Data from Firebase Realtime Database is retreived, but not set when returned from a function?-CodePudding

I'm trying to get data from Firebase and return it as an array. So far this works, however the data remains to be the default data that I had set it.

Here is the code (I added a few comments to clarify where it works and where id doesn't):

fun getRandomQuestion() : ArrayList<ArrayList<String>>{
    val random = Random(1234567)
    val randQuestionId = random.nextInt(0, 27)

    var dataRequestResult : String = "empty"

    var requestSide1 : String = "empty"
    var requestSide2 : String = "empty"
    var requestCount1 : String = "empty"
    var requestCount2 : String = "empty"

    //Get a random question from the DB
    Firebase.database.reference.child(Constants.wyrDBQuestionLocation)
        .child(randQuestionId.toString())
        .get()
        .addOnFailureListener { task ->
            Log.i("Get WYR Question Status", "Failed: "   task.message.toString())
            requestSide1 = "empty"
            requestSide2 = "empty"
            requestCount1 = "empty"
            requestCount2 = "empty"
        }.addOnSuccessListener {
            Log.i("Get WYR Question Status", "Success")

            //Logging this data returns the correct information from firebase, no issue here
            Log.i("Request Data Result", it.value.toString())

            dataRequestResult = it.value.toString()
        }

    requestSide1 = dataRequestResult.substringAfter("side1Question=").substringBefore("}")
    requestSide2 = dataRequestResult.substringAfter("side2Question=").substringBefore(",")
    requestCount1 = dataRequestResult.substringAfter("sideCount1=").substringBefore(",")
    requestCount2 = dataRequestResult.substringAfter("sideCount2=").substringBefore(",")

    //Return the value
    return arrayListOf(arrayListOf(requestSide1, requestCount1), arrayListOf(requestSide2, requestCount2))
}

Activity where I try to get the returned value:

    val myQuestion = databaseController.getRandomQuestion()

    //This is where the issue lies
    //When I get the returned data from the function above, it gives me all of the defaults I had set in the function (which is "empty")
    //It logs:`[["empty", "empty"],["empty", "empty"]]`
    Log.i("My Question", myQuestion.toString())

The data in the success listener is correct, but the return value is still default. Here is an image of the logs:

"Request Log Result", don't mind the result, I'm making a WYR app.

I have noticed that the request from the listener had come last, not first

I think this had to do with the data not even being completely gathered yet because in another app I made, all i had to do was give it a delay and it would be set, but in this case, I have no idea what to do.

Any help would be greatly appreciated! :)

CodePudding user response：

this is my solution

Method 1

fun getRandomQuestion() : ArrayList<ArrayList<String>>{
    val random = Random(1234567)
    val randQuestionId = random.nextInt(0, 27)

    var dataRequestResult : String = "empty"

    var requestSide1 : String = "empty"
    var requestSide2 : String = "empty"
    var requestCount1 : String = "empty"
    var requestCount2 : String = "empty"

    //Get a random question from the DB
    Firebase.database.reference.child(Constants.wyrDBQuestionLocation)
        .child(randQuestionId.toString())
        .get()
        .addOnFailureListener { task ->
            Log.i("Get WYR Question Status", "Failed: "   task.message.toString())
            requestSide1 = "empty"
            requestSide2 = "empty"
            requestCount1 = "empty"
            requestCount2 = "empty"
        }.addOnSuccessListener {
            Log.i("Get WYR Question Status", "Success")

            //Logging this data returns the correct information from firebase, no issue here
            Log.i("Request Data Result", it.value.toString())

            dataRequestResult = it.value.toString()
    requestSide1 = dataRequestResult.substringAfter("side1Question=").substringBefore("}")
    requestSide2 = dataRequestResult.substringAfter("side2Question=").substringBefore(",")
    requestCount1 = dataRequestResult.substringAfter("sideCount1=").substringBefore(",")
    requestCount2 = dataRequestResult.substringAfter("sideCount2=").substringBefore(",")

        }

    
    //Return the value
    return arrayListOf(arrayListOf(requestSide1, requestCount1), arrayListOf(requestSide2, requestCount2))
}

Method 2

fun getRandomQuestion() : ArrayList<ArrayList<String>>{
    val random = Random(1234567)
    val randQuestionId = random.nextInt(0, 27)

    var dataRequestResult : String = "empty"

    var requestSide1 : String = "empty"
    var requestSide2 : String = "empty"
    var requestCount1 : String = "empty"
    var requestCount2 : String = "empty"

    //Get a random question from the DB
    Firebase.database.reference.child(Constants.wyrDBQuestionLocation)
        .child(randQuestionId.toString())
        .get()
        .addOnFailureListener { task ->
            Log.i("Get WYR Question Status", "Failed: "   task.message.toString())
            requestSide1 = "empty"
            requestSide2 = "empty"
            requestCount1 = "empty"
            requestCount2 = "empty"
        }.addOnSuccessListener {
            Log.i("Get WYR Question Status", "Success")

            //Logging this data returns the correct information from firebase, no issue here
            Log.i("Request Data Result", it.value.toString())

            dataRequestResult = it.value.toString()
    requestSide1 = dataRequestResult.substringAfter("side1Question=").substringBefore("}")
    requestSide2 = dataRequestResult.substringAfter("side2Question=").substringBefore(",")
    requestCount1 = dataRequestResult.substringAfter("sideCount1=").substringBefore(",")
    requestCount2 = dataRequestResult.substringAfter("sideCount2=").substringBefore(",")
//Return the value
    return arrayListOf(arrayListOf(requestSide1, requestCount1), arrayListOf(requestSide2, requestCount2))
        }

return null    
    
}

CodePudding user response：

The listeners you use when requesting database data will fire later than the getRandomQuestion function returns ArrayList. You can use Kotlin coroutines to work with callbacks and suspend a coroutine until callback is fired, for example:

suspend fun getRandomQuestion() = suspendCoroutine<String> { continuation ->
    Firebase.database.reference.child(Constants.wyrDBQuestionLocation)
    .child(randQuestionId.toString())
    .get()
    .addOnFailureListener { task ->
        continuation.resume("empty")
    }.addOnSuccessListener {
        continuation.resume(it.value.toString())
    }
}

// somewhere in your code (e.g. in ViewModel, Activity or Fragment):

someCoroutineScope.launch {
    val question: String = getRandomQuestion();
    // ... use question to update UI
}

In Activity or Fragment you can use lifecycleScope, in ViewModel - viewModelScope.

suspendCoroutine builder function is used to wait for the callbacks. suspendCoroutine suspends coroutine in which it executed until we decide to continue by calling appropriate methods - Continuation.resume....