通过 navGraphViewModel 导航返回时保留 ViewModel 实例

Question

So I am following Google's approach where you use single activity for the whole app and multiple view models, which are scoped to your nested nav graphs or fragments.

And I have a problem with this approach. Let's use example provided by google docs so it would be easier for me.

Here when user reached destination C, the back stack contains one instance of each destination(A, B, C) and if we want to return to destination A without multiplying instances of our destinations inside back stack - we need to add popUpTo="@+id/a" and popUpToInclusive="true" to clear all instances inside the back stack.

Here is the problem - if we will use this then every old instance of destinations will be cleared and new instance of A will be created hence if we are using by navGraphViewModel(R.id.A) then our old instance of ViewModel will be cleared as well!

So how to avoid such behavior? Is there a way to open destination A without creating a new instance so that our view model won't be cleared as well?

I have tried multiple ways to prevent that, but nothing seemed to work (in this case A - could be a nested graph also and that would work the same way)

Answer 1

Edit: I Just read your question again and I think I misunderstood it the first time. My answer might not be of much help.

Is your requirement to have sub (nested) NavGraphs? If not, I don't think navGraphViewModels() applies to you. From what I read, viewmodels created using this approach are cleared when their corresponding NavGraph is completed.

navGraphViewModels is very useful when we are creating fragment for some modules and we don't want these viewModels to be persist throughout the whole activity. Hence, we can create our own custom sub-navGraph with navGraphViewModels().

That is, if you simply want to achieve a single activity / multiple fragments architecture, why not use by activityViewModels() inside each fragment. Using this, the viewmodel instance is persisted as long as the host activity is alive. Thus allowing you to share data between the activity and all the fragments.

Reference: https://skynight1996.medium.com/navigation-component-comparison-between-viewmodels-activityviewmodels-and-ae0145734228