Is there a way to keep fragment alive when using B

问题:

I'm trying to use the new navigation component. I use a BottomNavigationView with the navController : NavigationUI.setupWithNavController(bottomNavigation, navController)

But when I'm switching fragments, they are each time destroy/create even if they were previously used.

Is there a way to keep alive our main fragments link to our BottomNavigationView?

回答1:

Try this.

Navigator

Create custom navigator.

@Navigator.Name("custom_fragment")  // Use as custom tag at navigation.xml
class CustomNavigator(
    private val context: Context,
    private val manager: FragmentManager,
    private val containerId: Int
) : FragmentNavigator(context, manager, containerId) {

    override fun navigate(destination: Destination, args: Bundle?, navOptions: NavOptions?) {
        val tag = destination.id.toString()
        val transaction = manager.beginTransaction()

        val currentFragment = manager.primaryNavigationFragment
        if (currentFragment != null) {
            transaction.detach(currentFragment)
        }

        var fragment = manager.findFragmentByTag(tag)
        if (fragment == null) {
            fragment = destination.createFragment(args)
            transaction.add(containerId, fragment, tag)
        } else {
            transaction.attach(fragment)
        }

        transaction.setPrimaryNavigationFragment(fragment)
        transaction.setReorderingAllowed(true)
        transaction.commit()

        dispatchOnNavigatorNavigated(destination.id, BACK_STACK_DESTINATION_ADDED)
    }
}

NavHostFragment

Create custom NavHostFragment.

class CustomNavHostFragment: NavHostFragment() {
    override fun onCreateNavController(navController: NavController) {
        super.onCreateNavController(navController)
        navController.navigatorProvider += PersistentNavigator(context!!, childFragmentManager, id)
    }
}

navigation.xml

Use custom tag instead of fragment tag.

<navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto" android:id="@+id/navigation"
    app:startDestination="@id/navigation_first">

    <custom_fragment
        android:id="@+id/navigation_first"
        android:name="com.example.sample.FirstFragment"
        android:label="FirstFragment" />
    <custom_fragment
        android:id="@+id/navigation_second"
        android:name="com.example.sample.SecondFragment"
        android:label="SecondFragment" />
</navigation>

activity layout

Use CustomNavHostFragment instead of NavHostFragment.

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="com.example.sample.CustomNavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintBottom_toTopOf="@+id/bottom_navigation"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/navigation" />

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottom_navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:menu="@menu/navigation" />

</androidx.constraintlayout.widget.ConstraintLayout>

---

Update

I created sample project. link

I don't create custom NavHostFragment. I use navController.navigatorProvider += navigator.

回答2:

Google samples link Just copy NavigationExtensions to your application and configure by example. Works great.

回答3:

After many hours of research I found solution. It was all the time right in front of us :) There is a function: popBackStack(destination, inclusive) which navigate to given destination if found in backStack. It returns Boolean, so we can navigate there manually if the controller won't find the fragment.

if(findNavController().popBackStack(R.id.settingsFragment, false)) {
        Log.d(TAG, "SettingsFragment found in backStack")
    } else {
        Log.d(TAG, "SettingsFragment not found in backStack, navigate manually")
        findNavController().navigate(R.id.settingsFragment)
    }

回答4:

I've used the link provided by @STAR_ZERO and it works fine. For those who having problem with the back button, you can handle it in the activity / nav host like this.

override fun onBackPressed() {
        if(navController.currentDestination!!.id!=R.id.homeFragment){
            navController.navigate(R.id.homeFragment)
        }else{
            super.onBackPressed()
        }
    }

Just check whether current destination is your root / home fragment (normally the first one in bottom navigation view), if not, just navigate back to the fragment, if yes, only exit the app or do whatever you want.

Btw, this solution need to work together with the solution link above provided by STAR_ZERO, using keep_state_fragment.

回答5:

The solution provided by @piotr-prus helped me, but I had to add some current destination check:

if (navController.currentDestination?.id == resId) {
    return       //do not navigate
}

without this check current destination is going to recreate if you mistakenly navigate to it, because it wouldn't be found in back stack.

回答6:

Not available as of now.

As a workaround you could store all your fetched data into view model and have that data readly available when you recreate the fragment. Make sure you get the view using activities context.

You can use LiveData to make your data lifecycle-aware observable

回答7:

If you have trouble passing arguments add:

fragment.arguments = args

in class KeepStateNavigator