multiple async-await in kotlin

obj in promoType = [list of string]
its more like 10 firebase queries are running here, looking in 10 particular set of nodes and going down further.

what i'm not sure, whether i require to put on async / await on each of the queries, but all i want is 10 of these queries to run and then result me in whether a couponKey is empty or not. All i want to do is to display whether a coupon entered was correct or not.

further, in changeUserType(couponKey, couponFoundAtKey), some database write operations occur.

fun checkPromo(promoCodeET: String) = async(UI) {
try {
val database = PersistentFirebaseUtil.getDatabase().reference
val job = async(CommonPool) {

for (obj in promoType) {
val query = database.child("promos").child(obj).orderByChild("promoCode").equalTo(promoCodeET)

query.addListenerForSingleValueEvent(object :
ValueEventListener {
override fun onDataChange(dataSnapshot: DataSnapshot) {
if (dataSnapshot.exists()) {
couponKey = dataSnapshot.key.toString()
couponFoundAtKey = dataSnapshot.children.first().key.toString()
if (couponKey.isNotEmpty())
changeUserType(couponKey, couponFoundAtKey)
flag = true
}
}

override fun onCancelled(error: DatabaseError) {
// Failed to read value
}
})
if (flag) break
}
}
job.await()

}
catch (e: Exception) {
}
finally {
if (couponKey.isEmpty()){
Toast.makeText(this@Coupon, "Invalid coupon", Toast.LENGTH_LONG).show()
}
flag = true
}
}


1 Answer
1

There are several things I find wrong with your code:

async(UI)

async(CommonPool)

await

async

Your code should be much simpler. You should declare a suspend fun whose return value is the pair (couponKey, coupon):

suspend fun

(couponKey, coupon)

suspend fun fetchPromo(promoType: String, promoCodeET: String): Pair<String, String>? =
suspendCancellableCoroutine { cont ->
val database = PersistentFirebaseUtil.getDatabase().reference
val query = database.child("promos").child(promoType)
.orderByChild("promoCode").equalTo(promoCodeET)
query.addListenerForSingleValueEvent(object : ValueEventListener {
override fun onDataChange(dataSnapshot: DataSnapshot) {
cont.resume(
dataSnapshot
.takeIf { it.exists() }
?.let { snapshot ->
snapshot.key.toString()
.takeIf { it.isNotEmpty() }
?.let { key ->
Pair(key, snapshot.children.first().key.toString())
}
}
)
}

override fun onCancelled(error: DatabaseError?) {
if (error != null) {
cont.resumeWithException(MyException(error))
} else {
cont.cancel()
}
}
})
}


To call this function, use a launch(UI) at the call site. Change the user type once you get a non-null value:

launch(UI)

launch(UI) {
var found = false
for (type in promoType) {
val (couponKey, coupon) = fetchPromo(type, "promo-code-et") ?: continue
found = true
withContext(CommonPool) {
changeUserType(couponKey, coupon)
}
break
}
if (!found) {
Toast.makeText(this@Coupon, "Invalid coupon", Toast.LENGTH_LONG).show()
}
}


You say that changeUserType performs some database operations, so I wrapped them in a withContext(CommonPool).

changeUserType

withContext(CommonPool)

Note also that I extracted the loop over promo types outside the function. This will result in queries being performed sequentially, but you can just write different calling code to achieve parallel lookup:

var numDone = 0
var found = false
promoType.forEach { type ->
launch(UI) {
fetchPromo(type, "promo-code-et")
.also { numDone++ }
.takeIf { it != null }
?.also { (couponKey, coupon) ->
found = true
launch(CommonPool) {
changeUserType(couponKey, coupon)
}
}
?: if (numDone == promoType.size && !found) {
Toast.makeText(this@Coupon, "Invalid coupon", Toast.LENGTH_LONG).show()
}
}
}


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