在onresume之后，ViewPager中的片段为null

Question

I'm writing an application that has 2 Tabs in its MainActivity . I created the Activity Template with the Android Plugin for Eclipse and adapted it. Anyway when I resume the Application after a while the Fragments will be null.

Here is the code of my Activity (I removed "unimportant" 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 said me that its not good to keep the two Fragments as members in my Activity , because Android eventually would create a new instance of the Fragments onResume and this is why my references are null. But he could not tell me how to fix my problem. I was thinking about writing a getFragment() method that would use a FragmentManager to get the fragments, but I don't know how to get my fragments in a state, where I can read them with a FragmentManager .

Thank you in advance!

Answer 1

Your colleague is right: it's not a good practice to use references to fragments in your activity's variables. Your activity will be destroyed, eg when you rotate the screen, and the references may lead to memory leaks.

As for your problem, move the code that initialises the ViewPager (your initSectionsPagerAdapter() method) to onResume . It will be called when the activity is first started as well as when it becomes visible, eg when another app that was on top is closed. You don't need your current code in onResume .

EDIT:

When you create a fragment, don't store the reference to it in activity's 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) {
    }
}