How to stop the AsyncTask if the result of fetching data is null?

Question

I want to cancel the asyncTask if my result string is null. ( I get a username and password from the user in a login activity and if this username and password don't exist in the database, the asyncTask has to cancel and ready to start the same task.) I read and applied something but they didn't run. Here is my AsyncTask :

class ProductConnect extends AsyncTask<Boolean, String, String> {

   public AsyncResponse delegate=null;

   private Activity activity;


   public void MyAsyncTask(Activity activity) {
        this.activity = activity;
    }


    @Override
    protected String doInBackground(Boolean... params) {

        String result = null;

        StringBuilder sb = new StringBuilder();

        try {

            // http post
            HttpClient httpclient = new DefaultHttpClient();

            HttpGet httppost = new HttpGet( "http://191.165.2.235/getProducts.php?login=1&user_name="+UserName+"&user_pass="+Password);



            HttpResponse response = httpclient.execute(httppost);

            if (response.getStatusLine().getStatusCode() != 200) {
                Log.d("MyApp", "Server encountered an error");
            }

          BufferedReader reader = new BufferedReader(
                    new InputStreamReader(
                            response.getEntity().getContent(), "UTF8"));  



            sb = new StringBuilder();

            sb.append(reader.readLine() + "\n");

            if(reader.readLine() == null){

                asyncTask.cancel(true);
            }

            String line = null;

            while ((line = reader.readLine()) != null) {

                sb.append(line + "\n");

                if (isCancelled()) break;
            }


            result = sb.toString();

            Log.d("test", result);

        } 
        catch (Exception e) {

            Log.e("log_tag", "Error converting result " + e.toString());

        }

        return result;
    }




    @Override
    protected void onPostExecute(String result) {

        Intent passValue=new Intent(MainActivity.this, second.class);



        try {

            JSONArray jArray = new JSONArray(result);
            JSONObject json_data;
            for (int i = 0; i < jArray.length(); i++) {
                json_data = jArray.getJSONObject(i);

                t = json_data.getString("name");
                names.add(t);                                             

                latitude=json_data.getString("lat");
                lats.add(latitude);                                       

                longtitude=json_data.getString("lon");
                longts.add(longtitude);                                   

            }


            passValue.putStringArrayListExtra("latitudes", (ArrayList<String>) lats);

            passValue.putStringArrayListExtra("veri", (ArrayList<String>) names);

            passValue.putStringArrayListExtra("longtitudes", (ArrayList<String>) longts);

            startActivity(passValue);   

        } catch (JSONException e1) {
            e1.printStackTrace();
        } catch (ParseException e1) {
            e1.printStackTrace();
        }
        super.onPostExecute(result);
    }

     protected void onPreExecute()                          {
            super.onPreExecute();
            ProgressDialog pd = new ProgressDialog(MainActivity.this);
            pd.setTitle("Lütfen Bekleyiniz");
            pd.setMessage("Authenticating..");
            pd.show();

                                                                     }
}

How should i follow a way ? Which methods should i use?

Answer 1

You are missing logic here, you should only call Async task if your data is not null. Like this

b.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub

            EditText username=(EditText)findViewById(R.id.editText2);

            String UserName=username.getText().toString().trim();

            EditText password=(EditText)findViewById(R.id.editText1);
            String Password=password.getText().toString().trim(); 

            if (UserName.length()>0&& Password.length()>0) {
                asyncTask.execute(true); } 
        }
    });

Answer 2

AsyncTask is mainly using for operations which we wants to do in background thread, like webservice call. In this scenario, you are using AsyncTask for saving username and password through webservice. So before executing AsyncTask you must validate and have a null-check for both edit-texts for username and password. If everything is fine and get the password according to the defined password policy, just invoke the AsyncTask with these two values for httppost.

Answer 3

I notice this old post does not have the answer. So I will post this, hopefully it will help others.

To cancel a task simply call cancel()

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

Parameters mayInterruptIfRunning true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete.

Returns false if the task could not be cancelled, typically because it has already completed normally; true otherwise

---

So in your case:

result = sb.toString();

if(result==null){
   this.cancel(true);
}
Log.d("test", result);

to cancel outside of the task:

myTask.cancel(true);

You can use isCancel() to check , it returns true if this task was cancelled before it completed normally.

Additonally you can override asynctask method onCancelled() to take action when it's cancelled.

@Override
protected void onCancelled() {

}