Question

I am very new to android. I got two activities A, B . Activity A parse the data from the sever and iterate through the levels. and calls the activity B through intent. Activity B takes some time to display the data so I am trying to display the progress bar. Here is my code.

 public class Display extends Activity  {

        ProgressDialog dialog;


        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.attributequestions);

            new asynctask().execute();


        }

        class asynctask extends AsyncTask<Context,Void,Void>{
            Survey[] surveyque=null;
    // i hace created seperated class forsurvey that has info about data
            String list[];  


            private ProgressDialog Dialog;
            @Override
            protected void onPreExecute()
            {
                Dialog=ProgressDialog.show(Display.this, "Parsing Data", "Please wait..........");


            }
            @Override
            protected void onPostExecute(Void unused)    
            {
                try
                {
                    if(Dialog.isShowing())
                    {
                        Dialog.dismiss();
                    }
                    Intent intent=getIntent();

                }
                catch(Exception e)
                {
    Log.d("Onsitev4", "error");
                }
            }

            @Override
            protected Void doInBackground(Context... params) {
                try {

                    LinearLayout layout1 = (LinearLayout) findViewById(R.id.linearLayout1);

    //getting exception here. I dont understant why
    // I have declared layout params and displaying activities in another class 

                    ButtonView c = new ButtonView();                
                    c.layout=layout1;
                    c.context =getBaseContext();


                    DbCoreSqlSurveys surveys=new DbCoreSqlSurveys(getBaseContext());
                    Document doc =surveys.getSurveySet();
                    surveyquestions= GetSurveyLevels(doc,c );


                } catch (TransformerFactoryConfigurationError e) {
                    e.printStackTrace();
                }
                return null;
            } 

        }

        public SurveyObject[] GetSurveyLevels(Document doc, ButtonView c) {
            NodeList nlQuestions = doc.getElementsByTagName("Survey");
            SurveyObject[] allsurveys = new SurveyObject[nlQuestions.getLength()];


            for (int i = 0; i < nlQuestions.getLength(); i++){

                Node survey =  nlQuestions.item(i);
                String f =survey.getNodeName();
                Log.d("OnsiteV4", "survey " + f);

                NodeList surveyChildNodes = survey.getChildNodes();
                SurveyObject s=new SurveyObject();

                for (int j = 0; j < surveyChildNodes.getLength(); j++){

                    Node surveyChild =  surveyChildNodes.item(j);               
                    String h =surveyChild.getNodeName();
                    Log.d("OnsiteV4", "survey child node = " + h);

                    if (h !="#text"){
                        Surveys t = Surveys.valueOf(h); 

                        switch(t){
                        case KeySurvey:
                            s.KeySurvey=surveyChild.getTextContent();
                            displaySurveyLink(s.SurveyDescription,"",c,0,s.SurveyDescription,"","","","");
                            break;
                        case SurveyDescription:
                            s.SurveyDescription=surveyChild.getTextContent();
                            displaySurveyLink(s.SurveyDescription,"",c,0,s.SurveyDescription,"","","","");
                            break;
                        case SurveyUserCode:
                            s.SurveyUserCode=surveyChild.getTextContent();
                            break;
                        case Level1:
                            if(surveyChild.hasChildNodes()){
                                s.Level1=   processLevel1Nodes(surveyChild,c,s.SurveyDescription);
                            }
                            break;
                        default:
                            break;
                        }
                    }

                    allsurveys[i]=s;
                }
            }

            return allsurveys;
        }

    // methods iterating through levels that is not showed

        private  void displaySurveyLink(final String description, String tag, ButtonView c, int indentation, final String surveyDescription, final String level1description, final String level2description, final String level3description, final String level4description)
        {
            if (description == null || tag == null){
                return;
            }
            final TextView tv = c.addButton(description,tag,indentation);


            tv.setOnClickListener(new OnClickListener(){
                public void onClick(View v) {

                    final Intent intent = new Intent();
                    intent.setClass(v.getContext(),ActivityB.class);
                    intent.putExtra("KeyLevel",tv.getTag().toString()); 

                    intent.putExtra("SurveyDescription",surveyDescription);
                    intent.putExtra("level1description",level1description);
                    intent.putExtra("level2description",level2description);
                    intent.putExtra("level3description",level3description);
                    intent.putExtra("level4description",level4description);
                    intent.putExtra("Description",description);

                    if (tv.getTag() != null){
                        if (tv.getTag().toString() != ""){
                            startActivity(intent);
                        }


                    }
                }


            });
        }
    }  

I am getting exception in doinbackground. I am confused . please help me..

Answer 1

You are getting an exception because you are accessing UI elements on a non-UI thread. The main thread that the application creates is the UI thread, and that’s where all of your visual elements are created and therefore the only thread in which you should access them.

To appropriately use AsyncTask, you run your long-running operations in doInBackground, and you use onPreExecute, onPostExecute and onProgressUpdated to work with the UI (show/hide progress dialogs, update views, etc). Whenever I use an AsyncTask and I want to show progress, I override onProgressUpdated giving it parameter type Integer and I call publishProgress from doInBackground. This would require a change of the base class signature from AsyncTask<Context,Void,Void> to AsyncTask<Context,Integer,Void>. You can use other object types for this as well…I just use Integer as an example if you want to show the percentage of the task that is complete, for example.