About *Navigation*: what does it actually happen w...

# compose

[Daniele B]

About Navigation:
what does it actually happen when you call NavHostController.navigate() as in:

val navController = rememberNavController()
navController.navigate("detail")

It seems no recomposition is triggered.
What is the basis on which Compose is able to update the screen without a recomposition?
And how can we achieve the same in a Compose non-Android app?

[Ian Lake]

Well, for one, you should never be calling imperative code, like navigate, as part of composition. I assume your code is just for demonstration purposes and you aren't actually doing that

[Ian Lake]

That being said, the source of truth is the NavController's currentBackStackEntryAsState() - that state is updated when you navigate and that is what causes your UI to recompose (only on the parts of the hierarchy that depend on said state)

[Ian Lake]

That includes the NavHost itself, which certainly recomposes when you change destinations

[Daniele B]

you should never be calling imperative code, like navigate, as part of composition

yes, of course, that code was a simplification. I am always calling navigate from onClick functions

[Daniele B]

Ok, I understand it now. So it’s just recomposing anything that is inside NavHost. Thanks

[Ian Lake]

Or any bottom nav, etc. that uses currentBackStackEntryAsState() - see the example in the docs: https://developer.android.com/jetpack/compose/navigation#bottom-nav

[Lucien Guimaraes]

About the BottomNav, is there a way to avoid the view to recompose when switching between tabs ?

Let's say I have a BottomBar with Tab A being a list, and a Tab B being a mapView. In Tab A I scroll the list, then open Tab B and I move the map. Then switch to Tab A, I would like my list to have the position as it where before. Same if I go back to the mapView, I don't want to lose the map moved by the user.

[Ian Lake]

When it comes to saving state for each bottom nav item, I have good news: https://twitter.com/ianhlake/status/1391908231845322752

[Lucien Guimaraes]

That's amazing 🎉

[Lucien Guimaraes]

Thanks for your hardwork for bringing this alive!