How to handle Back Button press in android fragments

Issue

I have a class which extends fragment. It receives a data from server and loads on the fragment layout. The data comes from AsyncTask doInBackground. But I am facing a problem that if asynctask is still doing in background and user presses back button, my app goes to another activity. But when the asynctask finishes, it does not find the needed layout and hence crashes the app. How to cancel this async task when user presses the back button in the middle of do in background, and more importantly, how to call on back pressed in fragments.

Here is my code:

TabsPagerAdapter.java

public class TabsPagerAdapter extends FragmentPagerAdapter {

    public TabsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int index) {

        switch (index) {
        case 0:
            // Top Rated fragment activity
            return new TopStoriesFragment();
        case 1:
            // Games fragment activity
            return new NationalFragment();
        case 2:
            // Movies fragment activity
            return new InternationalFragment();

        case 3:
            // Movies fragment activity
            return new SportsFragment();

        case 4:
            // Movies fragment activity
            return new TechnologyFragment();
        case 5:
            // Movies fragment activity
            return new BusinessFragment();
        case 6:
            // Movies fragment activity
            return new EntertainmentFragment();
        }


        return null;
    }

    @Override
    public int getCount() {
        // get item count - equal to number of tabs
        return 7;
    }

    public void stop() {
        // get item count - equal to number of tabs
        return;
    }

}

MainActivity.java

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener {

    private ViewPager viewPager;
    private TabsPagerAdapter mAdapter;
    private ActionBar actionBar;
    // Tab titles
    private String[] tabs = { "Top Stories", "National", "International", "Sports", "Technology", "Business", "Entertainment"};

    @SuppressLint("NewApi")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        viewPager = (ViewPager) findViewById(R.id.pager);
        actionBar = getActionBar();
        mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

        viewPager.setAdapter(mAdapter);
        actionBar.setHomeButtonEnabled(false);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);        

        // Adding Tabs
        for (String tab_name : tabs) {
            actionBar.addTab(actionBar.newTab().setText(tab_name)
                    .setTabListener(this));
        }

        /**
         * on swiping the viewpager make respective tab selected
         * */
        viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

            @Override
            public void onPageSelected(int position) {
                // on changing the page
                // make respected tab selected
                actionBar.setSelectedNavigationItem(position);
            }

            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {
            }

            @Override
            public void onPageScrollStateChanged(int arg0) {
            }
        });
    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {
    }

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        // on tab selected
        // show respected fragment view
        viewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
    }

}

BusinessFragment.java

public class BusinessFragment extends Fragment {

    public String thehindu = "xxxxx";
    public String toi = "xxxxx";
    public String hindustantimes = "xxxxx";
    public String indianexpress = "xxxxxx";
    public String reuters = "xxxxxxx";

    public String dainikjagaran = "xxxxxx";
    public String dainikbhaskar = "xxxxx";
    public String amarujala = "xxxxxx";

    public String currenturl;
    AlertDialog levelDialog;
    public LayoutInflater inflater;
    ListView listView;
    int mCurCheckPosition;
    SharedPreferences sp;
    String papername;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_data, container,
                false);

        sp = getActivity().getSharedPreferences("myfile",
                getActivity().MODE_PRIVATE);
        papername = sp.getString("Newspaper_name", null);

        feedTitle = (TextView) rootView.findViewById(R.id.feedtitle);
        feedDescribtion = (TextView) rootView
                .findViewById(R.id.feeddescribtion);

        getCurrentURL();

        listView = (ListView) rootView.findViewById(android.R.id.list);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View v,
                    int position, long id) {

                Intent intent = new Intent(getActivity(), ShowDetails.class);
                Bundle bundle = new Bundle();
                bundle.putString("keyTitle", myRssFeed.getItem(position)
                        .getTitle());
                bundle.putString("keyDescription", myRssFeed.getItem(position)
                        .getDescription());
                bundle.putString("keyLink", myRssFeed.getItem(position)
                        .getLink());
                bundle.putString("keyPubdate", myRssFeed.getItem(position)
                        .getPubdate());
                intent.putExtras(bundle);
                startActivity(intent);

            }
        });

        startReadRss();

        return rootView;
    }

    public class RssLoadingTask extends AsyncTask<Void, Void, Void> {

        @Override
        protected void onPostExecute(Void result) {
            // TODO Auto-generated method stub
            displayRss();
        }

        @Override
        protected void onPreExecute() {
            // TODO Auto-generated method stub
            preReadRss();
        }

        @Override
        protected void onProgressUpdate(Void... values) {
            // TODO Auto-generated method stub
            // super.onProgressUpdate(values);
        }

        @Override
        protected Void doInBackground(Void... arg0) {
            // TODO Auto-generated method stub
            readRss();
            return null;
        }

    }

    private RSSFeed myRssFeed = null;

    TextView feedTitle;
    TextView feedDescribtion;

    // TextView feedPubdate;
    // TextView feedLink;

    public class MyCustomAdapter extends ArrayAdapter<RSSItem> {

        public MyCustomAdapter(Context context, int textViewResourceId,
                List<RSSItem> list) {
            super(context, textViewResourceId, list);
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            // TODO Auto-generated method stub
            // return super.getView(position, convertView, parent);

            View row = convertView;

            if (row == null) {
                inflater = LayoutInflater.from(getActivity());
                row = inflater.inflate(R.layout.row, parent, false);
            }

            TextView listTitle = (TextView) row.findViewById(R.id.listtitle);
            listTitle.setText(myRssFeed.getList().get(position).getTitle());
            TextView listPubdate = (TextView) row
                    .findViewById(R.id.listpubdate);
            listPubdate.setText(myRssFeed.getList().get(position).getPubdate());

            return row;
        }
    }

    private void startReadRss() {
        new RssLoadingTask().execute();
    }

    private void getCurrentURL() {

        if (papername.equalsIgnoreCase("The Times of India")) {
            currenturl = toi;
        } else if (papername.equalsIgnoreCase("The Hindu")) {
            currenturl = thehindu;
        } else if (papername.equalsIgnoreCase("The Hindustan Times")) {
            currenturl = hindustantimes;
        } else if (papername.equalsIgnoreCase("The Indian Express")) {
            currenturl = indianexpress;
        } else if (papername.equalsIgnoreCase("Reuters")) {
            currenturl = reuters;
        } else if (papername.equalsIgnoreCase("दैनिक जागरण")) {
            currenturl = dainikjagaran;
        } else if (papername.equalsIgnoreCase("दैनिक भास्कर")) {
            currenturl = dainikbhaskar;
        } else if (papername.equalsIgnoreCase("अमर उजाला")) {
            currenturl = amarujala;
        }
        // else if(papername.equalsIgnoreCase("Times of India"))
        // {
        //
        // }
        // else if(papername.equalsIgnoreCase("Times of India"))
        // {
        //
        // }
        //

    }

    private void preReadRss() {
        feedTitle.setText("--- wait ---");
        feedDescribtion.setText("");
        // feedPubdate.setText("");
        // feedLink.setText("");
        listView.setAdapter(null);

        // Toast.makeText(getActivity(), "Reading RSS, Please wait.",
        // Toast.LENGTH_LONG)
        // .show();
    }

    private void readRss() {

        try {
            URL rssUrl = new URL(currenturl);
            SAXParserFactory mySAXParserFactory = SAXParserFactory
                    .newInstance();
            SAXParser mySAXParser = mySAXParserFactory.newSAXParser();
            XMLReader myXMLReader = mySAXParser.getXMLReader();
            RSSHandler myRSSHandler = new RSSHandler();
            myXMLReader.setContentHandler(myRSSHandler);
            InputSource myInputSource = new InputSource(rssUrl.openStream());
            myXMLReader.parse(myInputSource);

            myRssFeed = myRSSHandler.getFeed();

        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ParserConfigurationException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (SAXException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    private void displayRss() {
        if (myRssFeed != null) {
            Calendar c = Calendar.getInstance();
            // String strCurrentTiime = "\n(Time of Reading - "
            // + c.get(Calendar.HOUR_OF_DAY)
            // + " Hrs. : "
            // + c.get(Calendar.MINUTE) + " Mins.)\n" +c.get(Calendar.AM_PM);

            SimpleDateFormat sdf = new SimpleDateFormat(
                    "MMMM-dd-yyyy HH:mm:ss a");
            String strCurrentTiime = sdf.format(c.getTime());

            feedTitle.setText(papername);
            feedDescribtion.setText(strCurrentTiime);
            // feedPubdate.setText(myRssFeed.getPubdate());
            // feedLink.setText(myRssFeed.getLink());

            MyCustomAdapter adapter = new MyCustomAdapter(getActivity(),
                    R.layout.row, myRssFeed.getList());
            listView.setAdapter(adapter);

        }
    }

}

Solution

I think you can do this:

Create a generic Fragment:

public abstract class extends BaseFragment {

    public abstract boolean  onBackPressed();

}

In his Fragments, extends this BaseFragment, example:

public class TopStoriesFragment extends BaseFragment {

    ...


    @Override
    public boolean onBackPressed()
    {
       //if you return false, then it is because you have not consumed 
       //the event, but if it          returns true, which is consumed by this event.
        return false;
    }
}

In his MainActivity activity, you should check in onBackPressed event, if the event was consumed or not, to decide what to do.

It is very important that you save the current tab!

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener {

       //It is very important that you save the current tab!
       private BaseFragment currentTab;

     // ...  implementation 



public class TabsPagerAdapter extends FragmentPagerAdapter {

    public TabsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int index) {

         currentTab = null;

        switch (index) {
        case 0:
            // Top Rated fragment activity
            currentTab = new TopStoriesFragment();
       break;
        case 1:
            // Games fragment activity
            return new NationalFragment();
        case 2:
            // Movies fragment activity
            currentTab = new InternationalFragment();
          break;
        case 3:
            // Movies fragment activity
            currentTab = new SportsFragment();
           break;
        case 4:
            // Movies fragment activity
            currentTab = new TechnologyFragment();
        case 5:
            // Movies fragment activity
            currentTab = new BusinessFragment();
         break;
        case 6:
            // Movies fragment activity
            currentTab = new EntertainmentFragment();
        break;
        }


        return currentTab;
    }

    @Override
    public int getCount() {
        // get item count - equal to number of tabs
        return 7;
    }

    public void stop() {
        // get item count - equal to number of tabs
        return;
    }

}


@Override
public void onBackPressed() 
{
  //If the event returned false, then call the super.
  if(currentTab == null || !currentTab.onBackPressed())
  super.onBackPressed();
}

}

Change to your case!

Answered By - Cícero Moura