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POreorder pages in FragmentStatePagerAdapter using getItemPosition(Object object)
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  1. POreorder pages in FragmentStatePagerAdapter using getItemPosition(Object object)
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    copied!<p>I believe that FragmentStatePagerAdapter does not behave correctly when overriding <code>getItemPosition(Object object)</code> with the purpose of reordering the pages.</p> <p>Below is a simple example. In the initial state, the order of the pages is {A, B, C}. Upon calling <code>toggleState()</code>, the order of the pages changes to {A, C, B}. By overriding <code>getItemPosition(Object object)</code>, we ensure that the current page being viewed (A, B, or C) does not change.</p> <pre><code>public static class TestPagerAdapter extends FragmentStatePagerAdapter { private boolean mState = true; public TestPagerAdapter(FragmentManager fragmentManager) { super(fragmentManager); } @Override public int getCount() { return 3; } private void toggleState() { mState = !mState; notifyDataSetChanged(); } private String getLabel(int position) { switch (position) { case 0: return "A"; case 1: return mState ? "B" : "C"; default: return mState ? "C" : "B"; } } @Override public int getItemPosition(Object object) { String label = ((TestFragment) object).getLabel(); if (label.equals("A")) { return 0; } else if (label.equals("B")) { return mState ? 1 : 2; } else { return mState ? 2 : 1; } } @Override public CharSequence getPageTitle(int position) { return getLabel(position); } @Override public Fragment getItem(int position) { return TestFragment.newInstance(getLabel(position)); } } </code></pre> <p>I have encountered two separate behaviours which seem incorrect.</p> <ol> <li><p>If I immediately call <code>toggleState()</code> (while viewing page A, before swiping to any other page), the app crashes.</p> <pre><code>java.lang.IndexOutOfBoundsException: Invalid index 2, size is 2 at java.util.ArrayList.throwIndexOutOfBoundsException(ArrayList.java:251) at java.util.ArrayList.set(ArrayList.java:477) at android.support.v4.app.FragmentStatePagerAdapter.destroyItem(FragmentStatePagerAdapter.java:136) at android.support.v4.view.ViewPager.populate(ViewPager.java:867) at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:469) at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:441) at android.support.v4.view.ViewPager.dataSetChanged(ViewPager.java:766) at android.support.v4.view.ViewPager$PagerObserver.onChanged(ViewPager.java:2519) at android.database.DataSetObservable.notifyChanged(DataSetObservable.java:37) at android.support.v4.view.PagerAdapter.notifyDataSetChanged(PagerAdapter.java:276) at com.ugglynoodle.test.testfragmentstatepageradapter.MainActivity$TestPagerAdapter.toggleState(MainActivity.java:55) ... </code></pre> <p>Looking at the source of <code>FragmentStatePagerAdapter</code>, this would be fixed by first checking the size of <code>mFragments</code> (as in lines 113-115) before calling <code>set()</code> in line 136.</p></li> <li><p>If I first swipe to page B, then <code>getItem(2)</code> is called, page C is created, and <code>mFragments</code> now has a size of 3 (this will prevent the crash above from happening in a moment). Then I swipe back to page A, and page C is destroyed, as it should be (since it is 2 pages away, and I'm using the default offscreen page limit of 1). Now, I call <code>toggleState()</code>. Page B is now destroyed. However, page C is NOT recreated! This means, when I now swipe to the right, I get an empty page.</p></li> </ol> <p>First, it would be nice to know whether I'm correct and these are in fact bugs, or whether I'm doing something wrong. If they are bugs, can anyone suggest a workaround (other than debugging and rebuilding the support library myself)? Surely somebody must have overridden <code>getItemPosition(Object object)</code> successfully (apart from setting everything to <code>POSITION_NONE</code>)?</p> <p>I am using the current revision (10) of the support library.</p>
 

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