CODEWITHSUNDEEP
androidfirebasefirebase-realtime-database
ayelet
ayelet

Reputation: 17

Android - onDataChange() strange behavior

I create an app with Firebase. There is an issue that i can't solve, and didn't find it talked here.

In this method I want to check if some data is already in the server. If not - I want to add it (the code of adding works well. The Firebase database is being changed as I want). so I'm using onDataChange method as following: 

public boolean insertNewList(String title)
{
    System.out.println("start: ");

    final boolean[] result = new boolean[1];
    result[0]=false;

    final String group = title;

    mRootRef = some reference...



    mRootRef.addValueEventListener(new ValueEventListener()
    {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot)
        {

            System.out.println(0);

            if (dataSnapshot.child(group).getValue() != null)
            {
                System.out.println(group + " is already exist");
                System.out.println(1);

            }

            //write the data.
            else
            {
                mRootRef=mRootRef.child(group);
                mRootRef.push().setValue("some data");
                System.out.println(2);
                result[0]=true;
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError)
        {

        }
    });


    System.out.println(3);

    return result[0];
}

But what realy happens is this output:

begin:
3 (just skip on onDataChange method and return false).
some print after calling the function
0 (goes back to function and enter to onDataChange method)
2 (finally goes where I want it to go)
0 (for some reason enters twice  :(   )
1

And because of that i receive wrong results in this function. Can you help please?

Upvotes: 0

Views: 2007

Answers (2)

martin jakubik
martin jakubik

Reputation: 4148

The output that you showed us looks normal to me. Let me try to explain:

begin: // a) this comes from the System.out.println("begin")
3      // b) you DECLARE your value event listener and then you run
       // the System.out.print("3") statement
0      // c) you just added a listener so firebase calls you, and
       // your listener fires
2      // d) this is the first time your listener fires, so you go
       // into the block called "write the data"
0      // e) since you just wrote the data, firebase calls you again
1      // f) this time, since the data has been written, you go into
       // the block called "is alraedy exist"

This is normal behaviour for firebase. In c), firebase always calls you back one time when you declare a listener. In e), firebase calls you because the data changed. But in b), you are only declaring your listener, not yet running it, so the statements after this declaration are executed and you see "3" before anything else happens.

Upvotes: 0

Nirel
Nirel

Reputation: 1925

Replace

mRootRef.addValueEventListener(new ValueEventListener()

with

mRootRef.addListenerForSingleValueEvent(new ValueEventListener()

When you add the the value to firebase, "addValueEventListener" called again, not like addListenerForSingleValueEvent that shots only once anywhy.

Upvotes: 1

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