I came across this android example that runs an AsyncTask from a UI thread. The class ExportDatabaseTask is declared and instantiated in the Activity, and apparently it is possible to reference the activity's UI context from the onPreExecute and onPostExecute events, like this:

public class ManageData extends Activity { 

  private ExportDatabaseTask exportDatabaseTask; 
  [...]

  @Override 
  public void onCreate(final Bundle savedInstanceState) { 
    [...]
    ManageData.this.exportDatabaseTask = new ExportDatabaseTask(); 
    ManageData.this.exportDatabaseTask.execute(); 
    [...]
  } 

  private class ExportDatabaseTask extends AsyncTask<String, Void, Boolean> { 
    private final ProgressDialog dialog = new ProgressDialog(ManageData.this); 

    protected void onPreExecute() { 
      this.dialog.setMessage("Exporting database..."); 
      this.dialog.show(); 
    } 

    protected Boolean doInBackground(final String... args) { 
      [...]
    } 

    protected void onPostExecute(final Boolean success) { 
      if (this.dialog.isShowing()) { 
         this.dialog.dismiss(); 
      } 
    } 
} 

I am trying to refactor this so that the ExportDatabaseTask is declared in another class that is not the Activity, for various reasons, and I can't quite figure out how to make it work. I am lacking some basic Java concepts here, which I readily admit.

Specifically, myActivity is null in onPreExecute(). Why is that?

public void onClick(View v) {
  Exporter ex = new Exporter(getApplicationContext(), ActivityMain.this);
  ex.exportDatabaseTask.execute();
} 

public class Exporter {

public ExportDatabaseTask exportDatabaseTask;   

public Exporter(Context ctx, ActivityMain act) {
    myContext = ctx;
    myActivity = act;

    this.exportDatabaseTask = new ExportDatabaseTask(); 
}

     public class ExportDatabaseTask extends AsyncTask<Void, Void, Boolean> {
       private final ProgressDialog dialog = new ProgressDialog(myContext);

       // can use UI thread here?
       protected void onPreExecute() {
         // ====> this throws a Nullpointer exception:
         myActivity.dialog.setMessage("Exporting database...");
         myActivity.dialog.show();
       }

       protected Boolean doInBackground(final Void... args) {
      }

       protected void onPostExecute(final Boolean success) {
         if (myActivity.dialog.isShowing()) {
           myActivity.dialog.dismiss();
         }
       }
     }
}