Question

I am trying to implement the following logic:

User enters a search string in a text box (an activity) -> Web service is called to perform the search asynchronously -> Search results are displayed in a list (another activity)

So far I have the following:

An activity to enter the search string and click a “Search” button:

public class Search extends Activity
{
    // ...
    // called when "Search" button is clicked
    private void runSearch()
    {
        ProgressDialog progressDialog = ProgressDialog.show(
            this, "Search", "Search...");
        searchAsyncTask = new SearchAsyncTask();

        SearchAsyncTaskParam param = new SearchAsyncTaskParam();
        param.SearchString = getSearchCode(); // gets input from text box
        param.ProgressDialog = progressDialog;

        searchAsyncTask.execute(param);
    }
}

Then I have a class to perform the asynchronous search:

public class SearchAsyncTask extends AsyncTask<SearchAsyncTaskParam,
    Void, SearchAsyncTaskResult> {

    private SearchAsyncTaskParam param;

    @Override
    protected SearchAsyncTaskResult doInBackground(
        SearchAsyncTaskParam... params) {
        if (params.length > 0)
            param = params[0];
        SearchAsyncTaskResult result = new SearchAsyncTaskResult();

        // call Webservice and fill result object with status (success/failed)
        // and a list of hits (each hit contains name, city, etc.)

        return result;
    }

    @Override
    protected void onPostExecute(SearchAsyncTaskResult result) {
        param.ProgressDialog.dismiss();
        if (!result.Success)
            // throw an AlertBox
        else
        {
            // this part is incomplete and doesn't look good, does it?
            // And how would I pass my result data to the new activity?
            Intent intent = new Intent(param.ProgressDialog.getContext(),
                SearchResultList.class);
            param.ProgressDialog.getContext().startActivity(intent);
        }
    }
}

And the last element is an activity to display a list with the search results:

public class SearchResultList extends ListActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setListAdapter(new ArrayAdapter<String>(
            this, android.R.layout.simple_list_item_1, data));
        // String was only for testing, should be a class with holds the data
        // for each item, i.e. Name, City, etc. And where do I get "data" from?
    }
    // ...
}

Now my questions:

1. Is it a good idea to start the activity for the result list in the onPostExecute method of the AsyncTask instance? I have tested it with the simple test sketched above and it seems to work. But is this safe, am I on the right thread (UI thread) at all? If this is bad practice what is the alternative to start the result activity?

2. It looks especially weird to use the context of the ProgressDialog for the Intent and to start the activity. It is just the only context I have available in the AsyncTask instance. Can this be a problem because the ProgressDialog is already dismissed? What context else could I use?

3. How do I pass my result data into the SearchResultList activity?

4. In case of a successful search (the else case in the onPostExecute method above) I would like to close the Search activity with the text box so that, if the back button is hit, the user returns to the main screen (which the search activity was opened from) and not the Search activity. Is this possible and how can I do that?

Answer 1

1. Yes, because you are on the UI thread. The alternative would be if you aren’t using AsyncTask but just a simple thread. In that case you would use a Handler to notify the UI thread that work is complete. This is one of the reasons Asynctask exists, so you don’t have to do the latter version.

2. “It looks especially weird to use the context of the ProgressDialog
   for the Intent and to start the activity.”

   It’s not weird at all because when you created the ProgressDialog you passed the context of Search, so when you retrieve the context of the ProgressDialog you actually get the context of the Search activity. No problem at all.

3. This depends on what data you want to pass. If you just want to pass an array of Strings you could do it like this:

   String [] data = getData();
   Intent intent = new Intent(param.ProgressDialog.getContext(),
       SearchResultList.class);
   intent.putExtra("key1", data);
   param.ProgressDialog.getContext().startActivity(intent);
   

   Then in the SearchResultList activity you would do:

   String [] data = this.getIntent().getStringArrayExtra("key1");
   

   If you would like to pass an object of your own, see this question: How to declare global variables in Android?

4. Is it possible? Yes. There’s two ways to do this in your case. Either you can, as I would do it, write the SearchAsyncTask as a inner class in your Search class and the just call

   Search.this.finish();
   

   If some condition holds, or you can check out the accepted anwser to this question: android how to finish an activity from other activity

Good Luck!