MutableList remaining empty (?) even though i'm populating it-CodePudding

I'm trying to populate a mutable list so that I can use it for a recycler view. Unfortunately, although (i think) I'm populating the list, it's still remaining empty and the recycler view is not working (and I imagine it's because of the list issue). Please see below for the code:

        private val newList: MutableList<NewListModel> = mutableListOf()
        private val oldList = retrieveOldList()

        private fun retrieveAndPopulate() {
                
                for (i in 0 until oldList.size){
        
                    val oldItem = oldList[i]
                    
                    val itemOne = oldItem.itemOne
                    val itemTwo = oldItem.itemTwo
                    val itemThree = oldItem.itemThree
                    val itemFour = oldItem.itemFour
    
                    val newItemData =
                        NewListModel(
                            itemOne, itemTwo, itemThree, itemFour
                        )
    
                newList.add(newItemData)
                Log.d(
                    "RetrieveData",
                    "${newItemData.itemOne} has been added to the list."
                )
            }
        }

The class below is for the "NewListModel"

@Keep
@IgnoreExtraProperties
data class NewListModel (
    var itemOne: String ?= null,
    var itemTwo: String ?= null,
    var itemThree: String ?= null,
    var itemFour: String ?= null,
)

Below is how i try to populate the "oldList"

fun retrieveData(): MutableList<OldListModel> {

        val list: MutableList<OldListModel> = mutableListOf()
        val ref = FirebaseDatabase.getInstance().getReference("/storage")

        ref.addValueEventListener(object : ValueEventListener {
            override fun onDataChange(snapshot: DataSnapshot) {
                if (snapshot.exists()) {
                    ref.get()
                        .addOnSuccessListener {

                            for (listItem in snapshot.children) {
                                val listItem = snapshot.getValue(OldListModel::class.java)

                                if (listItem != null) {
                                    list.add(listItem)
                                }
                            }
                        }
                } else {
                    Log.d(
                        "Data",
                        "Retrieving data was unsuccessful."
                    )
                }
            }

            override fun onCancelled(error: DatabaseError) {
            }
        })
        return list
}

It's probably worth mentioning that I'm getting the data from one mutable list and adding it to another. Any help is much appreciated

(below is how i try to populate the recycler view)

val newList = retrieveAndPopulate()

val recyclerView = findViewById<View>(R.id.recyclerView) as RecyclerView
val layoutManager = LinearLayoutManager(this)
recyclerView.layoutManager = layoutManager
val adapterAdapter = AdapterAdapter(newList)
recyclerView.adapter = adapterAdapter

CodePudding user response：

Your problem is that you think you're running code sequentially when it's running asynchronously. See the numbered comments from your function to trace the order of execution:

fun retrieveData(): MutableList<OldListModel> {

    // 1. Here you create a list
    val list: MutableList<OldListModel> = mutableListOf()
    val ref = FirebaseDatabase.getInstance().getReference("/storage")

    // 2. Here a listener is added that will let you know LATER when the data is ready
    ref.addValueEventListener(object : ValueEventListener {
        // 4. LATER the data changed will get called
        override fun onDataChange(snapshot: DataSnapshot) {
            if (snapshot.exists()) {
                ref.get()
                    .addOnSuccessListener {
                        // 5. EVEN LATER this listener is called with data
                        for (listItem in snapshot.children) {
                            val listItem = snapshot.getValue(OldListModel::class.java)

                            // 6. FINALLY - you add to a list that has long since stopped being relevant
                            if (listItem != null) {
                                list.add(listItem)
                            }
                        }
                    }
            } else {
                Log.d(
                    "Data",
                    "Retrieving data was unsuccessful."
                )
            }
        }

        override fun onCancelled(error: DatabaseError) {
        }
    })
    return list // 3. Here you return the EMPTY list that was created
}

A solution - though likely not the best solution is to update your list once the callbacks complete:

fun retrieveData(): { // No longer returning anything

    // Remove this, no longer returning anything
    // val list: MutableList<OldListModel> = mutableListOf()

    val ref = FirebaseDatabase.getInstance().getReference("/storage")
    ref.addValueEventListener(object : ValueEventListener {
        override fun onDataChange(snapshot: DataSnapshot) {
            if (snapshot.exists()) {
                ref.get()
                    .addOnSuccessListener {
                        // Move list here
                        val list: MutableList<OldListModel> = mutableListOf()

                        for (listItem in snapshot.children) {
                            val listItem = snapshot.getValue(OldListModel::class.java)

                            if (listItem != null) {
                                list.add(listItem)
                            }
                        }
                        
                        // Since you're using Kotlin, you could use a map,
                        // but that's unrelated to this issue
                        // val list = snapshot.children.map { getValue(...) }.filterNotNull()
                        
                        // Now that we have a full list here, update:
                        updateAdapterWithNewData(list)
                    }
            } else {
                Log.d(
                    "Data",
                    "Retrieving data was unsuccessful."
                )
            }
        }

        override fun onCancelled(error: DatabaseError) {
        }
    })
}

Where updateAdapterWithNewData is a function you write to do as it says.

Please read up on asynchronous programming and make sure you understand how the code is flowing when using callbacks / listeners in frameworks like Firebase.