Adding/Removing views for PagerAdapter / ViewPager issue Android

Question

I would like to keep a list of 3 views at all times. The app starts at position 1 (out of positions 0,1,2). When someone scrolls to position 0, I would like to remove view 2, and create a view before position 0. This way, it appears to the user, that there are unlimited views. In the same way, when someone scrolls to position 2, I would like to remove the view at position 0 and add one at the end.

However I'm having problems with both adding and removing views. When I get to position 0, nothing changes unless I try scrolling past position 0 (to position -1, i.e. the boundary is hit). At that point, I can see that it is the boundary of my views, but then setCurrentItem(1,false) is triggered and I'm brought back to the middle of the views. When I scroll to position 2 I see that position 2 has been updated. However position 0 and 1 remain the same.

When I scroll to position 2, nothing happens. However if I try and scroll to the boundary, for some reason, position 0 gets updated and setCurrentItem(1,false) is triggered.

I have no idea why its happening like this. Can anyone shed some light on this?

Here is my code:

public class MainActivity extends Activity {


ArrayList<Integer> showThree = new ArrayList<Integer>();
    int focusedPage = 0;

public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        showThree.add(0,5); //adding integers 5,6,7 for positions 0,1,2
        showThree.add(1,6);
        showThree.add(2,7);

        final MyPagerAdapter adapter = new MyPagerAdapter(getApplicationContext(),showThree);
        final ViewPager myPager = (ViewPager) findViewById(R.id.mypanelpager);
        myPager.setAdapter(adapter);
        myPager.setCurrentItem(1);

myPager.setOnPageChangeListener(new OnPageChangeListener(){

        @Override
        public void onPageScrollStateChanged(int state) {
            if (state == ViewPager.SCROLL_STATE_IDLE) {

//when position= 0, change the 3 views from 5,6,7 to 4,5,6. 
                 if (focusedPage == 0) {
                     showThreeMonths.set(0,4);
                     showThreeMonths.set(1,5);
                     showThreeMonths.set(2,6);
                     adapter.notifyDataSetChanged();
                     adapter.startUpdate(myPager);

                     }
                 else if (focusedPage ==2){
                        //ignore, just testing focusPage=0 for now }

                 }

//set current page to the middle of the 3 new views, which would be 
//the same view at position 0 of the old 3 views. 
//Thus user doesn't experience the views changing despite being 3 new views.

             myPager.setCurrentItem(1,false); 

        }

        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            // TODO Auto-generated method stub

        }

        @Override
        public void onPageSelected(int position) {
            focusedPage = position; 

        }

    });
}

PagerAdapter

public class MyPagerAdapter extends PagerAdapter {

private ArrayList<Integer> showThreeMonths;
private Context ctx;

public MyPagerAdapter (Context ctx, ArrayList<Integer> showThree){
    this.ctx = ctx ;
    this.showThree = showThree;
}


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



public Object instantiateItem(ViewGroup collection, int position ){
    //NewCustomView is a class I made that takes parameters context and an integer and creates a view based on the integer
    NewCustomView MyOwnView = new NewCustomView(ctx, showThree.get(position)); 


    View customViewLayout = MyOwnView.newLayout; //part of the class object
    collection.addView(customViewLayout);
    return customViewLayout;
 }


@Override
 public void destroyItem(ViewGroup collection, int position, Object arg2) {
     ((ViewPager) collection).removeView((ViewGroup) arg2);}


 @Override
 public Parcelable saveState() {
     return null;}


@Override
public boolean isViewFromObject(View view, Object arg1) {
    return view==arg1;}
@Override
public void startUpdate(ViewGroup collection) {}

@Override
public void finishUpdate(ViewGroup collection) {}


}

Answer 1

The instantiateItem() method creates the 2 view pages in the memory by default. Therefore when you swipe to the second page then 0 page is recreated as it's outside the range of the 2 pages saved in the memory. Please try to use

myViewPager.setOffscreenPageLimit(numberOfPages) 

method that receives an integer as a parameter and declares how many pages it should be keeping before recycling them.