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

问题

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

回答1:

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.

回答2:

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.

来源：https://stackoverflow.com/questions/56505455/scoping-a-viewmodel-to-multiple-fragments-not-activity-using-the-navigation-co