What is a correct way to collect a flow using a ce...

# coroutines

[StragaSevera]

What is a correct way to collect a flow using a certain dispatcher? For example, I have a code like this that needs to run on the Dispatchers.IO:

summaryFlow().take(100).collect { summary ->
    fileWriter.write(Json.encodeToString(summary))
}

What is the correct way to do it? Should I use withContext inside of the lambda, or flowOn before it?

[Youssef Shoaib [MOD]]

I believe you just surround the collect call itself in withContext

[Zach Klippenstein (he/him) [MOD]]

• flowOn will change the context in which everything upstream of the operator runs.
• Surrounding the collect call with withContext will change the context in which the collect body runs, and may change the context upstream (but flowOn would override that).
• Putting withContext inside the collect will only change the context that the actual write and encodeToString functions run in.

[Zach Klippenstein (he/him) [MOD]]

Generally it's best to only switch to the IO dispatcher for actual blocking IO call, so the best way would probably be:

summaryFlow().take(100).collect { summary ->
  // This is CPU-bound work, so don't switch to IO yet.
  val encoded = Json.encodeToString(summary)

  // This is the blocking IO call.
  withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
    fileWriter.write(encoded)
  }
}

[Zach Klippenstein (he/him) [MOD]]

But in practice running the encode on the IO dispatcher is probably fine too

[StragaSevera]

Got it, thanks for the explaination!