Android - Adding and showing items to ListView one at a time using an ArrayAdapter

Question

I'm using an ArrayAdapter to add items to a custom ListView and showing the results in my Android app. The problem I'm having is that the ArrayAdapter seems to wait until all items are in it before it shows the view. That is to say, when I'm adding the items to the ArrayAdapter and I call notifyDataSetChanged, it does not update the ListView to show the added item. It waits until all items are added and GetView is called before showing the items.

What I would like it to do is to show the item immediately after adding it to the ListView. Is this possible?

I believe the relevant code is the following:

r_adapter = new ReminderAdapater(Activity_ContentSearch.this, R.layout.search_listitem, myList);
listView.setAdapter(r_adapter);
...
r_adapter.notifyDataSetChanged();
r_adapter.clear();
for(int i = 0; i < myList.size(); i++)
{
    r_adapter.add(myList.get(i));
    r_adapter.notifyDataSetChanged();
}

As you can see, even though I am calling notifyDataSetChanged after the add method, it does not actually update the view. After it has finished the above loop the view is finally updated (based on what I know, that's because GetView isn't called until after this section of the code is done).

I tried to override the add method of my custom ArrayAdapter with no luck, since I don't have access to the view in that method.

Any help would be welcome :)

Bara

Answer 1

Android's UI is single-threaded. You are not giving control back to Android from the main application thread each time you add an entry to the adapter. Hence, Android does not get a chance to display your entries until you return control, and you're not doing that until you have populated your adapter in its entirety.

Here is an example showing the use of an AsyncTask to fill an ArrayAdapter progressively via a background thread.

/***
  Copyright (c) 2008-2012 CommonsWare, LLC
  Licensed under the Apache License, Version 2.0 (the "License"); you may not
  use this file except in compliance with the License. You may obtain   a copy
  of the License at http://www.apache.org/licenses/LICENSE-2.0. Unless required
  by applicable law or agreed to in writing, software distributed under the
  License is distributed on an "AS IS" BASIS,   WITHOUT WARRANTIES OR CONDITIONS
  OF ANY KIND, either express or implied. See the License for the specific
  language governing permissions and limitations under the License.

  From _The Busy Coder's Guide to Android Development_
    http://commonsware.com/Android
*/

package com.commonsware.android.async;

import android.app.ListActivity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.SystemClock;
import android.widget.ArrayAdapter;
import android.widget.Toast;
import java.util.ArrayList;

public class AsyncDemo extends ListActivity {
  private static final String[] items={"lorem", "ipsum", "dolor",
                                      "sit", "amet", "consectetuer",
                                      "adipiscing", "elit", "morbi",
                                      "vel", "ligula", "vitae",
                                      "arcu", "aliquet", "mollis",
                                      "etiam", "vel", "erat",
                                      "placerat", "ante",
                                      "porttitor", "sodales",
                                      "pellentesque", "augue",
                                      "purus"};
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    setListAdapter(new ArrayAdapter<String>(this,
                        android.R.layout.simple_list_item_1,
                        new ArrayList<String>()));

    new AddStringTask().execute();
  }

  class AddStringTask extends AsyncTask<Void, String, Void> {
    @Override
    protected Void doInBackground(Void... unused) {
      for (String item : items) {
        publishProgress(item);
        SystemClock.sleep(200);
      }

      return(null);
    }

    @SuppressWarnings("unchecked")
    @Override
    protected void onProgressUpdate(String... item) {
      ((ArrayAdapter<String>)getListAdapter()).add(item[0]);
    }

    @Override
    protected void onPostExecute(Void unused) {
      Toast
        .makeText(AsyncDemo.this, "Done!", Toast.LENGTH_SHORT)
        .show();
    }
  }
}