Question

I am trying to design a helper class that implements methods using AsyncTask.

public interface ResultCallback
{
    public String processResult();
}

public class ServerAdapter
{

    // Required processResult to call this method. Kind of lousy but I do not know
    // how to throw exception from onPostExcecute in AsyncTask.
    public String getResult() throws AirplaneModeException, NoNetworkException
    {
       // code to get return value from Dowork throw exceptions on errors
    }

    public void getLicense(ResultCallback licenseCallback)
    {
        ...// Set url, outmessage
        new Dowork(url, outMessage, licenseCallback).execute();
    }

    public void queryServer(int queryId, ArrayList<String> args, ResultCallback queryCallback)
    {
        ...// Set url, outmessage
        new Dowork(url, outmessage, queryCallback);
    }

    private class Dowork extends AsyncTask<Void, Void, String>
    {
        ...
        private ResultCallback rc;

        public Dowork(String url, String outMessage, ResultCallback rc)
        {
            // code here
        }

        protected String doInBackground(Void... params)
        {
            try
            {
                 // code here
            }
            catch (AirplaneModeException e)
            {
                return "AirplaneModeException";
            }
            catch ...
         }

         protected void onPostExecute(String result)
         {
             this.result = result;
             cb.processResult();
         }
    }
}

// Client class
public class myclass extends Activity
{
    MyServerAdapter myAdapter;

    public void onCreate(Bundle savedInstanceState)
    {
        ...
        myAdapter = new ServerAdapter();
        myAdapter.getLicence(new MyLicenseCallback);
        myAdapter.queryServer(id, args, new MyQueryCallback);
        ...
    }

    public class MyLicenseCallback extends ResultCallback implements processResult
    {
        try
        {
            String result = myAdapter.getResult;
            ...
        }
        catch
        ... 
    }
    ...

}

I am new to Java and Android and have a couple of questions:

1- Would several ServerAdapter method calls cause synchronize problem? For example while code for MyLicense callback is running, if onPostExecute calls MyQueryCallback, do I have to handle it or Java handles it?

2- How to get exception thrown in Dowork thrown in the callback instead of work around like in the code above?

Answer 1

1. Android guarantees you that methods in your activity and AsyncTask.onPostExecute runs in the same main UI thread.

2. You could save the exception in the task instance variable the same way as you do for result (return, say null as the result in this case). Check if exception present or not later to handle the error situation.