[FIXED] Why my RecyclerView updated while I don't notify the adapter?

Issue

I have some trouble to understand why my recyclerView get update while I add the data only after setAdapter() and never call notifyDataSetChanged() !

In the same type: -> I add 3 String to my list -> Notify the adapter only for the first added (notifyItemInserted)

Result: 3 String displayed.

If anyone can help me understand I will be very grateful to him :)

https://www.noelshack.com/2018-04-5-1516969446-recycler.png

my code:

public class MainActivity extends AppCompatActivity {
List<String> mList = new ArrayList<>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //Call before set adapter so it's normal if show when the adapter is set
    mList.add("Before set the adapter");

    RecyclerView recyclerView = findViewById(R.id.rcv);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    final Adapter mAdapter = new Adapter(mList);
    recyclerView.setAdapter(mAdapter);


    Button button = findViewById(R.id.button);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            mList.add("Button add " +0);
            mAdapter.notifyItemInserted(mList.size()-1);

            //Again it's showing but never notify
            mList.add("Button add " +1);
            mList.add("Button add " +2);
        }
    });

    //Called after setAdapter and NEVER notify the adapter but it's still show the data
    addMoreData();

}


private void addMoreData() {
    for (int i = 0; i < 10; i++) {
        mList.add("addMoreData: " + i);
    }
}


class Adapter extends RecyclerView.Adapter<Adapter.ViewHolder> {
    class ViewHolder extends RecyclerView.ViewHolder {
        TextView nameTextView;

        ViewHolder(View itemView) {

            super(itemView);
            nameTextView = (TextView) itemView.findViewById(R.id.textview);
        }
    }

    @Override
    public Adapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.raw, parent, false);
        return new Adapter.ViewHolder(view);
    }

    private List<String> mList;

    Adapter(List<String> list) {
        mList = list;
    }

    @Override
    public void onBindViewHolder(Adapter.ViewHolder holder, int position) {
        holder.nameTextView.setText(mList.get(position));

    }

    @Override
    public int getItemCount() {
        return mList.size();
    }
}
}

Solution

I think, because of notifyDataSetChanged's working principle, this is the result.

A Google engineer said ;

When you call notifyDataSetChanged, RecyclerView invalidates the data but does not update the UI until the next animation frame. This is how android view system works. When a widget is invalidated (e.g. changing its data) it requests a layout which means it will be re-measured and re-laid out in the next view traversal. This is done so that we can batch all changes until the next time screen will be updated.

That means; nofiyDataSetChanged waits the UI Thread. So, doesn't matter the below lines of the notifyItemInserted from your code.

    mList.add("Button add " +0);
    mAdapter.notifyItemInserted(mList.size()-1);

    //Again it's showing but never notify
    mList.add("Button add " +1);
    mList.add("Button add " +2);

Answered By - ziLk
Answer Checked By - Cary Denson (JavaFixing Admin)