Is this considered to be executed on the main thread? How do you execute a coroutine on the backgro...

# android

[lawlorslaw]

Is this considered to be executed on the main thread? How do you execute a coroutine on the background thread? to be clear exchangeRepository.getExchanges() is the call to the suspend function.

Untitled.kt

[Jorge Domínguez]

I believe that viewModelScope runs by default on Dispatchers.Main, so it does run on the main thread. If you want to move the execution context to a background thread you could use withContext()

[Casey Brooks]

Yes, this will execute on the main thread as viewModelScope uses the Dispatchers.Main.immediate dispatcher by default, but this is actually how it should be set up, preferably. The “structured concurrency” expectation is that if any given suspend method needs to be executed on a different dispatcher, that it should move itself there, rather than requiring the callers to be on the proper dispatcher.

So make sure the API call is done on <http://Dispatchers.IO|Dispatchers.IO> internally by wrapping its implementation with withContext(<http://Dispatchers.IO|Dispatchers.IO>), and you shouldn’t have to worry about changing dispatchers for it in the ViewModel

[lawlorslaw]

so in that case, then is this the correct way to set up the scheduler to be executed on a background thread?

Untitled.kt

[lawlorslaw]

or should it be this?

Untitled.kt

[Jorge Domínguez]

Following Casey's recommendation, you could instead do:

ExchangeRepository {
    suspend fun getExchange(...) = withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
        ....
    }
}

[lawlorslaw]

okay so set up the threading in the repository.

[lawlorslaw]

gotcha

[lawlorslaw]

probably makes sense if you need to invoke that method in multiple places

[Casey Brooks]

Neither, ideally you’d have withContext(<http://Dispatchers.IO|Dispatchers.IO>) in your Repository, not the ViewModel.

class ExchangeRepository {
    suspend fun getExchange() = withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
        // ...
    }
}

[lawlorslaw]

thanks for the helpful info

[lawlorslaw]

i appreciate it

[lawlorslaw]

what about if you need to return a value from the repo function?

[lawlorslaw]

how do you return within the withContext block?

[Jorge Domínguez]

An additional recommendation: to make your repository more testable, you should pass the CoroutineDispatcher as a dependency:

class ExchangeRepository(
    private val dispatcher: CoroutineDispatcher = <http://Dispatchers.IO|Dispatchers.IO>
) {
    suspend fun getExchange() = withContext(dispatcher) {
        ...
    }
}

[Casey Brooks]

It’s a lambda, the last line of the block will be returned from it

[lawlorslaw]

nevermind … it looks like i just have to remove the return keyword

[lawlorslaw]

and then whatever is the last statement executed will be returned

[lawlorslaw]

thanks again

[ephemient]

or use return@withContext if you want to be explicit