One Off the Slack: How Do We Have ViewModels Scoped to a Composable?

Tiago Araujo asked:

Is there already a way to scope view models by composable?

Google’s Adam Powell indicated that it was possible but wanted more details, so Tiago continued:

Let’s say I’ve a pager with the same composable and each needs their own instance of the same viewmodel. How would I do that?

Adam responded:

have each of your pages provide LocalViewModelStoreOwner and multiplex them into a host viewmodel; this is how androidx navigation-compose does it

you’ll have some decisions to make around the intended lifecycle of those viewmodels, how long to keep them alive and when to clear them if you don’t keep all of the pager’s pages composed at the same time

Later in the thread, Marco Romano had a bit of a “wait, wut?” moment around the phrase “multiplex them into a host viewmodel”, so Adam explained:

my apologies; you have a point where you need to create one ViewModelStore per page, each one establishes a namespace and scope for viewmodels obtained beneath it. You need a place to store those ViewModelStores and make sure they’re retained. Using another ViewModel is the most straightforward way to do this since it already provides all of the facilities and guarantees you want. You’ll create your own ViewModel that stores a map of some key that is unique for each page to a ViewModelStore, and you’ll need to manage the addition and removal of these ViewModelStores from that map. Then you’ll need to provide the correct ViewModelStore via LocalViewModelStoreOwner to the content of each page.

Tiago asked:

so it requires some work I see, I’ll have to create some custom implementation of LocalViewModelStoreOwner right? I need to check how the navigation does it then

Adam elaborated:

a little work, but not too much. the usual way to think about this is to have the composable function that provides the LocalViewModelStoreOwner fetch a container ViewModel that you write for this purpose. Then use a RememberObserver to remove+clear the ViewModelStore when a page leaves the composition if the host activity isChangingConfiguration isn’t true

(since if you’re tearing down the composition for a config change you want to leave them alone)

the reason to use RememberObserver directly instead of DisposableEffect is because you need to hook onAbandoned too; ViewModels are created during composition and need to be cleaned up if the composition they were created in doesn’t commit

shouldn’t be more than ~70-100 lines of utility code that you write once and don’t have to look at again 🙂

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