单值事件多次执行的侦听器

Question

sqlite = mDatabaseReference.child("Messages")
            .child(MessageSenderId).child(MessageRecieverId).orderByChild("Opened").equalTo(0);
valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        long count = 0;
        for (DataSnapshot ds : dataSnapshot.getChildren()) {
            count = dataSnapshot.getChildrenCount();

            MainData helper = new MainData(getApplicationContext()); //Change the name to your Helper Class name
            SQLiteDatabase db = helper.getWritableDatabase();
            String newId = "MyData";
            Cursor data = helper.getData();
            long newDatar = 0;
            long newDatat = 0;
            while (data.moveToNext()) {
                newId = data.getString(data.getColumnIndex("Data"));
                newDatar = data.getInt(data.getColumnIndex("TotalMessagesRecieved"));
                newDatat = data.getInt(data.getColumnIndex("TotalMessages"));
            }
            ContentValues contentValues = new ContentValues();
            contentValues.put(KEY_DATA, newId);
            contentValues.put(KEY_TOTAL_MESSAGES_RECIEVED, (newDatar + count));
            contentValues.put(KEY_TOTAL_MESSAGES, (newDatat + count));//Change the value of newData(which is actually your old value) by incrementing
            long returnVariable = db.update(TABLE_MAIN_DATA, contentValues, null, null);
            Log.d("FUCKU", String.valueOf(count) + "/" + String.valueOf(newDatar) + "/" + String.valueOf(newDatat));

            if (returnVariable == -1) {
                Toast.makeText(getApplication(), "Nope", Toast.LENGTH_LONG).show();
                //-1 means there was an error updating the values
            } else {
                Toast.makeText(getApplication(), "r", Toast.LENGTH_SHORT).show();
            }
        }
        Log.d("CMONNN", String.valueOf(count)); //Will print the number of seen messages
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        Log.d("CMONNN", databaseError.getMessage()); //Don't ignore errors!
    }
};
sqlite.addListenerForSingleValueEvent(valueEventListener);

您可以看到它执行了多次，但不是随机执行，而是执行的次数等于count的值。如果count为1，则执行一次。如果为2，则执行两次，如果三次，则执行三次。但是，即使它是单个值事件侦听器，为什么也会发生这种情况？