Wait for all volley request in a for loop - kotlin

Question

In my function, I need to return a list that is populated by a for loop with some Volley Request. So I need to wait that all of these requests to be terminated before return the list.
I think I need the async CoroutineScope to do this work but I don't know how can I wait for all of that response.
This is my code:
suspend fun getListOfAbility(pokemon: Pokemon) : MutableList<Ability> {
val listOfAbility: MutableList<Ability> = emptyList<Ability>() as MutableList<Ability>
CoroutineScope(Dispatchers.IO).launch {
/**
* get the pokemon json
*/
val pokemonJsonObjectRequest = JsonObjectRequest(
Request.Method.GET,
"$pokemonUrl${pokemon.id}",
null,
{
/**
* onResponse
*
* get the list of pokemon abilities
*/
val abilitiesJO = it.getJSONObject("abilities")
val abilityObjectType = object : TypeToken<List<PokemonGson.AbilityObjectGson>>() { }.type
val abilityListGson = Gson().fromJson<List<PokemonGson.AbilityObjectGson>>(abilitiesJO.toString(), abilityObjectType)
/**
* for each ability listed on pokemon info get the full Ability Object
*/
for((index, abilityObjectGson) in abilityListGson.withIndex()) {
val abilityJsonObjectRequest = JsonObjectRequest(
Request.Method.GET,
abilityObjectGson.ability.url,
null,
{
abilityJson ->
/**
* onResponse
*
* get the full ability info
*/
val abilityType = object : TypeToken<AbilityGson>() { }.type
val abilityGson = Gson().fromJson<AbilityGson>(abilityJson.toString(), abilityType)
/**
* fill the Ability entry of listOfAbility with the correct language
*/
val ability = Ability(abilityGson, abilityListGson[index].is_hidden)
listOfAbility.add(ability)
},
{
/**
* onError
*/
Log.d("POKEMON", "Pokemon ability error")
}
)
requestQueue.add(abilityJsonObjectRequest)
}
},
{
/**
* onError
*/
Log.d("POKEMON", "Pokemon request error")
}
)
requestQueue.add(pokemonJsonObjectRequest)
}
//wait
return listOfAbility
}

Answer 1

To use callback-based code in a suspend function, you need to convert it to a suspend function using suspendCoroutine or suspendCancellableCoroutine. So in this case to replace the action of creating a JSONObjectRequest and listener, queuing it to the RequestQueue, and waiting for it somehow, I would create a suspend function like this:
suspend inline fun RequestQueue.getJSONObjectOrNull(
method: Int,
url: String,
jsonRequest: JSONObject?,
crossinline onError: (VolleyError)->Unit = {}
): JSONObject? = suspendCancellableCoroutine { continuation ->
val request = JsonObjectRequest(
method,
url,
jsonRequest,
{ result: JSONObject -> continuation.resume(result) },
{ error ->
onError(error)
continuation.resume(null)
}
)
add(request)
continuation.invokeOnCancellation { request.cancel() }
}
It directly returns the JSONObject result, or null if there's a failure. You can optionally run a callback on errors in case you want to log it.
Then you can use it to write a more sequential version of your function instead of the callback-based version. You can use the pattern of coroutineScope { async { list.map { ... } } }.awaitAll() to convert each item of a list to something else using parallel coroutines.
Here is an untested version of your function. I am having it return an empty list on failure. You could alternatively return null on failure, which might be more useful so the calling function can decide to do something differently when there's a failure.
private fun VolleyError.logDebug() {
Log.d("POKEMON", "Pokemon request error: $this")
}
suspend fun getListOfAbility(pokemon: Pokemon): List<Ability> {
val pokemonJsonObject = requestQueue.getJSONObjectOrNull(Request.Method.GET, "$pokemonUrl${pokemon.id}", null, VolleyError::logDebug)
pokemonJsonObject ?: return emptyList()
val abilitiesJO = pokemonJsonObject.getJSONObject("abilities")
val abilityObjectType = object : TypeToken<List<PokemonGson.AbilityObjectGson>>() {}.type
val abilityListGson: List<Wrapper> = Gson().fromJson<List<PokemonGson.AbilityObjectGson>>(
abilitiesJO.toString(),
abilityObjectType
)
return coroutineScope {
abilityListGson.map {
async {
requestQueue.getJSONObjectOrNull(Request.Method.GET, it.ability.url, null, VolleyError::logDebug)
}
}
}
.awaitAll()
.filterNotNull()
.map { abilityJson ->
val abilityType = object : TypeToken<AbilityGson>() {}.type
val abilityGson = Gson().fromJson<AbilityGson>(abilityJson.toString(), abilityType)
Ability(abilityGson, abilityListGson[index].is_hidden)
}
}