Question

I am new to using a ViewPager, but the initial screen on my app is a ListView that the user can add/remove new items to, then clicking on one of the items brings them to a “details” fragment based on an id that is passed. I’d like for the user to also be able to swipe from the listing through to each of the details.

I have the ViewPager working, except the id’s are always off by one. This might be my lack of understanding of ViewPagers, but if I put a breakpoint in the onCreateView of the details fragment, the breakpoint is hit when the app loads and the id that is passed is the first id. So, say the ids are 1,2,3,4, when the app loads, the id on app start-up in onCreateView is 1. When I perform the initial swipe from the listing to the first details fragment, the id is 2 (when I would expect it to be 1).

This is what I have so far:

Main.class (this initial activity on app start-up)

public class Main extends SherlockFragmentActivity
{
    private static int NUMBER_OF_PAGES;  
    private ViewPager mViewPager;  
    private MyFragmentPagerAdapter mMyFragmentPagerAdapter; 
    private static List<Fragment> fragments;

    @Override
    public void onCreate(final Bundle icicle)
    {    
        super.onCreate(icicle);

        setContentView(R.layout.main);

        mViewPager = (ViewPager)findViewById(R.id.viewpager);  
        mMyFragmentPagerAdapter = new MyFragmentPagerAdapter(getSupportFragmentManager());  
        mViewPager.setAdapter(mMyFragmentPagerAdapter);

        final List<Integer> ids = GetIds(); //loads ids to popular the viewpager

        NUMBER_OF_PAGES = ids.size();

        fragments = new ArrayList<Fragment>();

        fragments.add(new ListingFragment()); //initial screen

        for(Integer id : ids)
            fragments.add(DetailsFragment.newInstance(id));
    }

    private static class MyFragmentPagerAdapter extends FragmentStatePagerAdapter  {            

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

        @Override  
        public Fragment getItem(int index) {

            return fragments.get(index);
        }

        @Override  
        public int getCount() {  

             return NUMBER_OF_PAGES;  
        }
   }
}

DetailsFragment.class

public class DetailsFragment extends SherlockFragmentActivity
{
    private int detailId;
    private List<Integer> items;

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);         
    }

    @Override
    public void onActivityCreated(final Bundle icicle)
    {    
        super.onActivityCreated(icicle);
    }

    public static DetailsFragment newInstance(int id) {

        DetailsFragment lf = new DetailsFragment();
        Bundle bundle = new Bundle();
        bundle.putInt("id", id);
        lf.setArguments(bundle);
        return lf;
    }

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

        View view = inflater.inflate(R.layout.details, container, false);

        detailId = getArguments().getInt("id"); //this id is always off

        items = GetItems();

        return view;
    }
}

Answer 1

I have the ViewPager working, except the id’s are always off by one.
This might be my lack of understanding of ViewPagers, but if I put a
breakpoint in the onCreateView of the details fragment, the breakpoint
is hit when the app loads and the id that is passed is the first id.
So, say the ids are 1,2,3,4, when the app loads, the id on app
start-up in onCreateView is 1. When I perform the initial swipe from
the listing to the first details fragment, the id is 2 (when I would
expect it to be 1).

Maybe I don’t understand the problem you’re facing but the code you posted(again, if that is the full code you use(currently it will not compile as your DetailsFragments extends SherlockFragmentActivity (?!))) can’t do what you say. The way you setup the fragments will make the them have the proper ids.

What you’re seeing it’s most likely due to the way the ViewPager works, which can be misleading. When you first start the app you’ll see the ListingFragment. Now, the ViewPager, in order to provide a smooth swipe for the user will also load(by default) one additional fragment on each side of the current visible fragment(so when the app starts the ViewPager will load page 0(ListingFragment) and the next(to the right as we don’t have anywhere to go left)) page, 1(the DetailsFragment with the id 1). When you swipe from the ListingFragment to the first DetailsFragment(which has the id 1) the ViewPager will automatically create the second DetailsFragment(which has the id 2) in advance(again, in order to be able to swipe right away to it). Now, if you have a Log statement in the onCreateView in the DetailsFragment(to see the id) it’s normal that you see the second id as you swipe from the ListingFragment to the first DetailsFragment because that Log will not appear from the first DetailsFragment(with id 1, which is already created) being created, it will appear because the second DetailsFragment(with id 2) is being created in advance.

Also, revise your code, as you introduced a subtle bug. You set the size of the adapter to
NUMBER_OF_PAGES which is initialized with ids.size() but then you add the ListingFragment which will make the DetailsFragment with the last id(from ids) not appear at all.