Coroutine called by event that requires another coroutine to finish - kotlin

Question

In my ViewModel I have a function that is called from UI. It can be called many times per second.
Data comes from barcode scanner ViewModel. I'm passing it from one ViewModel to another thru UI for simplicity and to reuse barcode scanner ViewModel.
For simplicity lets assume that I have something like this:
// called from the fragment (that observes some property on the another viewmodel)
public fun processScannedBarcode(barcode : String){
// process data after necessaryData is ready (not null?)
val item = findItemByBarcode(barcode)
}
private var dataFromApi: ArrayList<Item>?
private fun loadNecessaryDataFromTheApi(){
viewModelScope.launch {
canReload = false
dataFromApi = myapi.getDataFromApi() // suspend fun inside
canReload = true
}
}
// called from the fragment button click
public fun reloadNecessaryDataFromTheApi(){
loadNecessaryDataFromTheApi()
}
init {
loadNecessaryDataFromTheApi()
}
My data required to process may be not ready yet because it comes from the API. I have to deferr that processing and resume when data is ready.
I could simply solve this with some Queue and a Thread, but maybe it is possible to do that with Kotlin coroutines?

Answer 1

As your loadNecessaryDataFromTheApi() only sets properties, it is not that easy to observe it or wait for it. You need to use some synchronization utility like locks, channels, etc. In this case it will be probably the easiest to keep a Deferred of the necessary data:
public suspend fun processScannedBarcode(barcode : String){
val data = dataFromApi.await()
val item = findItemByBarcode(barcode)
}
private val dataFromApi = CompletableDeferred<ArrayList<Item>>()
private fun loadNecessaryDataFromTheApi(){
viewModelScope.launch {
canReload = false
dataFromApi.complete(myapi.getDataFromApi())
canReload = true
}
}
Some notes:
We have to make processScannedBarcode suspend if it is going to wait for something.
If you want reloadNecessaryDataFromTheApi() to defer processing barcodes again, simply replace dataFromApi with a new deferred.