I believe that FragmentStatePagerAdapter does not behave correctly when
overriding getItemPosition(Object object) with the purpose of reordering
the pages.
Below is a simple example. In the initial state, the order of the pages is
{A, B, C}. Upon calling toggleState(), the order of the pages changes to
{A, C, B}. By overriding getItemPosition(Object object), we ensure that the
current page being viewed (A, B, or C) does not change.
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));
}
}
I have encountered two separate behaviours which seem incorrect.
1. If I immediately call toggleState() (while viewing page A, before
swiping to any other page), the app crashes.
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)
...
Looking at the source (FragmentStatePagerAdapter.java), this would be
fixed by first checking the size of mFragments (as in lines 113-115) before
calling set() in line 136.
2. If I first swipe to page B, then getItem(2) is called, page C is
created, and mFragments 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 toggleState(). 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.
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
getItemPosition(Object object) successfully (apart from setting everything
to POSITION_NONE)?
I am using the current revision (10) of the support library.
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