Question

I have a ListView which I need to populate using a background thread. The list needs to update as each item is retrieved. Below is a very simplified example of how I implement this.

  public class DownloadTask extends AsyncTask <MyUserObject, Integer, String>
  {
    @Override
    protected MyUserObject doInBackground(MyUserObject... myUserObj) 
    {
        MyUserObject muo = null;
        int nCount = myUserObj.length;
        if( nCount > 0 )
            muo = myUserObj[0];

        muo.DownloadStuff();
        return muo.getUserName();
    }

    protected void onPostExecute(String userName)
    {
        adapter.names.add(userName);
        adapter.notifyDataSetChanged();
    }
}

public class MyAdapterClass extends BaseAdapter 
{
    private ArrayList<String>names;

    public MyAdapterClass(Context context)
    {
        names = new ArrayList<String>();
    }

    public fillList()
    {
         for( int i=0; i<users.length; i++ )
         {
             DownloadTask task = new DownloadTask();
             task.execute(users[i]);
         }
    }

In the above example, ‘adapter’ is an object of type MyAdapterClass, and its fillList() method is what launches the threads. Calling notifyDataSetChanged() in onPostExecute() is what updates my ListView as data arrives.

The problem is, that I am accessing my sqlite database in “DownloadStuff()’ which is called in ‘doInBackground’, and having multiple threads accessing the DB causes it to crash. (If I comment out all DB activities in here, then it runs fine). Below is how I try to workaround this problem, however it still crashes. Any advice on how I can have my ListView update as data is retrieved from a background thread?

 Semaphore semaphore = new Semaphore(1, true);

 public synchronized void DownloadStuff()
 {
     semaphore.acquire(1);
     // ... DB operations ... //
     semaphore.release(1);
 }

Answer 1

I think your approach is wrong from it’s beginning. Why do you want to start separate AsyncTask for each item you have to add to your adapter. Use onProgressUpdate to notify the gui for newly added items in the adapter. In this case you want have concurrent access to your db.