Question

(I figured out a solution – please see my post in the Answer section below.)

In my app, the user will start with a single view of his data. I’d like to add a ViewPager and allow the user to add more views as desired. How do I do this? (I dont’ want to use the FragmentPagerAdapter.)

I’ve read countless posts and overviews but am still missing something. Here’s what I think I understand:

MainActivity creates a ViewPager and PagerAdapter:

ViewPager pager = null;
MainPagerAdapter adapter = null;
public void onCreate (Bundle savedInstanceState)
{
  super.onCreate (savedInstanceState);
  pager = new ViewPager (this);
  setContentView (pager);

  adapter = new MainPagerAdapter();
  pager.setAdapter (adapter); 

  View v0 = code_to_create_initial_view();
  adapter.add (v0, 0);      
}

Use a PagerAdapter to provide the sets of view. For this it seems I need methods to add and remove views, something like this; obviously more is needed to tell the ViewPager stuff has changed and how to show the change:

class MainPagerAdapter extends PagerAdapter
{
  // This holds all the currently displayable views, in order from left to right.
  private ArrayList<View> views = new ArrayList<View>();

  public void addView (View v, int position)
  {
    views.add (position, v);
  }

  public void removeView (int position)
  {
    views.remove (position);
  }
}

In addition, I need to implement the following vitual methods. I’m lost here – what calls them and what are they supposed to do (ok, getCount is obvious)?

  public object instantiateItem (ViewGroup pager, int position);
  public void destroyItem (ViewGroup, int, Object);
  public int getCount ();
  public boolean isViewFromObject (View, Object);

• What are the ViewGroup params for – isn’t the containing group the ViewPager itself?
• What does isViewFromObject do – how does an object get associated with a view in the first place?
• Should I be calling startUpdate and finishUdate when I add or remove views?

Thanks.

Answer 1

After figuring out which ViewPager methods are called by ViewPager and which are for other purposes, I came up with a solution. I present it here since I see a lot of people have struggled with this and I didn’t see any other relevant answers.

First, here’s my adapter; hopefully comments within the code are sufficient:

public class MainPagerAdapter extends PagerAdapter
{
  // This holds all the currently displayable views, in order from left to right.
  private ArrayList<View> views = new ArrayList<View>();

  //-----------------------------------------------------------------------------
  // Used by ViewPager.  "Object" represents the page; tell the ViewPager where the
  // page should be displayed, from left-to-right.  If the page no longer exists,
  // return POSITION_NONE.
  @Override
  public int getItemPosition (Object object)
  {
    int index = views.indexOf (object);
    if (index == -1)
      return POSITION_NONE;
    else
      return index;
  }

  //-----------------------------------------------------------------------------
  // Used by ViewPager.  Called when ViewPager needs a page to display; it is our job
  // to add the page to the container, which is normally the ViewPager itself.  Since
  // all our pages are persistent, we simply retrieve it from our "views" ArrayList.
  @Override
  public Object instantiateItem (ViewGroup container, int position)
  {
    View v = views.get (position);
    container.addView (v);
    return v;
  }

  //-----------------------------------------------------------------------------
  // Used by ViewPager.  Called when ViewPager no longer needs a page to display; it
  // is our job to remove the page from the container, which is normally the
  // ViewPager itself.  Since all our pages are persistent, we do nothing to the
  // contents of our "views" ArrayList.
  @Override
  public void destroyItem (ViewGroup container, int position, Object object)
  {
    container.removeView (views.get (position));
  }

  //-----------------------------------------------------------------------------
  // Used by ViewPager; can be used by app as well.
  // Returns the total number of pages that the ViewPage can display.  This must
  // never be 0.
  @Override
  public int getCount ()
  {
    return views.size();
  }

  //-----------------------------------------------------------------------------
  // Used by ViewPager.
  @Override
  public boolean isViewFromObject (View view, Object object)
  {
    return view == object;
  }

  //-----------------------------------------------------------------------------
  // Add "view" to right end of "views".
  // Returns the position of the new view.
  // The app should call this to add pages; not used by ViewPager.
  public int addView (View v)
  {
    return addView (v, views.size());
  }

  //-----------------------------------------------------------------------------
  // Add "view" at "position" to "views".
  // Returns position of new view.
  // The app should call this to add pages; not used by ViewPager.
  public int addView (View v, int position)
  {
    views.add (position, v);
    return position;
  }

  //-----------------------------------------------------------------------------
  // Removes "view" from "views".
  // Retuns position of removed view.
  // The app should call this to remove pages; not used by ViewPager.
  public int removeView (ViewPager pager, View v)
  {
    return removeView (pager, views.indexOf (v));
  }

  //-----------------------------------------------------------------------------
  // Removes the "view" at "position" from "views".
  // Retuns position of removed view.
  // The app should call this to remove pages; not used by ViewPager.
  public int removeView (ViewPager pager, int position)
  {
    // ViewPager doesn't have a delete method; the closest is to set the adapter
    // again.  When doing so, it deletes all its views.  Then we can delete the view
    // from from the adapter and finally set the adapter to the pager again.  Note
    // that we set the adapter to null before removing the view from "views" - that's
    // because while ViewPager deletes all its views, it will call destroyItem which
    // will in turn cause a null pointer ref.
    pager.setAdapter (null);
    views.remove (position);
    pager.setAdapter (this);

    return position;
  }

  //-----------------------------------------------------------------------------
  // Returns the "view" at "position".
  // The app should call this to retrieve a view; not used by ViewPager.
  public View getView (int position)
  {
    return views.get (position);
  }

  // Other relevant methods:

  // finishUpdate - called by the ViewPager - we don't care about what pages the
  // pager is displaying so we don't use this method.
}

And here’s some snips of code showing how to use the adapter.

class MainActivity extends Activity
{
  private ViewPager pager = null;
  private MainPagerAdapter pagerAdapter = null;

  //-----------------------------------------------------------------------------
  @Override
  public void onCreate (Bundle savedInstanceState)
  {
    super.onCreate(savedInstanceState);
    setContentView (R.layout.main_activity);

    ... do other initialization, such as create an ActionBar ...

    pagerAdapter = new MainPagerAdapter();
    pager = (ViewPager) findViewById (R.id.view_pager);
    pager.setAdapter (pagerAdapter);

    // Create an initial view to display; must be a subclass of FrameLayout.
    LayoutInflater inflater = context.getLayoutInflater();
    FrameLayout v0 = (FrameLayout) inflater.inflate (R.layout.one_of_my_page_layouts, null);
    pagerAdapter.addView (v0, 0);
    pagerAdapter.notifyDataSetChanged();
  }

  //-----------------------------------------------------------------------------
  // Here's what the app should do to add a view to the ViewPager.
  public void addView (View newPage)
  {
    int pageIndex = pagerAdapter.addView (newPage);
    // You might want to make "newPage" the currently displayed page:
    pager.setCurrentItem (pageIndex, true);
  }

  //-----------------------------------------------------------------------------
  // Here's what the app should do to remove a view from the ViewPager.
  public void removeView (View defunctPage)
  {
    int pageIndex = pagerAdapter.removeView (pager, defunctPage);
    // You might want to choose what page to display, if the current page was "defunctPage".
    if (pageIndex == pagerAdapter.getCount())
      pageIndex--;
    pager.setCurrentItem (pageIndex);
  }

  //-----------------------------------------------------------------------------
  // Here's what the app should do to get the currently displayed page.
  public View getCurrentPage ()
  {
    return pagerAdapter.getView (pager.getCurrentItem());
  }

  //-----------------------------------------------------------------------------
  // Here's what the app should do to set the currently displayed page.  "pageToShow" must
  // currently be in the adapter, or this will crash.
  public void setCurrentPage (View pageToShow)
  {
    pager.setCurrentItem (pagerAdapter.getItemPosition (pageToShow), true);
  }
}

Finally, you can use the following for your activity_main.xml layout:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.view.ViewPager
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/view_pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

</android.support.v4.view.ViewPager>