id.action_fooFragment_to_barFragment) [email protected]("After Call Navigate") } V/FooFragment: Click NavigateBar V/FooFragment: Before Call Navigate V/FooFragment: After Call Navigate V/BarFragment: onAttach V/BarFragment: onCreate V/BarFragment: onViewCreated V/BarFragment: onActivityCreated V/BarFragment: onStart V/BarFragment: onResume V/FooFragment: onPause V/FooFragment: onStop V/FooFragment: onDestroyView FragmentTransaction will not be performed immediately FooFragment.kt: Pending input events are canceled at onStop 

The View system is completely independent from any higher level framework such as Fragments or Navigation, so there's not much we can do. https://issuetracker.google.com/issues/136024230 

button_navigate_bar.setOnClickListener { val navController = findNavController() navController.navigate(R.id.action_fooFragment_to_barFragment) } Check to see if we haven't already navigated away 

button_navigate_bar.setOnClickListener { val navController = findNavController() if (navController.currentDestination?.id == R.id.fooFragment) { navController.navigate(R.id.action_fooFragment_to_barFragment) } } Check to see if we haven't already navigated away 

fun NavController.safeNavigate( @IdRes currentId: Int, @IdRes destId: Int, ) { if (currentDestination?.id == currentId) { navigate(destId) } } NavigationExt.kt: 

BaseFragment.kt: abstract class BaseFragment : Fragment() { @IdRes open val navId: Int = 0 protected fun navigate(@IdRes destId: Int) { require(navId != 0) { "Need fragment id in navGraph for safe navigation" } findNavController().safeNavigate(desId) } } class FooFragment : BaseFragment() { override val navId: Int = R.id.fooFragment override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) button_navigate_bar.setOnClickListener { navigate(R.id.action_fooFragment_to_barFragment) } } } FooFragment.kt: 

V/FooFragment: Click ShowDialog V/FooFragment: Before Call Navigate V/FooFragment: After Call Navigate V/DialogFragment: onAttach V/DialogFragment: onCreate V/DialogFragment: onCreateDialog V/DialogFragment: onActivityCreated V/DialogFragment: onStart V/DialogFragment: onResume button_navigate_bar.setOnClickListener { [email protected]("Click NavigateBarMaybeCrash") [email protected]("Before Call Navigate") findNavController().navigate(R.id.action_fooFragment_to_barFragment) [email protected]("After Call Navigate") } Fragment will will not enter onStop when showing a dialogFragment 

button_navigate_bar.setOnClickListener { findNavController().safeNavigate( R.id.fooFragment, R.id.action_fooFragment_to_barFragment ) } Show dialog: 

Throttle the navigation destination 

button_navigate_bar.setOnClickListener { findNavController().navigate(R.id.action_fooFragment_to_barFragment) } Show dialog: 

V/FooFragment: Click NavigateBaz V/FooFragment: Before Call Navigate V/FooFragment: After Call Navigate V/FooFragment: onPause V/BazActivity: onCreate V/BazActivity: onStart V/BazActivity: onResume V/FooFragment: onStop V/FooFragment: Click NavigateBaz V/FooFragment: Before Call Navigate V/FooFragment: After Call Navigate V/FooFragment: onPause V/BazActivity: onCreate V/BazActivity: onStart V/BazActivity: onResume V/FooFragment: Click NavigateBaz V/FooFragment: Before Call Navigate V/FooFragment: After Call Navigate V/BazActivity: onPause V/BazActivity: onCreate V/BazActivity: onStart V/BazActivity: onResume V/BazActivity: onStop V/FooFragment: onStop Navigate to Activity once: Navigate to Activity twice: - No Crash - CurrentDestinationId not changed 

BaseFragment.kt: abstract class BaseFragment : Fragment() { @IdRes open val navId: Int = 0 protected fun navigate(@IdRes destId: Int) { require(navId != 0) { "Need fragment id in navGraph for safe navigation" } findNavController().safeNavigate(desId) } } 

BaseFragment.kt: abstract class BaseFragment : Fragment() { @IdRes open val navId: Int = 0 protected fun navigate(@IdRes destId: Int) { require(navId != 0) { "Need fragment id in navGraph for safe navigation" } if (viewLifecycleOwner.lifecycle.currentState.isAtLeast(Lifecycle.State.RESUMED)) { findNavController().safeNavigate(desId) } } } 

button_navigate_baz.setOnClickListener { navigate(R.id.action_fooFragment_to_bazActivity) navigate(R.id.action_fooFragment_to_bazActivity) } Two Activities will still be launched … 

button_navigate_baz.setOnClickListener { navigate(R.id.action_fooFragment_to_bazActivity) } Good enough to deal with the general situation 

• Check current destination ID before navigation to avoid the crash • Do navigation after onResume to avoid launch duplicate Activities Recap 

Thank you!