加载列表Firebase时显示进度条

Question

我想在Firebase中寻找列表时在屏幕上显示进度条。我怎样才能做到这一点？

现在的代码不会显示进度条。如果你能帮助我，谢谢你。

@Override
protected void onResume() {
    super.onResume();
    openProgressBar();
    callConnection();
    mDatabaseReference.child("bakeries").addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.getChildrenCount() > 0) {
                closeProgressBar();
                adapter = new FirebaseRecyclerAdapter<Bakery, BakeryViewHolder>(
                        Bakery.class,
                        R.layout.card_bakery,
                        BakeryViewHolder.class,
                        //referencing the node where we want the database to store the data from our Object
                        mDatabaseReference.child("bakeries").getRef()
                ) {
                    @Override
                    protected void populateViewHolder(BakeryViewHolder viewHolder, final Bakery model, final int position) {
                        if (tvNoBakeries.getVisibility() == View.VISIBLE) {
                            tvNoBakeries.setVisibility(View.GONE);
                        }
                        if (icBakery.getVisibility() == View.VISIBLE) {
                            icBakery.setVisibility(View.GONE);
                        }
                        viewHolder.tvBakeryName.setText(model.getFantasyName());
                        viewHolder.tvPhone.setText(model.getFone());
                        viewHolder.tvDistrict.setText(model.getAdress().getDistrict());
                        distance = GeoLocation.distanceCalculate(userLatitude, userLongitude,
                                model.getAdress().getLatitude(), model.getAdress().getLongitude());
                        viewHolder.tvDistance.setText(String.valueOf(distance));
                        //Picasso.with(MainActivity.this).load(model.getMoviePoster()).into(viewHolder.ivMoviePoster);

                        viewHolder.mView.setOnClickListener(new View.OnClickListener() {
                            @Override
                            public void onClick(View view) {
                                Log.w(TAG, "You clicked on " + model.getId());
                                bakeryID = model.getId();
                                String name = model.getFantasyName();
                                params.putString("bakeryID", bakeryID);
                                params.putString("name", name);
                                Intent intentProductList = new Intent(BakeryListActivity.this, ProductListActivity.class);
                                intentProductList.putExtras(params);
                                startActivity(intentProductList);
                            }
                        });
                    }
                };
                rvBakery.setAdapter(adapter);
            } else {
                tvNoBakeries.setVisibility(View.VISIBLE);
                icBakery.setVisibility(View.VISIBLE);
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.w(TAG, "getUser:onCancelled", databaseError.toException());
        }
    });

}

调用进度条活动Common Activity

protected void openProgressBar(){
    progressBar.setVisibility( View.VISIBLE );
}

protected void closeProgressBar(){
    progressBar.setVisibility( View.GONE );
}