Question

I’m working with a FragmentStatePagerAdapter using this example.

The MyAdapter class is implemented as follows:

public static class MyAdapter extends FragmentStatePagerAdapter {
        public MyAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public int getCount() {
            return NUM_ITEMS;
        }

        @Override
        public Fragment getItem(int position) {
            return ArrayListFragment.newInstance(position);
        }
    }

The ListFragment class includes the following method to create a new instance:

    /**
     * Create a new instance of CountingFragment, providing "num"
     * as an argument.
     */
    static ArrayListFragment newInstance(int num) {
        ArrayListFragment f = new ArrayListFragment();

        // Supply num input as an argument.
        Bundle args = new Bundle();
        args.putInt("num", num);
        f.setArguments(args);

        return f;
    }

When I create a new fragment state pager adapter in my activity, getItem is called, which in turn calls the newInstance method in the ListFragment class. This is great when I want to create a new fragment.

But it’s not clear to me how to modify getItem (if even needed) to get the fragment object when it already exists and the user pages from, for example, page 2 to page 1. I’d like my Activity to retrieve that existing, previously created fragment so that it can run an inner class AsyncMethod, which resides in the fragment class.

Answer 1

I have implemented something similar to what you have. I extended the FragmentPagerAdapter class like so:

public class ContactsFragmentPagerAdapter extends FragmentPagerAdapter {
    ActionBar mActionBar;
    private List<Fragment> mFragments;

    public ContactsFragmentPagerAdapter(FragmentManager fm, List<Fragment> fragments) {
        super(fm);
        mFragments = fragments;
    }

    @Override
    public int getCount() {
        return mFragments.size();
    }

    @Override
    public Fragment getItem(int position) {
        return mFragments.get(position);
    }

    public void setActionBar(ActionBar bar) {
        mActionBar = bar;
    }
}

Notice I have added an argument to the constructor to pass in the List of Fragment objects. This way the getItem() method of this class can return any class that extends Fragment or any of its subclasses and not just one specific class ArrayListFragment like you have done.

In the Activity where I instantiate my subclass of FragmentPagerAdapter I have passed in the list of Fragment objects:

Class the instantiates the FragmentPagerAdapter

public final class ContactManager extends Activity {
    private ContactsFragmentPagerAdapter mAdapter;
    private ViewPager mPager;
    public ActionBar mActionBar;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.contact_manager);

        List<Fragment> fragments = new Vector<Fragment>();
        fragments.add(Fragment.instantiate(this, ContactsListFragment.class.getName()));
        fragments.add(Fragment.instantiate(this, GroupsListFragment.class.getName()));
        mAdapter = new ContactsFragmentPagerAdapter(this.getFragmentManager(), fragments);

        mPager = (ViewPager) findViewById(R.id.pager);
        mPager.setAdapter(mAdapter);

        mPager.setOnPageChangeListener(new OnPageChangeListener() {
            @Override
            public void onPageScrollStateChanged(int arg0) {}

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

            @Override
            public void onPageSelected(int arg0) {
                mActionBar.getTabAt(arg0).select();
            }
        });

    }

}

By accessing the variable “fragments”, you can access a previously created Fragment so that you can run methods of that Fragment.