Android RecyclerView 未在 adapter.notifyDataSetChanged 上刷新

Question

Classic problem here: I'm fetching some data from a database into a recyclerView using an ArrayList of custom objects (and this happens in a Fragment, not in the Main Activity). Everything works like a charm until I try to refresh the recyclerView using a spinner that changes how the data is sorted. I know the data is fed to the recyclerView correctly. What am I doing wrong? API level is 19.

This is how the fetch is done:

    public ArrayList<LEGOSet> getSets(String conditions) {
        ArrayList<LEGOSet> sets = new ArrayList<>();

        Cursor cursor = database.rawQuery("SELECT * FROM Sets " + conditions, null);
        cursor.moveToFirst();
        while (!cursor.isAfterLast()) {
                LEGOSet set = new LEGOSet(cursor.getInt(0), cursor.getString(1), cursor.getInt(2), cursor.getInt(3), cursor.getString(4), cursor.getDouble(5),
                    cursor.getDouble(6), cursor.getDouble(7), cursor.getDouble(8), cursor.getDouble(9), cursor.getDouble(10), cursor.getString(11),
                    cursor.getString(12), cursor.getString(13), cursor.getString(14), cursor.getInt(15), cursor.getInt(16), cursor.getInt(17));
            sets.add(set);
            cursor.moveToNext();
        }
        cursor.close();
        return sets;
    }

The fragment with the data being pulled, the recyclerView being set up and with the spinner onItemSelected code:

@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View root = inflater.inflate(R.layout.fragment_catalog, container, false);

        // pull data from database
        sets = dbManager.getSets("order by pieces desc");

        // set up the RecyclerView
        final RecyclerView recyclerView = root.findViewById(R.id.rvSets);
        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        adapter = new MyRecyclerViewAdapter(getActivity(), sets, portrait);
        adapter.setClickListener(this);
        recyclerView.setAdapter(adapter);

        // add dividers
        DividerItemDecoration dividerItemDecoration = new DividerItemDecoration(recyclerView.getContext(), 1);
        dividerItemDecoration.setDrawable(new ColorDrawable(getResources().getColor(R.color.colorSubtle)));
        recyclerView.addItemDecoration(dividerItemDecoration);

        // spinner Product Sorting
        spinnerProductSorting = root.findViewById(R.id.spinnerProductSorting);
        spinnerProductSorting.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parentView, View selectedItemView, int position, long id) {
                String s = spinnerProductSorting.getSelectedItem().toString();

                switch(s)
                {
                    case "Biggest first":
                        sets = dbManager.getSets("order by pieces desc");
                        Toast.makeText(getContext(),sets.get(0).getName(), Toast.LENGTH_SHORT).show();
                        break;
                    case "Smallest first":
                        sets = dbManager.getSets("order by pieces asc");
                        Toast.makeText(getContext(),sets.get(0).getName(), Toast.LENGTH_SHORT).show();
                        break;
                    default:
                }
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onNothingSelected(AdapterView<?> parent) {}
        });


        disclaimer = root.findViewById(R.id.disclaimer);
        disclaimer.setMovementMethod(LinkMovementMethod.getInstance());

        return root;
    }

What I've noticed is that I can get refreshed recyclerView if I replace:

adapter.notifyDataSetChanged();

with

adapter = new MyRecyclerViewAdapter(getActivity(), sets, portrait);
recyclerView.setAdapter(adapter);

But that doesn't seem right. I should be able to refresh the existing adapter with new data instead of creating a brand new adapter, correct?

Answer 1

This is a classic issue faced by many developers frequently.

Quick Fix:

Replace

sets = dbManager.getSets("order by pieces desc");

with

sets.clear()
sets.addAll(dbManager.getSets("order by pieces desc"));

And same for Ascending Order also.

Explanation:

When you initialize the Adapter, you pass an Arraylist whose instance is stored by the Adapter. When you call notifyDataSetChanged(), Adapter reads the instance and refreshes the layout as per the new ArrayList. However, when you reinitialize the ArrayList with sets = dbManager.getSets("order by pieces desc"); , the adapter loses the reference to the new list and is unable to refresh the layout. This can be fixed by keeping the instance the same and replacing the values which are done using clear() and addAll(list) .

Feel free to ask for any doubts in comments and please mark this answer correct if I am able to solve your problem.

Answer 2

Inside your adapter class create method:

setData(ArrayList<LEGOSet> sets) {  // new list here
    this.sets = sets;  // assing ArrayList from database to your list which is inside adapter class
    notifyDataSetChanged();
}

and then just replace adapter.notifyDataSetChanged() with adapter.setData(sets)