Hello devs, Is it possible to create custom Room database return types like rxjava3's flowables/obse...

# android

[FunkyMuse]

Hello devs,
Is it possible to create custom Room database return types like rxjava3's flowables/observable/single, suspend and flowables, paging's data source etc..

I want room to return a type of my sealed class, just like you can do with retrofit's custom call adapters?

[jefbit]

Im newer to Android dev but may be able to help you. I would return a flow from your Dao class. Collect the flow in your repository class and return the result of the flow as your sealed class.

[gildor]

I don’t think custom “call” adapters are supported.
I think all supported types for code generation are hardcoded to annotation processor, but I’m not 100% sure, but it looks the case


Just curious, what is your case for Sealed class for data wrapping?

[FunkyMuse]

Make db call

State;
Loading
Success with result
Call error with throwable
Empty data if result is null

That's my sealed class, so instead of a flow/flowable being mapped to a sealed class i wanted it directly coming out of room

[gildor]

what is Loading state in case of sql request? just initiali?
You can encode it on level of flow, it probably would be just easier

[gildor]

honestly it looks that just use suspend function would be easier in this case, but maybe you have own specific case

[FunkyMuse]

That's what I'm currently doing

But the return type is flow
Then i have to collect that flow and map the flow into a sealed class result.

Instead of doing that i wanted the converter/adapter to do this for me without any hussle like now

[gildor]

But the return type is flow

So you want a flow of those sealed classes?

[gildor]

Loading will never work properly for you, it also look a bit overkill. Error is quite rare thing, it’s essentially request error or fail some constraint checks (possible, but I wouldn’t handle it on level of every subscription, it’s probably better just fail in such case), so the only what you get is empty or not empty data, which look a bit as more hustle to handle, Kotlin nullable type is good enough for it. empty list is a bit different case, but still, even for empty list it doesn’t look necessary

[FunkyMuse]

Yup, I'm getting flow of those sealed classes anyway

Consider this scenario:

I make an insert to the database
I need to show database operation indicator like loading
I need to catch the throwable if it happens
I need to know when the operation ends

Currently I'm doing

stateflow.value= Sealed.Loading
viewModelScope.launch(ioDispatcher + SupervisorJob)
{

try {

dao.insert(model)

} catch (t: Throwable){
stateflow.value= Sealed.CallError(t)
} finally {
stateflow.value= Sealed.CallFinished
}

}

[gildor]

I need to catch the throwable if it happens

I really not sure that you should catch it though, probably crash is just fine for this case

[gildor]

So this case is actually pretty different, it’s not Flow as standard Room flow, because standard room flow will not work for insert, isn’t it?

[gildor]

I don’t think you need all this viewModelScope.launch if insert alredy suspend function

[FunkyMuse]

I just pointed out what I'm currently doing for insert/update/delete

[gildor]

so it becomes
stateflow.value = Sealed.Loading
dao.insert(model)
stateflow.value= Sealed.CallFinished

[gildor]

or with try/catch, if you really want it

[FunkyMuse]

Should i use withcontext only?

[gildor]

no, why switch context if it already suspend function?

[gildor]

if function is not blocking, there is no reason to do this (if suspend function implemented correctly, which the case for Room)

[FunkyMuse]

Yes it's suspend update/delete/insert

You must call suspend from a coroutine scope anyway

[gildor]

yeah, sure, but you also switch context

[FunkyMuse]

Not if u launch it with ioDispatcher

[gildor]

so I would just create a special coroutine builder, which automatically updates loading, finished and error cases

[gildor]

after all you still should launch this operation

[FunkyMuse]

Yeah how else u need to make the suspend call from within a scope...

[gildor]

so it can work like:

CoroutineScope.launchWithState(state: StateFlow, block: suspend () -> Unit): Job {
  stateflow.value= Sealed.Loading
   try {
 block()
} catch (t: Throwable){
stateflow.value= Sealed.CallError(t)
} finally {
stateflow.value= Sealed.CallFinished
}
}

[gildor]

so your insert becomes:

viewModelScope.launchWithState(stateflow) {
   dao.insert(model)
}

[FunkyMuse]

Exactly, you still need the scope
Which is viewModelScope in my case

[gildor]

but you need scope in any case

[gildor]

even if you would be able to create this Flow with sealed class

[gildor]

Just to clarify, i’m not against scopes, I’m pointed out above just that you don’t need context switching like ioDispatcher + SupervisorJob

[FunkyMuse]

Supervisor job/scope is needed in case the job fails (which is rarely) and doesn't cancel the whole scope/other jobs

[gildor]

viewModelScope already uses SupervisorJob

[gildor]

when you just do SupervisorJob() you coroutine violates structured concurrence and leaks (it’s not attached to any scope anymore)!

[FunkyMuse]

ViewModel.viewModelSupervisorIOSupervised(crossinline function: suspend () -> Unit) {
    viewModelScope.launch(ioDispatcher) {
        supervisorScope {
            function()
        }
    }
}

what about this?

[gildor]

you don’t need supervisorScope, it’s useless, if you know that you alreaady run from viewModel

[gildor]

also usually you don’t need ioDispatcher, it needed only to wrap blocking calls, and suspend function never should be blocking