CODEWITHSUNDEEP
androidandroid-fragmentsandroid-jetpackandroid-viewmodelandroid-jetpack-navigation
martinseal1987
martinseal1987

Reputation: 2419

Scoping a viewmodel to multiple fragments (not activity) using the navigation component

I'm using the navigation component, I want a view model to be shared between a few fragments but they should be cleared when I leave the fragments (hence not scoping them to the activity) I'm trying to take the one activity many fragments approach. I have managed to achieve this using multiple nav hosts and scoping the fragments to it using getParentFragment but this just leads to more issues having to wrap fragments in other parent fragments, losing the back button working seamlessly and other hacks to get something to work that should be quite simple. Does anyone have a good idea on how to achieve this? I wondered if theres anything with getViewModelStore I could be using, given the image below I want to scope a view model to createCardFragment2 and use it in anything after it (addPredictions, editImageFragment, and others i haven't added yet), but then if I navigate back to mainFragment I want to clear the view models.

BTW I cant just call clear on mainFragment view model store as there are other view models here that shouldn't be cleared, I guess i want a way to tell the nav host what the parent fragment should be which I'm aware isn't going to be a thing, or a way to make the view model new if I'm navigating from mainFragment or cardPreviewFragment

nav graph

Upvotes: 32

Views: 13711

Answers (4)

Schadenfreude
Schadenfreude

Reputation: 1910

So based on the answers here I made a function that lazily returns a ViewModel scoped to the current navigation graph.

     private val scopedViewModel by lazy { getNavScopedViewModel(arg) }


    /**
     * The [navGraphViewModels] function is not entirely lazy, as we need to pass the graph id
     * immediately, but we cannot call [findNavController] to get the graph id from, before the
     * Fragment's [onCreate] has been called. That's why we wrap the call in a function and call it lazily.
     */
    fun getNavScopedViewModel(arg: SomeArg): ScopedViewModel {
        // The id of the parent graph. If you're currently in a destination within this graph
        // it will always return the same id
        val parentGraphScopeId = findNavController().currentDestination?.parent?.id
            ?: throw IllegalStateException("Navigation controller should already be initialized.")
        val viewModel by navGraphViewModels<ScopedViewModel>(parentGraphScopeId) {
            ScopedViewModelFactory(args)
        }

        return viewModel
    }

It's not the prettiest implementation but it gets the job done

Upvotes: 1

martinseal1987
martinseal1987

Reputation: 2419

so when i posted this the functionality was there but didn't quite work as expected, since then i now use this all the time and this question keeps getting more attention so thought i would post an up to date example,

using 

//Navigation
implementation "androidx.navigation:navigation-fragment:2.2.0-rc04"
// Navigation UI
implementation "androidx.navigation:navigation-ui:2.2.0-rc04"

i get the view model store owner like this 

private ViewModelStoreOwner getStoreOwner() {

        NavController navController = Navigation
                .findNavController(requireActivity(), R.id.root_navigator_fragment);
        return navController.getViewModelStoreOwner(R.id.root_navigator);
}

im using the one activity multiple fragments implementation, but using this i can effectively tie my view models to just the scoped fragments and with the new live data you can even limit that too

the first id comes from the nav graphs fragment

<?xml version="1.0" encoding="utf-8"?>
  <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <fragment
      android:id="@+id/root_navigator_fragment"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:name="androidx.navigation.fragment.NavHostFragment"
      app:defaultNavHost="true"
      app:navGraph="@navigation/root_navigator"/>

  </FrameLayout>

and the second comes from the id of the nav graph

  <?xml version="1.0" encoding="utf-8"?>
  <navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/root_navigator"
    app:startDestination="@id/mainNavFragment">

and then you can use it like so

private void setUpSearchViewModel() {
    searchViewModel = new ViewModelProvider(getStoreOwner()).get(SearchViewModel.class);
}

Upvotes: 5

Miko Chu
Miko Chu

Reputation: 1382

Here's a concrete example of Alex H's accepted answer.

In your build.gradle (app)

dependencies {
    def nav_version = "2.1.0"
    implementation "androidx.navigation:navigation-fragment-ktx:$nav_version"
}

Example of view model

class MyViewModel : ViewModel() { 
    val name: MutableLiveData<String> = MutableLiveData()
}

In your FirstFlowFragment.kt define

val myViewModel: MyViewModel by navGraphViewModels(R.id.your_nested_nav_id)
myViewModel.name.value = "Cool Name"

And in your SecondFlowFragment.kt define

val myViewModel: MyViewModel by navGraphViewModels(R.id.your_nested_nav_id)
val name = myViewModel.name.value.orEmpty()
Log.d("tag", "welcome $name!")

Now the ViewModel is scoped in this nested fragment, shared state will be destroyed when nested nav is destroyed as well, no need to manually reset them.

Upvotes: 17

Alex H
Alex H

Reputation: 797

Yes, it's possible to scope a viewmodel to a navgraph now starting with androidx.navigation:*:2.1.0-alpha02. See the release notes here and an example of the API here. All you need to give is the R.id for your navgraph. I find it a bit annoying to use, though, because normally viewmodels are initialized in onCreate, which isn't possible with this scope because the nav controller isn't guaranteed to be set by your nav host fragment yet (I'm finding this is the case with configuration changes).

Also, if you don't want your mainFragment to be part of that scope, I would suggest taking it out and maybe using a nested nav graph.

Upvotes: 14

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