I'm having trouble seeing how the parts fit togeth...

# compose

[robnik]

I'm having trouble seeing how the parts fit together. Eg, I have a NavHost(...) { composable(..., arguments = ...) { ... inside that I extract at argument (id: String), now I can want to query my Room database, like db.getThing(id):Flow<Thing> and pass it to a @Composable fun ThingView(thing) but that DB function is not callable outside of a coroutine context. So... where do I get the coroutine context, and then how to I hook the result obtained within that back to some MutableState that Compose can observe, in ThingView.

[Ian Lake]

Room methods that return a Flow<Thing> should not be suspend methods, so you just call it directly and call collectAsState() on the Flow as per https://developer.android.com/jetpack/compose/libraries#streams

[robnik]

Okay, my code seems to work. One thing looks awkward - maybe it's normal but tutorials tend not to show it – the DB returns Flow<Thing?>, like most "get by ID" functions. So all these composables have a some boilerplate checking the state value for null (and Kotlin's smart casts won't work on the val state by flow.collectAsState(initial=null) so there's an extra assignment to a local. But maybe that's just reality. The UI does need to wait for a DB query. It's just that the docs tend to use simpler examples.

[robnik]

I'm trying to understand what thread actually sets that state value when the DB gets a result. I'm finding coroutines harder to understand than good old threads.

[Ian Lake]

Yeah, that null check actually makes it incredibly easy to write whatever loading spinner, etc. you want as the null branch of your if statement - you output one set of composables when it is null vs another set when it is non-null

[dewildte]

Coroutines are like baby threads in adult threads.