如何将项目动态添加到 ListView 的自定义适配器

Question

I made adapter for ListView. If I fill the elements array before creating the adapter and linking it to the listView, the elements are displayed. But if I use the updateItems () method to add items when the button is clicked, nothing happens. Code of adapter:

public class ListAdapter extends ArrayAdapter<Lf> {
private LayoutInflater inflater;
private int layout;
private List<Lf> lfs;

public ListAdapter(Context context, int resource, List<Lf> lfs) {
    super(context, resource, lfs);
    this.lfs = lfs;
    this.layout = resource;
    this.inflater = LayoutInflater.from(context);
}
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder viewHolder;
    if(convertView==null){
        convertView = inflater.inflate(this.layout, parent, false);
        viewHolder = new ViewHolder(convertView);
        convertView.setTag(viewHolder);
    }
    else{
        viewHolder = (ViewHolder) convertView.getTag();
    }
    Lf lf = lfs.get(position);

    viewHolder.name.setText(lf.getLf());
    viewHolder.freq.setText((int)lf.getFreq() + "");

    return convertView;
}

public void updateItems(List<Lf> lfs) {
    this.lfs.clear();
    this.lfs.addAll(lfs);
    this.notifyDataSetChanged();
}

private class ViewHolder {
    final TextView name, freq;
    ViewHolder(View view){
        name = (TextView) view.findViewById(R.id.text_item_1);
        freq = (TextView) view.findViewById(R.id.text_item_2);
    }
}
}

Code of MainActivity:

public class MainActivity extends AppCompatActivity {
    EditText editText1;
    ListView listView;
    ListAdapter adapter;
    List<Lf> elements = new ArrayList<>();

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

        editText1 = (EditText) findViewById(R.id.edit_text);
        listView = (ListView) findViewById(R.id.fragment_list);
        // Working... Elements print on screen
        for(int i = 0; i < 10; i++) {
            Lf temp = new Lf();
            temp.setLf("mean");
            temp.setFreq(100);
            elements.add(temp);
        }

        adapter = new ListAdapter(this, R.layout.list_item, elements);
        listView.setAdapter(adapter);

    }


    public void activity_button(View view) {
        adapter.updateItems(elements);
    }
}

If I click on the button, the existing items on the screen are cleared instead of a new one added. But in debug I see that elements normally passed to ListAdapter.

Answer 1

The problem is that your adapter's lfs 's field and your activity's elements field both refer to the same List instance. This happens because you pass elements to the ListAdapter constructor, and then simply assign this.lfs = lfs .

So let's look at what happens when you pass elements to updateItems() ...

public void updateItems(List<Lf> lfs) {
    this.lfs.clear(); // this.lfs and the input lfs are the same list, so this clears both
    this.lfs.addAll(lfs); // input lfs is now empty, so addAll() does nothing
    this.notifyDataSetChanged();
}

Probably the best thing to do is to create a copy of the list in your adapter's constructor.

this.lfs = new ArrayList<>(lfs);

Now your adapter and activity will reference different lists, so this.lfs.clear() won't accidentally clear out the very list you're passing to it.