Code does not wait for FirebaseDatabase to be read

Question

So I have this app which uses Firebase. The problem is that it takes some input (suppose X), reads some data from the FirebaseDatabase (suppose Y), and then compares them. If some elements of Y match with corresponding elements of X, it considers them a duplicate and only updates the value of Y.

If the elements of the two do not match, then it creates a new entry in the database with X.

Now, my code has a button which calls the checkDataNew() method which checks the aforementioned values for similarities. It is responsible for reading from the database and then comparing it with the input values. It then uses the result from that function to update/add the values in the FirebaseDB.

If I keep the activity open and then enter similar values, it works perfectly and only updates the value in the FirebaseDB.

However, if I close the activity and then open it again, then it doesn't enter the addValueEventListener function to read data from the FirebaseDB.

I know because I have debugged it. In the first scenario, the code goes to the lines written inside in the rootRef.addValueEventListener code, but in the second, it doesn't.

Needless to say, this is causing issues of duplicate items in my app. Can anyone explain as to why the addValueEventListener code executes when the activity is open and input is provided multiple times, but fails to execute if I close the activity and reopen it?

The relevant code for that is written below (it's in Kotlin, but even if someone has a solution for Java, I can convert it to Kotlin):

checkInsulinBtn?.setOnClickListener()
{
        var isDataValidCheck:Boolean=validateData()
        if(isDataValidCheck==true)
        {
            checkDataNew() //calls Firebase DB read function

            var newKey=foundKey

            if(newKey=="")
            {
                var mLogBG:BGLevel= BGLevel(emailID,recentFood,recentEvent,BGLevel,insulinLevel,calendarTime)
                var key:String=mFirebaseDatabaseReference.push().toString()

                var parsedKeyList=key.split("/-")
                var parsedKey=parsedKeyList[1]
                parsedKey="-" + parsedKey


                mFirebaseDatabaseReference.child(parsedKey).setValue(mLogBG)

                var mFirebaseDatabaseReference2=mFirebaseDatabase.getReference("BG Keys")
                mFirebaseDatabaseReference2.push().setValue(parsedKey)
                Toast.makeText(applicationContext,"Your data has been saved to the cloud, and is viewable in the app calendar",Toast.LENGTH_SHORT).show()
            }

            else
            {
                var mLogBG:BGLevel= BGLevel(emailID,recentFood,recentEvent,BGLevel,insulinLevel,calendarTime)
                var newKey=foundKey
                mFirebaseDatabaseReference.child(foundKey).setValue(mLogBG)
                Toast.makeText(applicationContext,"Your previous data has been updated with the new one",Toast.LENGTH_LONG).show()
            }

        }
}

The code for the FirebaseDB read data and comparison is written below:

fun checkDataNew()
{

    var rootRef=FirebaseDatabase.getInstance().getReference("BG Data")
    // Read from the database
    rootRef.addValueEventListener(object : ValueEventListener {
        override fun onDataChange(dataSnapshot: DataSnapshot) {
            // This method is called once with the initial value and again
            // whenever data at this location is updated.
            for(data:DataSnapshot in dataSnapshot.children)
            {
                    var oldEvent=data.child("recentEvent").getValue().toString()
                    var oldDate:String=data.child("calendarTime").getValue().toString()
                    var oldEmailID:String=data.child("emailID").getValue().toString()

                    if(oldEvent.equals(recentEvent) && oldDate.equals(calendarTime) && oldEmailID.equals(emailID)) {
                        foundKey = data.key.toString()
                        isKeyFound = true
                        return
                    }
            }
        }

        override fun onCancelled(error: DatabaseError) {
            // Failed to read value
        }
    })
    isKeyFound=false
    return
}

programming basics: [AMI](https://en.wikipedia.org/wiki/Asynchronous_method_invocation) — Selvin, Jul 18 '18 at 11:29
I see, wasn't aware that it was an asynchronous method. how do I make it a blocking call then? — Usama Jawad, Jul 18 '18 at 11:31
[well ...are you aware of ANR?](http://selvin.pl/blockingcalls.png) — Selvin, Jul 18 '18 at 11:35
Yes, but then exactly what is the solution? If I wait for the data to be read, then I'll get ANR. If I don't, then my data won't be read. Both of these things are problems for my app. — Usama Jawad, Jul 18 '18 at 11:42

score 2 · Accepted Answer · answered Jul 18 '18 at 13:45

As Selvin commented: data is loaded from Firebase asynchronously. You can't reliably wait for the data to become available. See Setting Singleton property value in Firebase Listener.

The solution is to move the code that needs the data from Firebase into the onDataChange in checkDataNew:

fun checkDataNew() {
    var rootRef=FirebaseDatabase.getInstance().getReference("BG Data")
    // Read from the database
    rootRef.addValueEventListener(object : ValueEventListener {
        override fun onDataChange(dataSnapshot: DataSnapshot) {
            var isKeyFound = false; // local variables
            var foundKey;
            // This method is called once with the initial value and again
            // whenever data at this location is updated.
            for(data:DataSnapshot in dataSnapshot.children)
            {
                var oldEvent=data.child("recentEvent").getValue().toString()
                var oldDate:String=data.child("calendarTime").getValue().toString()
                var oldEmailID:String=data.child("emailID").getValue().toString()

                if(oldEvent.equals(recentEvent) && oldDate.equals(calendarTime) && oldEmailID.equals(emailID)) {
                    foundKey = data.key.toString()
                    isKeyFound = true
                }
            }

            // TODO: process the result here
            if (isKeyFound) {
              ...
            } else {
              ...
            }
        }

        override fun onCancelled(error: DatabaseError) {
            // Failed to read value
        }
    })
}

Alternatively you can define your own callback interface, pass that into checkDataNew and invoke it from within there. For an example of this, see getContactsFromFirebase() method return an empty list.

This is a quite common question. So in addition to the links I already provided, I recommend you check out some of these:

Code does not wait for FirebaseDatabase to be read

1 Answers1