Is it possible to navigate between activities? and...

# compose

[codeslubber]

Is it possible to navigate between activities? and have the bottom bar work? seems like the answer is no (for now?)…

[Ian Lake]

You mean have each bottom nav item open a brand new activity? That hasn't been a recommended pattern...well, ever lol

[codeslubber]

So one activity is a user profile and the other is a FAQ and those should be fragments that share a Viewmodel? that sounds great!

[Ian Lake]

Or, since you're in the #compose channel, composables, but yes

[codeslubber]

Yeah that’s sad (I was joking about it sounding great), I was so thrilled when Viewmodels arrived and the Blueprint stuff, but I guess the right way to do this is just put the various unrelated things in the Drawer. Thanks @Ian Lake

[codeslubber]

So I reread the docs, but kept searching and found this SO question (https://stackoverflow.com/questions/61038532/view-model-can-be-heavy-if-we-use-single-activity-based-architecture) that @Ian Lake answered, which gave me a surprise: Viewmodels can be scoped to Fragments! the documentation does not show how that’s done, though? in the docs they show a Master(sic)/Detail that shares data between the fragments (which is cool)…

[Ian Lake]

You can scope ViewModels to activities, fragments, navigation graphs or composables in a Navigation Compose graph, yup

[Ian Lake]

The fragment docs talk about using ViewModels in Fragments, FWIW: https://developer.android.com/guide/fragments/saving-state#nonconfig

[Ian Lake]

Same as how the Navigation docs talk about using navigation graph scoped ViewModels: https://developer.android.com/guide/navigation/navigation-programmatic#share_ui-related_data_between_destinations_with_viewmodel

[Ian Lake]

In a Compose world, you use = viewModel() - https://developer.android.com/jetpack/compose/interop#viewmodel

[Ian Lake]

Which, if you're using Navigation Compose (https://developer.android.com/jetpack/compose/navigation), will automatically scope your ViewModel to the current destination if used within the NavHost

[codeslubber]

so then to summarize, my mistake was to start with the Viewmodels page in the docs 😉 … this is great!! thanks!

[Ian Lake]

Yeah, ViewModels are one of the lowest level building blocks - lower level than fragments or Navigation. You'd have to go to the docs on the higher level components to find out how they expose those lower level building blocks

[codeslubber]

Well I would suggest that the Viewmodels page mention that they can be scoped to all those things. When they were first introduced they were scoped only to Activities.

[codeslubber]

But thanks, on the whole samples and docs in android are amazing…

[Ian Lake]

You mean when they were first introduced in the docs? Because ViewModels themselves had support for fragments on day 1 (way back before AndroidX was even a thing)

[Ian Lake]

Do you mind filing a documentation bug for the ViewModel page? I think there could at least be a few more 'learn more' links at the bottom pointing to the other pages. https://issuetracker.google.com/issues/new?component=192697

[codeslubber]

Yeah I am happy to do that.

[codeslubber]

On the first issue, I am not saying they did not have support for fragments, I am saying they were scoped to the activity so the share one viewmodel between one or more fragments, sure that worked, a different viewmodel for different fragments? I don’t think so.

[Ian Lake]

The ViewModelProviders.of(Fragment) method which allowed fragment scoped ViewModels is in the very first android.arch.lifecycle:lifecycle-extensions:1.0.0-alpha01 release 🙂

[codeslubber]

I was wrong.. perfect 🙂

[codeslubber]

That’s interesting, but since that was before the Single Activity reborn movement, not sure how germane…