Fragments in ViewPager vanish after onresume

Question

I am writing a two-tab application in MainActivity . I created an activity template with an Android plugin for Eclipse and adapted it. In any case, when I resume the application after a while, the Snippets will be empty.

Here is the code of my activity (I deleted the "non-essential" parts)

 public class MainActivity extends FragmentActivity implements ActionBar.TabListener { /** * The {@link android.support.v4.view.PagerAdapter} that will provide * fragments for each of the sections. We use a * {@link android.support.v4.app.FragmentPagerAdapter} derivative, which * will keep every loaded fragment in memory. If this becomes too memory * intensive, it may be best to switch to a * {@link android.support.v4.app.FragmentStatePagerAdapter}. */ SectionsPagerAdapter mSectionsPagerAdapter; private static final String ARG_SECTION_NUMBER = "section_number"; /** * Fragment to display markets on a {@link GoogleMap}. */ private GluehweinMapFragment mGluehweinMapFragment; /** * Fragment to display available markets as list. */ private GluehweinListFragment mGluehweinListFragment; /** * The {@link ViewPager} that will host the section contents. */ ViewPager mViewPager; /* * (non-Javadoc) * * @see android.support.v4.app.FragmentActivity#onCreate(android.os.Bundle) */ @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); initSectionsPagerAdapter(); //[...] } private void initSectionsPagerAdapter() { if (mSectionsPagerAdapter == null || mViewPager == null) { final ActionBar actionBar = getActionBar(); actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); // Create the adapter that will return a fragment for each of the // three // primary sections of the app. mSectionsPagerAdapter = new SectionsPagerAdapter( getSupportFragmentManager()); // Set up the ViewPager with the sections adapter. mViewPager = (ViewPager) findViewById(R.id.pager); mViewPager.setAdapter(mSectionsPagerAdapter); // When swiping between different sections, select the corresponding // tab. We can also use ActionBar.Tab#select() to do this if we have // a reference to the Tab. mViewPager .setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() { @Override public void onPageSelected(int position) { switch (position) { case 0: break; } refreshFragments(); actionBar.setSelectedNavigationItem(position); } }); actionBar.getSelectedNavigationIndex(); // For each of the sections in the app, add a tab to the action bar. for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) { // Create a tab with text corresponding to the page title // defined by // the adapter. Also specify this Activity object, which // implements // the TabListener interface, as the callback (listener) for // when // this tab is selected. actionBar.addTab(actionBar.newTab() .setText(mSectionsPagerAdapter.getPageTitle(i)) .setTabListener(this)); } } } /** * Hides or shows the closed markets on the list and map */ private void updateVisibility() { if (this.mGluehweinMapFragment != null) { mGluehweinMapFragment.updateVisibility(); } if (this.mGluehweinListFragment != null) { mGluehweinListFragment.updateVisibility(); } } @Override public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { mViewPager.setCurrentItem(tab.getPosition()); invalidateOptionsMenu(); } @Override public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { } @Override public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { } /** * A {@link FragmentPagerAdapter} that returns a fragment corresponding to * one of the sections/tabs/pages. */ public class SectionsPagerAdapter extends FragmentPagerAdapter { public SectionsPagerAdapter(FragmentManager fm) { super(fm); } @Override public Fragment getItem(int position) { // getItem is called to instantiate the fragment for the given page. // Return a DummySectionFragment (defined as a static inner class // below) with the page number as its lone argument. Fragment fragment = null; Bundle args = new Bundle(); args.putInt(ARG_SECTION_NUMBER, position + 1); switch (position) { case 0: // position 0 is the Map if (mGluehweinMapFragment == null) { mGluehweinMapFragment = new GluehweinMapFragment(); } fragment = mGluehweinMapFragment; break; case 1: // position 1 is the List if (mGluehweinListFragment == null) { mGluehweinListFragment = new GluehweinListFragment(); } fragment = mGluehweinListFragment; break; } fragment.setArguments(args); return fragment; } @Override public int getCount() { return 2; } @Override public CharSequence getPageTitle(int position) { Locale l = Locale.getDefault(); switch (position) { case 0: return getString(R.string.tab_map).toUpperCase(l); case 1: return getString(R.string.tab_list).toUpperCase(l); } return null; } } @Override protected void onResume() { // on resume, restart requesting location updates super.onResume(); //[..] initSectionsPagerAdapter(); //some calls on the fragments which are null here!!! refreshFragments(); } /** * Notifies the Fragments in the Tabs that data has changed */ private void refreshFragments() { if (this.mGluehweinMapFragment != null) { mGluehweinMapFragment.updateVisibility(); } if (this.mGluehweinListFragment != null) { mGluehweinListFragment.updateVisibility(); } } } 

I asked a colleague and he told me that it’s not good to keep both Fragments as members in my Activity , because Android will eventually create a new instance of Fragments onResume , and that’s why my links are null, But he couldn’t tell me how to fix my problem. I was thinking of writing a getFragment() method that would use the FragmentManager to get the fragments, but I don't know how to get my fragments in a state where I can read them using the FragmentManager .

Thank you in advance!

Answer 1

Your colleague is right: it's not good practice to use snippet references in your activity variables. Your activity will be destroyed, for example. when you rotate the screen, and links can lead to memory leak.

For your problem, move the code that initializes the ViewPager (your initSectionsPagerAdapter() method) to onResume . It will be called up when the activity first starts, and also when it becomes visible, for example. when another application that was on top is closed. You do not need the current code in onResume .

EDIT:

When you create a fragment, do not store the link to it in activity variables. To access the fragments later, you can use:

 FragmentManager fm = this.getFragmentManager(); GluehweinMapFragment f1 = (GluehweinMapFragment)fm.getFragments().get(0); // to get one fragment for (Fragment f : fm.getFragments()) { // to loop through fragments and checking their type if (f instanceof GluehweinMapFragment) { } }