Firebase检索的值在方法外部变为零

Question

当我使用侦听器从Firebase检索数据时，方法外的变量变为零。因此，值1总是会更新。如何将值从Firebase获取到变量中？我知道该方法是异步的，但是我仍然在方法内部分配了变量，如何处理它。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.poll);
    mFirebaseDatabase = FirebaseDatabase.getInstance();
    mVotesRef = mFirebaseDatabase.getReference().child("votes");

    mChildEventListner = new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            if(dataSnapshot.child("votes").exists()) {
                bjp = dataSnapshot.child("bjp").getValue(Integer.class);
                congress = dataSnapshot.child("congress").getValue(Integer.class);
                others = dataSnapshot.child("others").getValue(Integer.class);
                nato = dataSnapshot.child("nato").getValue(Integer.class);
            }
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
            bjp = dataSnapshot.child("bjp").getValue(Integer.class);
            congress = dataSnapshot.child("congress").getValue(Integer.class);
            others = dataSnapshot.child("others").getValue(Integer.class);
            nato = dataSnapshot.child("nato").getValue(Integer.class);
        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) { }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) { }

        @Override
        public void onCancelled(DatabaseError databaseError) { }
    };
    mVotesRef.addChildEventListener(mChildEventListner);

    ImageButton bjpButton = (ImageButton)findViewById(R.id.bjp);
    bjpButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mVotesRef.child("bjp").setValue(bjp+1);
        }
    });
    final ImageButton congressButton = (ImageButton)findViewById(R.id.congress);
    congressButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mVotesRef.child("congress").setValue(congress+1);
        }
    });