ExpendableListView无法使用firebase

Question

尝试使用ExpendableListView来显示firebase数据，但它没有显示任何内容，当使用静态数据时它正在工作。 my data，还是有其他方法可以列出像ExpendableListView这样的数据，但可以使用firebase。

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    // Inflate the layout for this fragment
    final View view = inflater.inflate(R.layout.fragment_appointment, container, false);

    if (user != null) {

        buttonAppoint = view.findViewById(R.id.bappoint);

        buttonAppoint.setOnClickListener(new View.OnClickListener() {

            public void onClick(View view) {

                Fragment newFragment = new Register();
                FragmentTransaction transaction = getFragmentManager().beginTransaction();
                transaction.replace(R.id.flContent, newFragment);
                transaction.addToBackStack(null);

                // Commit the transaction
                transaction.commit();
            }
        });
        // get the listview
        expListView = view.findViewById(R.id.explist);

        // preparing list data
        prepareListData();

        listAdapter = new ExpandableListAdapter(this.getContext(), listDataHeader, listDataChild);

        // setting list adapter
        expListView.setAdapter(listAdapter);

        // Listview Group click listener
        expListView.setOnGroupClickListener(new ExpandableListView.OnGroupClickListener() {

            @Override
            public boolean onGroupClick(ExpandableListView parent, View v,
                                        int groupPosition, long id) {
                // Toast.makeText(getApplicationContext(),
                // "Group Clicked " + listDataHeader.get(groupPosition),
                // Toast.LENGTH_SHORT).show();
                return false;
            }
        });

        // Listview Group expanded listener
        expListView.setOnGroupExpandListener(new ExpandableListView.OnGroupExpandListener() {

            @Override
            public void onGroupExpand(int groupPosition) {
                Toast.makeText(getActivity().getApplicationContext(),
                        listDataHeader.get(groupPosition) + " Expanded",
                        Toast.LENGTH_SHORT).show();
            }
        });

        // Listview Group collasped listener
        expListView.setOnGroupCollapseListener(new ExpandableListView.OnGroupCollapseListener() {

            @Override
            public void onGroupCollapse(int groupPosition) {
                Toast.makeText(getActivity().getApplicationContext(),
                        listDataHeader.get(groupPosition) + " Collapsed",
                        Toast.LENGTH_SHORT).show();

            }
        });

        // Listview on child click listener
        expListView.setOnChildClickListener(new ExpandableListView.OnChildClickListener() {

            @Override
            public boolean onChildClick(ExpandableListView parent, View v,
                                        int groupPosition, int childPosition, long id) {
                // TODO Auto-generated method stub
                Toast.makeText(
                        getActivity().getApplicationContext(),
                        listDataHeader.get(groupPosition)
                                + " : "
                                + listDataChild.get(
                                listDataHeader.get(groupPosition)).get(
                                childPosition), Toast.LENGTH_SHORT)
                        .show();
                return false;
            }
        });
        Log.d(TAG, "onAuthStateChanged:signed_in:" + user.getUid());
    } else {

        Toast.makeText(getActivity(), "Please login before view/make appointment",
                Toast.LENGTH_SHORT).show();
        Fragment newFragment = new LoginFragment();
        FragmentTransaction transaction = getFragmentManager().beginTransaction();
        transaction.replace(R.id.flContent, newFragment);
        transaction.addToBackStack(null);

        // Commit the transaction
        transaction.commit();
        Log.d(TAG, "onAuthStateChanged:signed_out");
    }

    return view;
}

private void prepareListData() {
    listDataHeader = new ArrayList<>();
    listDataChild = new HashMap<>();



    myRef.orderByChild("userID").equalTo(currentUser).addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String prevChildKey) {
            Appointment appointment = dataSnapshot.getValue(Appointment.class);
            listDataHeader.add(appointment.date+" "+ appointment.time);
            List<String> appoint = new ArrayList<>();
            appoint.add("Hours of Service: "+ appointment.hours);
            appoint.add("Person In Charge: "+ appointment.personInCharge);
            appoint.add("Address: "+ appointment.address);
            appoint.add("Contact: "+ appointment.contact);
            listDataChild.put(listDataHeader.get(i), appoint); // Header, Child data
            i++;
            listAdapter.notifyDataSetChanged();
        }

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

        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

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

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}