Android: custom ListView doesn't get populated immediately

I would like to create a custom ListView populated by a custom ArrayAdapter. The code works fine, but the ListView is not showing data immediately. In order to have the data shown I need to click on a random EditText of the same page, close the Keyboard and magically the ListView shows data.
I don't know if it this can have an impact but the adapter is used in a fragment,which retrieve the data from Firebase during the OnCreate method.
Below my code, I removed everything not necessary for this topic and simplified the array.

This is my Class of elements inside the array:

public class ListItem {
private String Description;
private String Price;
public ListItem (String Description, String Price) {
this.Description = Description;
this.Price = Price;
}
public String getDescription() { return Description; }
public String getPrice() {
return Price;
}
public void setDescription(String description) {
Description = description;
}
public void setPrice(String price) {
Price = price;
}
}

The adapter is the following:

public class CustomListAdapter extends ArrayAdapter<ListItem> {
private Context mContext;
private Integer mResource;
private static class ViewHolder {
TextView txt_description_item;
TextView txt_price_item;
}
public CustomListAdapter(@NonNull Context context, int resource, @NonNull ArrayList<ListItem> objects) {
super(context, resource, objects);
mContext = context;
mResource = resource;
}
@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
String Description = getItem(position).getDescription();
String Price = getItem(position).getPrice();
LayoutInflater inflater = LayoutInflater.from(mContext);
convertView = inflater.inflate(mResource, parent, false);
ViewHolder viewHolder = new ViewHolder();
viewHolder.txt_description_item = convertView.findViewById(R.id.txt_description_item);
viewHolder.txt_price_item = convertView.findViewById(R.id.txt_price_item);
viewHolder.txt_description_item.setText(Description);
viewHolder.txt_price_item.setText(Price);
return convertView;
}
}

Finally this is the Fragment code:

public class MyFragment extends Fragment {
private ListView view_list_items;
private FirebaseDatabase database;
private DatabaseReference refDatabase;
private static final String USER = "user";
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
final View v = inflater.inflate(R.layout.fragment_myprofile, container, false);
view_list_items = v.findViewById(R.id.list_items);
database = FirebaseDatabase.getInstance();
refDatabase = database.getReference(USER);
refDatabase.child("items").addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
final ArrayList<ListItem> arr_items = new ArrayList<>();
for (DataSnapshot ds : dataSnapshot.child("itemlist1").getChildren()) {
final String description = ds.getKey();
final String price = ds.getValue(String.class);
ListItem listItem = new ListItem(description,price);
arr_items.add(listItem);
}
CustomListAdapter adapter = new CustomListAdapter(getContext(),R.layout.layout_myItemList, arr_items);               
view_list_items.setAdapter(adapter);
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
}
});
}

EDIT:
I solved the issue by setting the adapter inside the 'for' cycle. Maybe it's not the most elegant solution but it works fine

for (DataSnapshot ds : dataSnapshot.child("itemlist1").getChildren()) {
final String description = ds.getKey();
final String price = ds.getValue(String.class);
ListItem listItem = new ListItem(description,price);
arr_items.add(listItem);
CustomListAdapter adapter = new 
CustomListAdapter(getContext(),R.layout.layout_myItemList, arr_items);               
view_list_items.setAdapter(adapter);
}

The firebase database is asynchronous i.e, the compiler doesn't wait for the code inside onDataChange() or onCancelled() to return a value. It would return all the values apart from those inside those 2 methods immediately, while for those 2 methods it would return the value when the value is available from the database. Below code might help you in better understanding

 refDatabase.child("items").addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
System.out.println("Inside onDataChange method");
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
}
});
System.out.println ("Outside onDataChange method");

The output will be like:

Outside onDataChange method
Inside onDataChange method

Notice that comes Outside onDataChange method first. This is the reason why your listView doesn't show data immediately.
What you can do to keep the user engaged is, add a spinner in your view which will be visible till the data is fetched from the database. To do this, try this code.

        database = FirebaseDatabase.getInstance();
refDatabase = database.getReference(USER);
spinner.setVisibility(View.VISIBLE);
refDatabase.child("items").addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
spinner.setVisibility(View.INVISIBLE);
//Your code goes here
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
//Error code goes here
}
});