Function returns Emptylist value before inner results are finished in Kotlin with firebase call [duplicate] - kotlin

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How to return a list from Firestore database as a result of a function in Kotlin?
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Closed 18 days ago.
Trying to retrieve some objects from firebase and manipulate them in this function. However the return of this statement is always an empty list. It skips the databse.child.... action which is the majority of the code and jumps straight to the return statement.
#RequiresApi(Build.VERSION_CODES.O)
fun getFirebaseModuleData(): ArrayList<Module> {
//Firebase Data
var userCloudModules : ArrayList<Module> = ArrayList()
database = Firebase.database.reference
val userId = Firebase.auth.currentUser!!.uid
database.child("Modules").child(userId).get().addOnSuccessListener {
Log.i("firebase", "Got value ${it.value}")
var children = it.children
for (child in children){
try {
val value = child.value as HashMap<String, Any>
val name = value["name"] as String
val colour = value["colour"] as String
val sessions = value["moduleSessions"] as List<Map<String, Any>>
val moduleSessions = ArrayList<ModuleSession>()
for (session in sessions) {
val location = session["location"] as String
val sessionType = session["sessionType"] as String
val dayOfTheWeek = session["dayOfTheWeek"] as String
val startTimeData : HashMap<String, String> = session["startTime"] as HashMap<String, String>
val endTimeData : HashMap<String, String> = session["endTime"] as HashMap<String, String>
var startTimeHour = startTimeData["hour"].toString()
var startTimeMinute = startTimeData["minute"].toString()
val startHourLong : Long = startTimeData["hour"] as Long
if(startHourLong < 10){
startTimeHour = "0"+startTimeHour
}
val startMinuteLong : Long = startTimeData["minute"] as Long
if(startMinuteLong < 10){
startTimeMinute = "0"+startTimeMinute
}
val startTime = "$startTimeHour:$startTimeMinute"
var endTimeHour = endTimeData["hour"].toString()
var endTimeMinute = endTimeData["minute"].toString()
val endHourLong : Long = endTimeData["hour"] as Long
if(endHourLong < 10){
endTimeHour = "0"+endTimeHour
}
val endMinuteLong : Long = endTimeData["minute"] as Long
if(endMinuteLong < 10){
endTimeMinute = "0"+endTimeMinute
}
val endTime = "$endTimeHour:$endTimeMinute"
val moduleSession = ModuleSession(location, sessionType, dayOfTheWeek, LocalTime.parse(startTime), LocalTime.parse(endTime))
moduleSessions.add(moduleSession)
}
val module = Module(name,colour,moduleSessions)
userCloudModules.add(module)
} catch (e: Exception) {
Log.e("firebase", "Error casting data", e)
}
}
}.addOnFailureListener{
Log.e("firebase", "Error getting data", it)
println("Error getting data")
}
return userCloudModules
}
I have done some println's and the data is being retrieved but is not added to the list.